@article{voigtlaender_houssais_bacik_bourg_burton_daniels_datta_del gado_deshpande_devauchelle_et al._2024, title={Soft matter physics of the ground beneath our feet}, ISSN={["1744-6848"]}, url={https://doi.org/10.1039/D4SM00391H}, DOI={10.1039/D4SM00391H}, abstractNote={Earth's surface materials constitute the basis for life and natural resources. Most of these materials can be catergorized as soft matter, yet a general physical understanding of the ground beneath our feet is still lacking. Here we provide some perspectives.}, journal={SOFT MATTER}, author={Voigtlaender, Anne and Houssais, Morgane and Bacik, Karol A. and Bourg, Ian C. and Burton, Justin C. and Daniels, Karen E. and Datta, Sujit S. and Del Gado, Emanuela and Deshpande, Nakul S. and Devauchelle, Olivier and et al.}, year={2024}, month={Jul} }
 @article{fazelpour_daniels_2023, title={Controlling rheology via boundary conditions in dense granular flows}, ISSN={1744-6848}, url={https://doi.org/10.1039/D2SM00683A}, DOI={10.1039/D2SM00683A}, abstractNote={Boundary shape, particularly roughness, strongly controls the amount of wall slip and internal pressure in dense granular flows.}, journal={SOFT MATTER}, author={Fazelpour, Farnaz and Daniels, Karen E.}, year={2023}, month={Feb} }
 @misc{daniels_fazelpour_2023, title={Data from: Controlling rheology via boundary conditions in dense granular flows}, url={https://datadryad.org/stash/dataset/doi:10.5061/dryad.866t1g1v8}, DOI={10.5061/DRYAD.866T1G1V8}, journal={Dryad}, publisher={Dryad}, author={Daniels, Karen E. and Fazelpour, Farnaz}, year={2023} }
 @misc{desai_fazelpour_handwerger_daniels_2023, title={Data from: Forecasting landslides using community detection on geophysical satellite data}, url={https://datadryad.org/stash/dataset/doi:10.5061/dryad.41ns1rnjf}, DOI={10.5061/DRYAD.41NS1RNJF}, publisher={Dryad}, author={Desai, Vrinda D. and Fazelpour, Farnaz and Handwerger, Alexander L. and Daniels, Karen E.}, year={2023} }
 @article{desai_fazelpour_handwerger_daniels_2023, title={Forecasting landslides using community detection on geophysical satellite data}, volume={108}, ISSN={2470-0045 2470-0053}, url={http://dx.doi.org/10.1103/PhysRevE.108.014901}, DOI={10.1103/PhysRevE.108.014901}, abstractNote={As a result of extreme weather conditions, such as heavy precipitation, natural hillslopes can fail dramatically; these slope failures can occur on a dry day, due to time lags between rainfall and pore-water pressure change at depth, or even after days to years of slow motion. While the prefailure deformation is sometimes apparent in retrospect, it remains challenging to predict the sudden transition from gradual deformation (creep) to runaway failure. We use a network science method-multilayer modularity optimization-to investigate the spatiotemporal patterns of deformation in a region near the 2017 Mud Creek, California landslide. We transform satellite radar data from the study site into a spatially embedded network in which the nodes are patches of ground and the edges connect the nearest neighbors, with a series of layers representing consecutive transits of the satellite. Each edge is weighted by the product of the local slope (susceptibility to failure) measured from a digital elevation model and ground surface deformation (current rheological state) from interferometric synthetic aperture radar (InSAR). We use multilayer modularity optimization to identify strongly connected clusters of nodes (communities) and are able to identify both the location of Mud Creek and nearby creeping landslides which have not yet failed. We develop a metric, i.e., community persistence, to quantify patterns of ground deformation leading up to failure, and find that this metric increased from a baseline value in the weeks leading up to Mud Creek's failure. These methods hold promise as a technique for highlighting regions at risk of catastrophic failure.}, number={1}, journal={Physical Review E}, publisher={American Physical Society (APS)}, author={Desai, Vrinda D. and Fazelpour, Farnaz and Handwerger, Alexander L. and Daniels, Karen E.}, year={2023}, month={Jul} }
 @article{hillaire_nithyanandam_song_nadimi_kiani_dickey_daniels_2023, title={Interfacial Tension Hysteresis of Eutectic Gallium-Indium}, volume={12}, ISSN={["1616-3028"]}, url={https://doi.org/10.1002/adfm.202311501}, DOI={10.1002/adfm.202311501}, abstractNote={Abstract}, journal={ADVANCED FUNCTIONAL MATERIALS}, author={Hillaire, Keith D. and Nithyanandam, Praneshnandan and Song, Minyung and Nadimi, Sahar Rashid and Kiani, Abolfazl and Dickey, Michael D. and Daniels, Karen E.}, year={2023}, month={Dec} }
 @misc{data from: gardner-like crossover from variable to persistent force contacts in granular crystals_2022, DOI={10.5061/dryad.kd51c5b97}, journal={Dryad}, year={2022} }
 @article{jules_reid_daniels_mungan_lechenault_2022, title={Delicate memory structure of origami switches}, volume={4}, ISSN={["2643-1564"]}, url={https://doi.org/10.1103/PhysRevResearch.4.013128}, DOI={10.1103/PhysRevResearch.4.013128}, abstractNote={Théo Jules ,∗ Austin Reid , Karen E. Daniels , Muhittin Mungan , and Frédéric Lechenault Université de Lyon, Ecole Normale Supérieure de Lyon, Université Claude Bernard, CNRS, Laboratoire de Physique, F-69342 Lyon, France Laboratoire de Physique de l’École Normale Supérieure, ENS, PSL Research University, CNRS, Sorbonne University, Université Paris Diderot, Sorbonne Paris Cité, 75005 Paris, France 3 Raymond and Beverly Sackler School of Physics and Astronomy, Tel Aviv University, Ramat Aviv, Tel Aviv, 69978, Israel Center for Exploration of Energy and Matter, Indiana University Department of Physics, North Carolina State University and Institut für angewandte Mathematik, Universität Bonn, Endenicher Allee 60, 53115 Bonn, Germany (Dated: November 5, 2021)}, number={1}, journal={Physical Review Research}, author={Jules, Theo and Reid, Austin and Daniels, Karen E. and Mungan, Muhittin and Lechenault, Frederic}, year={2022}, month={Feb} }
 @article{kool_charbonneau_daniels_2022, title={Gardner-like crossover from variable to persistent force contacts in granular crystals}, volume={106}, ISSN={2470-0053}, url={https://doi.org/10.1103/PhysRevE.106.054901}, DOI={10.1103/PhysRevE.106.054901}, abstractNote={We report experimental evidence of a Gardner-like crossover from variable to persistent force contacts in a two-dimensional bidisperse granular crystal by analyzing the variability of both particle positions and force networks formed under uniaxial compression. Starting from densities just above the freezing transition and for variable amounts of additional compression, we compare configurations to both their own initial state and to an ensemble of equivalent reinitialized states. This protocol shows that force contacts are largely undetermined when the density is below a Gardner-like crossover, after which they gradually transition to being persistent, being fully so only above the jamming point. We associate the disorder that underlies this effect with the size of the microscopic asperities of the photoelastic disks used, by analogy to other mechanisms that have been previously predicted theoretically.}, number={5}, journal={Physical Review E}, author={Kool, Lars and Charbonneau, Patrick and Daniels, Karen E.}, year={2022}, month={Nov} }
 @book{kool_charbonneau_daniels_2022, title={Gardner-like transition from variable to persistent force contacts in granular crystals}, url={https://arxiv.org/abs/2205.06794}, DOI={10.48550/ARXIV.2205.06794}, abstractNote={We report experimental evidence of a Gardner-like transition from variable to persistent force contacts in a two-dimensional, bidisperse granular crystal by analyzing the variability of both particle positions and force networks formed under uniaxial compression. Starting from densities just above the freezing transition, and for variable amounts of additional compression, we compare configurations to both their own initial state, and to an ensemble of equivalent, reinitialized states. This protocol shows that force contacts are largely undetermined when the density is below a Gardner-like transition, after which they gradually transition to being persistent, being fully so only above the jamming point. We associate the disorder that underlies this effect to the size of the microscopic asperities of the photoelastic disks used, by analogy to other mechanisms that have been previously predicted theoretically.}, institution={arXiv}, author={Kool, Lars and Charbonneau, Patrick and Daniels, Karen E.}, year={2022} }
 @article{amini_tuohey_long_zhang_morton_daniels_fazelpour_hapgood_2022, title={Photoelastic stress response of complex 3D-printed particle shapes}, volume={409}, ISSN={1873-328X}, url={https://doi.org/10.1016/j.powtec.2022.117852}, DOI={10.1016/j.powtec.2022.117852}, abstractNote={Stress visualization within 3-dimensional particles could provide significant insights for understanding of the behaviors of complex particles. However traditional photoelastic methods are only able to produce simple particle shapes. Recently, 3D-printing has created new possibilities for enhancing the scope of stress analysis of physically representative particles. We report the results of X-ray computed tomography and 3D-printing, combined with traditional photoelastic analysis, to visualize strain for particles ranging from simple 2D discs to complex 3D-printed coffee beans, including internal voids. The relative orientation of the print layers and the loading force can affect the optical response of the discs without affecting the mechanical properties. Furthermore, a semi-quantitative measurement of the generated stresses within 3D-printed complex particle is presented. Potential limitations and areas of future interest for stress visualization of 3-dimensional particles are also outlined. • 3D printing photoelastic UV-curable resin enables stress analysis of complex particles. • Ray tracing confirms shadow at multiple faces of complex particles is from refraction. • Print orientation affects optical properties of 3D printed discs with applied load. • Semi-quantitative stress measurement can be obtained from complex 3D printed particles.}, journal={POWDER TECHNOLOGY}, author={Amini, Negin and Tuohey, Josh and Long, John M. and Zhang, Jun and Morton, David A. and Daniels, Karen E. and Fazelpour, Farnaz and Hapgood, Karen P.}, year={2022}, month={Sep} }
 @article{kozlowski_zheng_daniels_socolar_2022, title={Stick-Slip Dynamics in a Granular Material With Varying Grain Angularity}, volume={10}, ISSN={["2296-424X"]}, DOI={10.3389/fphy.2022.916190}, abstractNote={Experiments, simulations, and theoretical treatments of granular materials typically feature circular or elliptical grains. However, grains found in natural systems often have flat faces that introduce local rotational constraints; these rotational constraints have been shown to affect, for example, the jamming transition, discontinuous shear thickening, and ordered states in colloids and thermalized grains. In this work, we experimentally investigate the effects of grain angularity on stick-slip dynamics. A weighted slider is pulled by a spring over a gravity-packed granular bed composed of polygonal grains with varying angularity. We find that packings of triangular or square grains have higher shear strengths than packings of pentagons, hexagons, heptagons, or disks. Additionally, as the number of sides increases, sticking periods, during which the slider remains motionless while the spring force on it increases, become shorter on average, with the material yielding at smaller applied stresses. Lastly, we find that dilation of the medium during sticking periods tends to be larger for grains with higher angularity, in part because of the presence of stilt-like columnar structures that prop the slider up. We report on measurements of the pulling force on the slider, particle dynamics during slip events, and properties of force-bearing contact networks identified via photoelasticity. Our findings indicate that high angularity of grains (pentagons, squares, triangles) leads to differences in grain-scale flow and macroscopic stick-slip dynamics of bulk granular materials. Our experiments also indicate a continuous change in dynamics with decreasing angularity as the circular grain limit is approached.}, journal={Frontiers in Physics}, author={Kozlowski, Ryan and Zheng, Hu and Daniels, Karen E. and Socolar, Joshua E. S.}, year={2022}, month={Jul} }
 @article{fazelpour_tang_daniels_2022, title={The effect of grain shape and material on the nonlocal rheology of dense granular flows}, volume={1}, ISSN={["1744-6848"]}, url={https://doi.org/10.1039/D1SM01237A}, DOI={10.1039/D1SM01237A}, abstractNote={Experiments using particles with a variety of particle shapes and stiffnesses reveal the success of nonlocal rheologies in new contexts, with the degree of nonlocality depending sensitively on particle properties.}, journal={Soft Matter}, author={Fazelpour, Farnaz and Tang, Zhu and Daniels, Karen E.}, year={2022}, month={Jan} }
 @article{song_daniels_kiani_rashid‐nadimi_dickey_2021, title={Interfacial Tension Modulation of Liquid Metal via Electrochemical Oxidation}, volume={3}, ISSN={2640-4567 2640-4567}, url={http://dx.doi.org/10.1002/aisy.202100024}, DOI={10.1002/aisy.202100024}, abstractNote={Herein, this progress report summarizes recent studies of electrochemical oxidation to modulate the interfacial tension of gallium‐based alloys. These liquid alloys have the largest interfacial tension of any liquid at room temperature. The ability to modulate the tension offers the possibility to create forces that change the shape and position of small volumes of liquid metal. It has been known since the late 1800s that electrocapillarity—the use of potential to modulate the electric double layer on the surface of metals in electrolyte—lowers the interfacial tension of liquid metals. This phenomenon, however, can only achieve modest changes in interfacial tension since it is limited to potentials that avoid Faradaic reactions. A recent discovery suggests reactions driven by the electrochemical oxidation of gallium alloys cause the interfacial tension to decrease from ≈500 mN m−1 at 0 V to ≈0 mN m−1 at less than 1 V. This change in interfacial tension is reversible, controllable, and goes well‐beyond what is possible via conventional electrocapillarity or surfactants. This report aims to introduce beginners to this field and address misconceptions. The report discusses applications that utilize modulations in interfacial tension of liquid metal and concludes with remaining opportunities and challenges needing further investigation.}, number={8}, journal={Advanced Intelligent Systems}, publisher={Wiley}, author={Song, Minyung and Daniels, Karen E. and Kiani, Abolfazl and Rashid‐Nadimi, Sahar and Dickey, Michael D.}, year={2021}, month={May}, pages={2100024} }
 @article{kozlowski_zheng_daniels_socolar_2021, title={Particle dynamics in two-dimensional point-loaded granular media composed of circular or pentagonal grains}, volume={249}, ISSN={2100-014X}, url={http://dx.doi.org/10.1051/epjconf/202124906010}, DOI={10.1051/epjconf/202124906010}, abstractNote={Granular packings exhibit significant changes in rheological and structural properties when the rotational symmetry of spherical or circular particles is broken. Here, we report on experiments exploring the differences in dynamics of a grain-scale intruder driven through a packing of either disks or pentagons, where the presence of edges and vertices on grains introduces the possibility of rotational constraints at edge-edge contacts. We observe that the intruder’s stick-slip dynamics are comparable between the disk packing near the frictional jamming fraction and the pentagonal packing at significantly lower packing fractions. We connect this stark contrast in packing fraction with the average speed and rotation fields of grains during slip events, finding that rotation of pentagons is limited and the flow of pentagonal grains is largely confined in front of the intruder, whereas disks rotate more on average and circulate around the intruder to fill the open channel behind it. Our results indicate that grain-scale rotation constraints significantly modify collective motion of grains on mesoscopic scales and correspondingly enhance resistance to penetration of a local intruder.}, journal={EPJ Web of Conferences}, publisher={EDP Sciences}, author={Kozlowski, Ryan and Zheng, Hu and Daniels, Karen E. and Socolar, Joshua E. S.}, editor={Aguirre, M.A. and Luding, S. and Pugnaloni, L.A. and Soto, R.Editors}, year={2021}, pages={06010} }
 @article{kollmer_featherstone_bullard_emm_jackson_reid_shefferman_dove_colwell_daniels_2021, title={Probing regolith-covered surfaces in low gravity}, volume={249}, ISSN={2100-014X}, url={http://dx.doi.org/10.1051/epjconf/202124902005}, DOI={10.1051/epjconf/202124902005}, abstractNote={The surfaces of many planetary bodies, including asteroids, moons, and planets, are composed of rubble-like grains held together by varying levels of gravitational attraction and cohesive forces. Future instrumentation for operation on, and interacting with, such surfaces will require efficient and effective design principles and methods of testing. Here we present results from the EMPANADA experiment (Ejecta-Minimizing Protocols for Applications Needing Anchoring or Digging on Asteroids) which flew on several reduced gravity parabolic flights. EMPANADA studies the effects of the insertion of a flexible probe into a granular medium as a function of ambient gravity. This is done for an idealized 2D system as well as a more realistic 3D sample. To quantify the dynamics inside the 2D granular material we employ photoelasticity to identify the grain-scale forces throughout the system, while in 3D experiments we use simulated regolith. Experiments were conducted at three different levels of gravity: martian, lunar, and microgravity. In this work, we demonstrate that the photoelastic technique provides results that complement traditional load cell measurements in the 2D sample, and show that the idealized system exhibits similar behaviour to the more realistic 3D sample. We note that the presence of discrete, stick-slip failure events depends on the gravitational acceleration.}, journal={EPJ Web of Conferences}, publisher={EDP Sciences}, author={Kollmer, Jonathan E. and Featherstone, Jack and Bullard, Robert and Emm, Tristan and Jackson, Anna and Reid, Riley and Shefferman, Sean and Dove, Adrienne and Colwell, Joshua and Daniels, Karen E.}, editor={Aguirre, M.A. and Luding, S. and Pugnaloni, L.A. and Soto, R.Editors}, year={2021}, pages={02005} }
 @article{liu_kollmer_daniels_schwarz_henkes_2021, title={Spongelike Rigid Structures in Frictional Granular Packings}, volume={126}, ISBN={1079-7114}, url={https://doi.org/10.1103/PhysRevLett.126.088002}, DOI={10.1103/PhysRevLett.126.088002}, abstractNote={We show how rigidity emerges in experiments on sheared two-dimensional frictional granular materials by using generalizations of two methods for identifying rigid structures. Both approaches, the force-based dynamical matrix and the topology-based rigidity percolation, agree with each other and identify similar rigid structures. As the system becomes jammed, at a critical contact number z_{c}=2.4±0.1, a rigid backbone interspersed with floppy, particle-filled holes of a broad range of sizes emerges, creating a spongelike morphology. While the pressure within rigid structures always exceeds the pressure outside the rigid structures, they are not identified with the force chains of shear jamming. These findings highlight the need to focus on mechanical stability arising through arch structures and hinges at the mesoscale.}, number={7}, journal={Physical Review Letters}, author={Liu, Kuang and Kollmer, Jonathan E. and Daniels, Karen E. and Schwarz, J. M. and Henkes, Silke}, year={2021} }
 @article{featherstone_bullard_emm_jackson_reid_shefferman_dove_colwell_kollmer_daniels_2021, title={Stick-slip Dynamics in Penetration Experiments on Simulated Regolith}, volume={2}, ISSN={2632-3338}, url={http://dx.doi.org/10.3847/PSJ/ac3de2}, DOI={10.3847/PSJ/ac3de2}, abstractNote={Abstract}, number={6}, journal={The Planetary Science Journal}, publisher={American Astronomical Society}, author={Featherstone, Jack and Bullard, Robert and Emm, Tristan and Jackson, Anna and Reid, Riley and Shefferman, Sean and Dove, Adrienne and Colwell, Joshua and Kollmer, Jonathan E. and Daniels, Karen E.}, year={2021}, month={Dec}, pages={243} }
 @article{kozlowski_zheng_daniels_socolar_2021, title={Stress propagation in locally loaded packings of disks and pentagons}, volume={10}, ISSN={["1744-6848"]}, url={https://doi.org/10.1039/D1SM01137E}, DOI={10.1039/D1SM01137E}, abstractNote={A granular medium composed of pentagons transmits stresses more directly from a local load to the walls of its container than does a medium composed of disks made of the same material.}, journal={Soft Matter}, publisher={Royal Society of Chemistry (RSC)}, author={Kozlowski, Ryan and Zheng, Hu and Daniels, Karen E. and Socolar, Joshua E. S.}, year={2021}, month={Oct} }
 @article{fazelpour_daniels_2021, title={The effect of boundary roughness on dense granular flows}, volume={249}, ISSN={2100-014X}, url={http://dx.doi.org/10.1051/epjconf/202124903014}, DOI={10.1051/epjconf/202124903014}, abstractNote={In the field of granular rheology, an important open question is to understand the influence of boundary conditions on granular flows. We perform experiments in a quasi-2D annular shear cell subject to 6 different boundaries with controlled roughness/compliance. We characterize the granular slip at the boundaries to investigate which aspects of a dense granular flow can be controlled by the choice of boundary condition. Photoelastic techniques are implemented to measure the stress fields P(r) and τ(r) throughout the material. A full inverse-analysis of the fringes within each disk provides the vector force at each contact. This allows us to measure the continuum stress field by coarse-graining internal forces. We have observed that boundary roughness and compliance strongly controls the flow profile v(r) and shear rate profile γ˙(r). We also observed that boundary roughness and compliance play a significant role in the pressure profile P(r) and shear stress profile τ(r).}, journal={EPJ Web of Conferences}, publisher={EDP Sciences}, author={Fazelpour, Farnaz and Daniels, Karen E.}, editor={Aguirre, M.A. and Luding, S. and Pugnaloni, L.A. and Soto, R.Editors}, year={2021}, pages={03014} }
 @article{rock_vadlakonda_figurskey_ledford_west_miller_pankow_daniels_horn_2020, title={Analysis of Self-Organized Patterned Surface Oxide Spots on Ejected Spatter Produced during Laser Powder Bed Fusion}, volume={35}, ISBN={2214-7810}, url={http://dx.doi.org/10.1016/j.addma.2020.101320}, DOI={10.1016/j.addma.2020.101320}, abstractNote={Spatter particles ejected from the melt pool after melting of 316 L stainless steel by laser powder bed fusion additive manufacturing (LPBF), were found to contain morphologies not observed in as-atomized 316 L powder. This spatter consisted of large, spherical particles, highly dendritic surfaces, particles with caps of accreted liquid, and agglomerations of multiple individual particles fixed together by liquid ligaments prior to solidification. The focus of this study is on an additional, unique spatter morphology consisting of larger, spherical particles with surface oxide spots exhibiting a wide distribution of surface configurations, including organized patterning. Spatter particles with organized surface oxide patterns were characterized for surface and internal particle features using multiple imaging techniques. The following observations are made: 1) spots resided at the spatter particle surface and did not significantly penetrate the interior, 2) the spot(s) were amorphous and rich in Silicon (Si)-Manganese (Mn)-Oxygen (O), 3) a two-part Chromium (Cr)-O rich layer exists between the particle and spot, 4) Cr-O rich morphological features were present at the top surface of the spots, 5) the spatter particle composition was consistent with 316 L but appeared to decrease in Si content into the spatter particle away from a spot, and 6) small Si-rich spherical particles existed within the spatter particle interior.}, journal={Additive Manufacturing}, publisher={Elsevier BV}, author={Rock, Christopher and Vadlakonda, Rashmi and Figurskey, Sullivan and Ledford, Christopher and West, Harvey and Miller, Victoria and Pankow, Mark and Daniels, Karen E. and Horn, Tim}, year={2020}, month={Oct}, pages={101320} }
 @article{bardall_chen_daniels_shearer_2020, title={Gradient-induced droplet motion over soft solids}, volume={85}, ISBN={1464-3634}, DOI={10.1093/imamat/hxaa015}, abstractNote={Abstract}, number={3}, journal={IMA Journal of Applied Mathematics}, publisher={Oxford University Press (OUP)}, author={Bardall, Aaron and Chen, Shih-Yuan and Daniels, Karen E. and Shearer, Michael}, year={2020}, month={Jun}, pages={495–512} }
 @article{daniels_elting_2020, title={Knitting Ripples}, volume={1}, ISSN={2666-3899}, url={http://dx.doi.org/10.1016/j.patter.2020.100034}, DOI={10.1016/j.patter.2020.100034}, abstractNote={Ripples are common in both biological systems and human clothes. Knitters have long exploited the ability of fabric to curl out of plane, by either removing or adding stitches to the material as it is created. Here, we present two knitting patterns for scarves to illustrate how ripples can arise through such additive processes. Ripples are common in both biological systems and human clothes. Knitters have long exploited the ability of fabric to curl out of plane, by either removing or adding stitches to the material as it is created. Here, we present two knitting patterns for scarves to illustrate how ripples can arise through such additive processes. Humans, plants, and marine creatures are all known to decorate their margins with a bit of frill: skirts, ornamental kale, and nudibranchs all catch our eye due to their rippling edges, whether created by fashion, plant breeding, or evolution. The unifying mechanism for these ripples was identified by the work of scientists working at the University of Texas Center for Nonlinear Dynamics during the early 2000s, with the patterns (sometimes fractal!) arising from both the elastic and geometric properties of thin sheets. The central idea is non-Euclidean: if the area of a thin sheet is larger than the space available to it, it must deform out of plane, as illustrated by the scarves in Figure 1. This forms the ripples, via spontaneous symmetry breaking. Where the material is not stiff, the ripples can be readily rearranged into other ripples after they are created, which is why they are appealing on clothes: they rearrange as you move. Knitters have long exploited non-Euclidean shapes in designing garments that curve inward around our heads (positive Gaussian curvature) or outward into ripples (negative Gaussian curvature). They do this by either removing or adding stitches to the material as it is created, line by line from one edge of the garment to the other. Using such techniques, it is possible to turn the corner on the heel of a sock, or to taper the sleeve of a sweater. When one of the authors first heard about the work of Sharon et al.,1Sharon E. Roman B. Marder M. Shin G.-S. Swinney H.L. Buckling cascades in free sheets.Nature. 2002; 419: 579https://doi.org/10.1038/419579aCrossref PubMed Scopus (152) Google Scholar, 2Marder M. Sharon E. Smith S. Roman B. Theory of edges of leaves.Europhys. Lett. 2003; 62https://doi.org/10.1209/epl/i2003-00334-5Crossref Scopus (48) Google Scholar, 3Sharon E. Marder M. Swinney H. Leaves, Flowers and Garbage Bags: Making Waves.Am. Sci. 2004; 92 (https://www.jstor.org/stable/27858394): 254-261Crossref Google Scholar she turned to knitting to create a concrete demonstration of the principle. In the resulting scarf (Figure 1A), there are about 130 rows along the central spine of the scarf, but 3 times as many along each edge. These extra stitches ripple out of the plane of flat material created along the central spine. In fact, due to the way knits are constructed (row by row, zig-zagging along the fabric; see Figure 2B), there are two ways to imagine creating such a rippled scarf: from long edge to long edge, or from short end to short end. In the scarf shown in Figure 1A, the latter orientation was chosen for design reasons: the knitter had only one ball of yarn (sentimentally gifted from a fellow physics graduate student) to use to make the scarf. Using the edge-to-edge method presented two advantages: maximal use of the ball of yarn to make the scarf as long as possible and the opportunity to more immediately see that the rippling principle was working as intended. This method involves casting on enough stitches to form the width of the scarf, then periodically knitting extra, partial rows that extend from the edge of the scarf only part way in toward the spine. The line defects created by these short rows introduce negative Gaussian curvature to the fabric. A short segment of scarf illustrating this technique is shown in Figure 3A.Figure 3Illustration of the Two MethodsShow full captionA small section of scarf, created using each of the two methods.(A) This sample was knit from edge to edge (black arrow), with line-defects (magenta lines) added through the technique known as short-rows, and corresponds to the scarf in Figure 1A.(B) This sample was knit from the center line outward toward each long edge, from end to end (black arrow), with line-defects (magenta lines) added through the technique known as increasing stitches, and corresponds to the scarf in Figure 1B.View Large Image Figure ViewerDownload (PPT) A small section of scarf, created using each of the two methods. (A) This sample was knit from edge to edge (black arrow), with line-defects (magenta lines) added through the technique known as short-rows, and corresponds to the scarf in Figure 1A. (B) This sample was knit from the center line outward toward each long edge, from end to end (black arrow), with line-defects (magenta lines) added through the technique known as increasing stitches, and corresponds to the scarf in Figure 1B. The second method (Figure 1B), more conventionally used by knitters to shape garments such as sweaters or hats, is to first cast on a chain of stitches as long as the edge of the scarf is intended to be and then knit outward toward each rippled edge by adding individual stitches at regular intervals while knitting each row. Gradually, the ripples develop as the number of stitches per row grows. A sample of this technique is shown in Figure 3B, illustrating the knitting direction and the line defects where the new stitches are added. Again, these line defects are what induce the negative Gaussian curvature. To generate the second side of the scarf, you pick up the stitches from the spine and begin the process again. In this method, using up all the yarn requires casting on the correct number of stitches from the beginning. The easiest way to surmount this challenge is to knit a sample of a small number of stitches, weigh the sample, and then scale up to the total amount of yarn. Note that these two examples yield samples with a different anisotropy: the orientation of the stitches is either along the scarf or perpendicular to it, which changes how the fabric drapes and flows.4Poincloux S. Adda-Bedia M. Lechenault F. Geometry and Elasticity of a Knitted Fabric.Phys. Rev. X. 2018; 8: 21075https://doi.org/10.1103/PhysRevX.8.021075Crossref Scopus (25) Google Scholar,5Markande S.G. Matsumoto E.A. Knotty knits are tangles on tori.arXiv. 2002; (2002.01497)https://arxiv.org/abs/2002.01497Google Scholar In fact, if you look carefully at Figure 2B, you will notice that even the flattest knitting is slightly anisotropic—the fabric curls toward the front side on the top and bottom (k1 < 0 but small) and toward the backside on the sides (k2 > 0 but small). This inherent curvature has an interesting effect on our two rippled-scarf methods. In the “short-row” method, knit from edge to edge, the fabric curls toward the purled side, making the knitted side (the one normally used as the “right” side) showing. In the “increasing stitches” method, knitted from the center line outward from end to end, the scarf curls toward the knit side, leaving the purled side (the conventional “wrong” side) showing. Below, we have included knitting instructions for both versions of the scarves. The adventurous reader/knitter might be interested in extending them in a variety of ways. For example, the amount of curvature can be increased by doing more short rows with fewer stitches between them in the edge-to-edge version, or by doing more increases in the end-to-end version. Changing the stitch pattern, such as by introducing horizontal or vertical ribbing or by knitting all rows on both sides, would also change the behavior of the final scarf. These patterns will also be posted on the fiber arts community Ravelry; please join us there to share your handiwork. As shown in Figures 1 and 3, both scarves are reversible and so technically have no right (knit) or wrong (purl) side. However, we’ve used the traditional “RS” and “WS” notation to help you keep track of which side is which on the short rows. In the patterns, we use “RS” to mean the side facing the knitter on row 1. Gauge: 5 stitches/inch Cast on 24 stitches.Row 1: Knit 12, place marker, purl to end of row.Row 2: Knit 12, purl 12. Short row section 1:∗Short row 1 [RS]: Knit 6, wrap and turn 1 stitch.Short row 2 [WS]: Purl back to start of row.Short row 3 [RS]: Knit 3, wrap and turn 1 stitch.Short row 4 [WS]: Purl back to start of row.Short row 5 [RS]: Knit to marker, working wrapped stitches together with their wraps. Purl to end of row. Short row section 2:Repeat from ∗, except this time the WS and RS will be reversed since you’re starting on the WS. Repeat short row sections 1 and 2 until you’re out of yarn or scarf is desired length. Bind off all stitches knitwise. Gauge: 5 stitches/inch Cast on 200 stitches (or as many as you want your scarf to be long).Rows 1–6: Stockinette stitch (knit the RS and purl the WS).Row 7: Knit front and back in every stitch. (400 st)Row 8: Purl.Row 9: Knit.Row 10: Purl.Row 11: ∗Knit 1, knit into front and back of next stitch repeat from ∗ across. (600 st)Row 12: Purl.Row 13: Knit.Row 14: Purl. With knit side of work facing, pick up 200 stitches from the cast on edge. Repeat rows 1–14 and bind off, as for the first half of the scarf. (Note: You will knit the first row, with the purl side of the previous work facing. This way, the scarf will be symmetric.) The original impetus for knitting ripples came from conversations with Eran Sharon at the KITP workshop “Pattern Formation in Physics and Biology” (2003). Various knitting skills, materials, and ideas came from Jane Daniels, Mary Petty, Deb Romani, Wendy McRae, Samuel Poincloux, Frédéric Lechenault, Mike Dimitriyev, and Sabetta Matsumoto. Professors Karen Daniels (@karenedaniels on Twitter and @kedaniels on Ravelry) and Mary Williard Elting (@mwelting on Twitter and @melting on Ravelry) can be found knitting in various seminars and meetings at the physics department at North Carolina State University. Their more traditional interests in soft matter physics span networks, gels, granular materials, and the biophysics of cells and tissues.}, number={2}, journal={Patterns}, publisher={Elsevier BV}, author={Daniels, Karen E. and Elting, Mary Williard}, year={2020}, month={May}, pages={100034} }
 @article{song_kartawira_hillaire_li_eaker_kiani_daniels_dickey_2020, title={Overcoming Rayleigh–Plateau instabilities: Stabilizing and destabilizing liquid-metal streams via electrochemical oxidation}, volume={117}, ISSN={0027-8424 1091-6490}, url={http://dx.doi.org/10.1073/pnas.2006122117}, DOI={10.1073/pnas.2006122117}, abstractNote={Significance}, number={32}, journal={Proceedings of the National Academy of Sciences}, publisher={Proceedings of the National Academy of Sciences}, author={Song, Minyung and Kartawira, Karin and Hillaire, Keith D. and Li, Cheng and Eaker, Collin B. and Kiani, Abolfazl and Daniels, Karen E. and Dickey, Michael D.}, year={2020}, month={Jul}, pages={19026–19032} }
 @article{yannarell_grandchamp_chen_daniels_shank_2019, title={A Dual-Species Biofilm with Emergent Mechanical and Protective Properties}, volume={201}, ISSN={0021-9193 1098-5530}, url={http://dx.doi.org/10.1128/jb.00670-18}, DOI={10.1128/jb.00670-18}, abstractNote={
            In the environment, many microbes form biofilms. However, the interspecies interactions underlying bacterial coexistence within these biofilms remain understudied. Here, we mimic environmentally relevant biofilms by studying a dual-species biofilm formed between
            Bacillus subtilis
            and
            Pantoea agglomerans
            and subjecting the coculture to chemical and physical stressors that it may experience in the natural world. We determined that both bacteria contribute structural elements to the coculture, which is reflected in its overall viscoelastic behavior. Existence within the coculture can be either beneficial or detrimental depending on the context. Many of the features and determinants of the coculture biofilm appear distinct from those identified in monoculture biofilm studies, highlighting the importance of characterizing multispecies consortia to understand naturally occurring bacterial interactions.
          }, number={18}, journal={Journal of Bacteriology}, publisher={American Society for Microbiology}, author={Yannarell, Sarah M. and Grandchamp, Gabrielle M. and Chen, Shih-Yuan and Daniels, Karen E. and Shank, Elizabeth A.}, editor={O'Toole, GeorgeEditor}, year={2019}, month={Mar} }
 @article{kollmer_daniels_2019, title={Betweenness centrality as predictor for forces in granular packings}, volume={15}, ISSN={1744-683X 1744-6848}, url={http://dx.doi.org/10.1039/C8SM01372A}, DOI={10.1039/C8SM01372A}, abstractNote={A load applied to a jammed frictional granular system is localized into a network of force chains making inter-particle connections throughout the system. While this force network is not unique, likely patterns can be predicted using metric known as betweenness centrality.}, number={8}, journal={Soft Matter}, publisher={Royal Society of Chemistry (RSC)}, author={Kollmer, Jonathan E. and Daniels, Karen E.}, year={2019}, pages={1793–1798} }
 @article{chen_bardall_shearer_daniels_2019, title={Distinguishing deformation mechanisms in elastocapillary experiments}, volume={15}, ISSN={1744-683X 1744-6848}, url={http://dx.doi.org/10.1039/C9SM01756A}, DOI={10.1039/C9SM01756A}, abstractNote={Soft materials are known to deform due to a variety of mechanisms, including capillarity, buoyancy, and swelling. The choice of liquid plays a significant role in the outcome of experiments.}, number={46}, journal={Soft Matter}, publisher={Royal Society of Chemistry (RSC)}, author={Chen, Shih-Yuan and Bardall, Aaron and Shearer, Michael and Daniels, Karen E.}, year={2019}, pages={9426–9436} }
 @article{kozlowski_carlevaro_daniels_kondic_pugnaloni_socolar_zheng_behringer_2019, title={Dynamics of a grain-scale intruder in a two-dimensional granular medium with and without basal friction}, volume={100}, ISBN={2470-0053}, DOI={10.1103/PhysRevE.100.032905}, abstractNote={We report on a series of experiments in which a grain-sized intruder is pushed by a spring through a two-dimensional granular material composed of photoelastic disks in a Couette geometry. We study the intruder dynamics as a function of packing fraction for two types of supporting substrates: A frictional glass plate and a layer of water for which basal friction forces are negligible. We observe two dynamical regimes: Intermittent flow, in which the intruder moves freely most of the time but occasionally gets stuck, and stick-slip dynamics, in which the intruder advances via a sequence of distinct, rapid events. When basal friction is present, we observe a smooth crossover between the two regimes as a function of packing fraction, and we find that reducing the interparticle friction coefficient causes the stick-slip regime to shift to higher packing fractions. When basal friction is eliminated, we observe intermittent flow at all accessible packing fractions. For all cases, we present results for the statistics of stick events, the intruder velocity, and the force exerted on the intruder by the grains. Our results indicate the qualitative importance of basal friction at high packing fractions and suggest a possible connection between intruder dynamics in a static material and clogging dynamics in granular flows.}, number={3}, journal={Physical Review E}, author={Kozlowski, Ryan and Carlevaro, C. Manuel and Daniels, Karen E. and Kondic, Lou and Pugnaloni, Luis A. and Socolar, Joshua E. S. and Zheng, Hu and Behringer, Robert P.}, year={2019} }
 @article{zadeh_bares_brzinski_daniels_dijksman_docquier_everitt_kollmer_lantsoght_wang_et al._2019, title={Enlightening force chains: a review of photoelasticimetry in granular matter}, volume={21}, ISBN={1434-7636}, DOI={10.1007/s10035-019-0942-2}, abstractNote={A photoelastic material will reveal its internal stresses when observed through polarizing filters. This eye-catching property has enlightened our understanding of granular materials for over half a century, whether in the service of art, education, or scientific research. In this review article in honor of Robert Behringer, we highlight both his pioneering use of the method in physics research, and its reach into the public sphere through museum exhibits and outreach programs. We aim to provide clear protocols for artists, exhibit-designers, educators, and scientists to use in their own endeavors. It is our hope that this will build awareness about the ubiquitous presence of granular matter in our lives, enlighten its puzzling behavior, and promote conversations about its importance in environmental and industrial contexts. To aid in this endeavor, this paper also serves as a front door to a detailed wiki containing open, community-curated guidance on putting these methods into practice (Abed-Zadeh et al. in Photoelastic methods wiki https://git-xen.lmgc.univ-montp2.fr/PhotoElasticity/Main/wikis/home, 2019).}, number={4}, journal={Granular Matter}, author={Zadeh, Aghil Abed and Bares, Jonathan and Brzinski, Theodore A. and Daniels, Karen E. and Dijksman, Joshua and Docquier, Nicolas and Everitt, Henry O. and Kollmer, Jonathan E. and Lantsoght, Olivier and Wang, Dong and et al.}, year={2019}, month={Nov} }
 @article{thomas_tang_daniels_vriend_2019, title={Force fluctuations at the transition from quasi-static to inertial granular flow}, volume={15}, ISSN={1744-683X 1744-6848}, url={http://dx.doi.org/10.1039/C9SM01111K}, DOI={10.1039/C9SM01111K}, abstractNote={We analyse the rheology of gravity-driven, dry granular flows in experiments where individual forces within the flow bulk are measured.}, number={42}, journal={Soft Matter}, publisher={Royal Society of Chemistry (RSC)}, author={Thomas, A. L. and Tang, Zhu and Daniels, Karen E. and Vriend, N. M.}, year={2019}, pages={8532–8542} }
 @article{berthier_porter_daniels_2019, title={Forecasting failure locations in 2-dimensional disordered lattices}, volume={116}, ISSN={0027-8424 1091-6490}, url={http://dx.doi.org/10.1073/pnas.1900272116}, DOI={10.1073/pnas.1900272116}, abstractNote={Significance}, number={34}, journal={Proceedings of the National Academy of Sciences}, publisher={Proceedings of the National Academy of Sciences}, author={Berthier, Estelle and Porter, Mason A. and Daniels, Karen E.}, year={2019}, month={Aug}, pages={16742–16749} }
 @article{bililign_kollmer_daniels_2019, title={Protocol Dependence and State Variables in the Force-Moment Ensemble}, volume={122}, ISSN={0031-9007 1079-7114}, url={http://dx.doi.org/10.1103/physrevlett.122.038001}, DOI={10.1103/physrevlett.122.038001}, abstractNote={Stress-based ensembles incorporating temperaturelike variables have been proposed as a route to an equation of state for granular materials. To test the efficacy of this approach, we perform experiments on a two-dimensional photoelastic granular system under three loading conditions: uniaxial compression, biaxial compression, and simple shear. From the interparticle forces, we find that the distributions of the normal component of the coarse-grained force-moment tensor are exponential tailed, while the deviatoric component is Gaussian distributed. This implies that the correct stress-based statistical mechanics conserves both the force-moment tensor and the Maxwell-Cremona force-tiling area. As such, two variables of state arise: the tensorial angoricity (α[over ^]) and a new temperaturelike quantity associated with the force-tile area which we name keramicity (κ). Each quantity is observed to be inversely proportional to the global confining pressure; however, only κ exhibits the protocol independence expected of a state variable, while α[over ^] behaves as a variable of process.}, number={3}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Bililign, Ephraim S. and Kollmer, Jonathan E. and Daniels, Karen E.}, year={2019}, month={Jan} }
 @article{berthier_kollmer_henkes_liu_schwarz_daniels_2019, title={Rigidity percolation control of the brittle-ductile transition in disordered networks}, volume={3}, ISBN={2475-9953}, url={http://dx.doi.org/10.1103/physrevmaterials.3.075602}, DOI={10.1103/PhysRevMaterials.3.075602}, abstractNote={In ordinary solids, material disorder is known to increase the size of the process zone in which stress concentrates at the crack tip, causing a transition from localized to diffuse failure. Here, we report experiments on disordered 2D lattices, derived from frictional particle packings, in which the mean coordination number $\langle z \rangle$ of the underlying network provides a similar control. Our experiments show that tuning the connectivity of the network provides access to a range of behaviors from brittle to ductile failure. We elucidate the cooperative origins of this transition using a frictional pebble game algorithm on the original, intact lattices. We find that the transition corresponds to the isostatic value $\langle z \rangle = 3$ in the large-friction limit, with brittle failure occurring for structures vertically spanned by a rigid cluster, and ductile failure for floppy networks containing nonspanning rigid clusters. Furthermore, we find that individual failure events typically occur within the floppy regions separated by the rigid clusters.}, number={7}, journal={Physical Review Materials}, author={Berthier, Estelle and Kollmer, Jonathan E. and Henkes, Silke E. and Liu, Kuang and Schwarz, J. M. and Daniels, Karen E.}, year={2019} }
 @article{jerolmack_daniels_2019, title={Viewing Earth’s surface as a soft-matter landscape}, volume={1}, ISSN={2522-5820}, url={http://dx.doi.org/10.1038/s42254-019-0111-x}, DOI={10.1038/s42254-019-0111-x}, abstractNote={Earth's surface is composed of a staggering diversity of particulate–fluid mixtures: dry to wet, dilute to dense, colloidal to granular and attractive to repulsive particles. This material variety is matched by the range of relevant stresses and strain rates, from laminar to turbulent flows and steady to intermittent forcing, leading to anything from rapid and catastrophic landslides to the slow relaxation of soil and rocks over geologic timescales. From a physics point of view, virtually all Earth and planetary landscapes are composed of soft matter, in the sense that they are both deformable and sensitive to collective effects. Geophysical materials, however, often involve compositions and flow geometries that have not yet been examined in physics. In this Review, we explore how a soft-matter physics perspective has helped to illuminate, and even predict, the rich dynamics of earth materials and their associated landscapes. We also highlight phenomena of geophysical flows that challenge, and will hopefully inspire, work on more fundamental aspects of soft matter. Earth and planetary landscapes are composed of soft matter: amorphous materials that deform in response to broad-spectrum excitations, from fluid turbulence to plate tectonics. This Review surveys complex behaviours of earth materials that challenge existing physics frameworks and may inspire new approaches.}, number={12}, journal={Nature Reviews Physics}, publisher={Springer Science and Business Media LLC}, author={Jerolmack, Douglas J. and Daniels, Karen E.}, year={2019}, month={Oct}, pages={716–730} }
 @article{papadopoulos_porter_daniels_bassett_2018, title={Network analysis of particles and grains}, volume={6}, DOI={10.1093/comnet/cny005}, abstractNote={The arrangements of particles and forces in granular materials and particulate matter have a complex organization on multiple spatial scales that range from local structures to mesoscale and system-wide ones. This multiscale organization can affect how a material responds or reconfigures when exposed to external perturbations or loading. The theoretical study of particle-level, force-chain, domain, and bulk properties requires the development and application of appropriate mathematical, statistical, physical, and computational frameworks. Traditionally, granular materials have been investigated using particulate or continuum models, each of which tends to be implicitly agnostic to multiscale organization. Recently, tools from network science have emerged as powerful approaches for probing and characterizing heterogeneous architectures in complex systems, and a diverse set of methods have yielded fascinating insights into granular materials. In this paper, we review work on network-based approaches to studying granular materials (and particulate matter more generally) and explore the potential of such frameworks to provide a useful description of these materials and to enhance understanding of the underlying physics. We also outline a few open questions and highlight particularly promising future directions in the analysis and design of granular materials and other particulate matter.}, number={4}, journal={Journal of Complex Networks}, author={Papadopoulos, Lia and Porter, Mason A. and Daniels, Karen E. and Bassett, Danielle S.}, year={2018}, pages={485–565} }
 @article{tang_brzinski_shearer_daniels_2018, title={Nonlocal rheology of dense granular flow in annular shear experiments}, volume={14}, ISSN={1744-683X 1744-6848}, url={http://dx.doi.org/10.1039/c8sm00047f}, DOI={10.1039/c8sm00047f}, abstractNote={Experimental measurements of boundary stresses and flow fields of a quasi-2D granular material under steady shear validate two nonlocal rheological models.}, number={16}, journal={Soft Matter}, publisher={Royal Society of Chemistry (RSC)}, author={Tang, Zhu and Brzinski, Theodore A. and Shearer, Michael and Daniels, Karen E.}, year={2018}, pages={3040–3048} }
 @article{brzinski_daniels_2018, title={Sounds of Failure: Passive Acoustic Measurements of Excited Vibrational Modes}, volume={120}, ISSN={["1079-7114"]}, DOI={10.1103/physrevlett.120.218003}, abstractNote={Granular materials can fail through spontaneous events like earthquakes or brittle fracture. However, measurements and analytic models which forecast failure in this class of materials, while of both fundamental and practical interest, remain elusive. Materials including numerical packings of spheres, colloidal glasses, and granular materials have been known to develop an excess of low-frequency vibrational modes as the confining pressure is reduced. Here, we report experiments on sheared granular materials in which we monitor the evolving density of excited modes via passive monitoring of acoustic emissions. We observe a broadening of the distribution of excited modes coincident with both bulk and local plasticity, and evolution in the shape of the distribution before and after bulk failure. These results provide a new interpretation of the changing state of the material on its approach to stick-slip failure.}, number={21}, journal={PHYSICAL REVIEW LETTERS}, author={Brzinski, Theodore A., III and Daniels, Karen E.}, year={2018}, month={May} }
 @article{workamp_ramirez_daniels_dijksman_2018, title={Symmetry-reversals in chiral active matter}, volume={14}, ISSN={["1744-6848"]}, url={https://doi.org/10.1039/C8SM00402A}, DOI={10.1039/c8sm00402a}, abstractNote={A swarm of active-spinner particles displays a reversal of their swarming direction as their packing density is increased, an effect that can be enhanced by adding geometric friction between the particles.}, number={27}, journal={SOFT MATTER}, publisher={Royal Society of Chemistry (RSC)}, author={Workamp, Marcel and Ramirez, Gustavo and Daniels, Karen E. and Dijksman, Joshua A.}, year={2018}, month={Jul}, pages={5572–5580} }
 @article{kollmer_daniels_2017, title={An experimental investigation of the force network ensemble}, volume={140}, ISSN={2100-014X}, url={http://dx.doi.org/10.1051/EPJCONF/201714002024}, DOI={10.1051/EPJCONF/201714002024}, abstractNote={We present an experiment in which a horizontal quasi-2D granular system with a fixed neighbor network is cyclically compressed and decompressed over 1000 cycles. We remove basal friction by floating the particles on a thin air cushion, so that particles only interact in-plane. As expected for a granular system, the applied load is not distributed uniformly, but is instead concentrated in force chains which form a network throughout the system. To visualize the structure of these networks, we use particles made from photoelastic material. The experimental setup and a new data-processing pipeline allow us to map out the evolution subject to the cyclic compressions. We characterize several statistical properties of the packing, including the probability density function of the contact force, and compare them with theoretical and numerical predictions from the force network ensemble theory.}, journal={EPJ Web of Conferences}, publisher={EDP Sciences}, author={Kollmer, Jonathan E. and Daniels, Karen E.}, editor={Radjai, F. and Nezamabadi, S. and Luding, S. and Delenne, J.Y.Editors}, year={2017}, pages={02024} }
 @article{grzelka_bostwick_daniels_2017, title={Capillary fracture of ultrasoft gels: variability and delayed nucleation}, volume={13}, ISSN={["1744-6848"]}, DOI={10.1039/c7sm00257b}, abstractNote={A droplet of surfactant spreading on an ultrasoft (E ≲ 100 Pa) gel substrate will produce capillary fractures at the gel surface; these fractures originate at the contact-line and propagate outwards in a starburst pattern. There is an inherent variability in both the number of fractures formed and the time delay before fractures form. In the regime where single fractures form, we observe a Weibull-like distribution of delay times, consistent with a thermally-activated process. The shape parameter is close to 1 for softer gels (a Poisson process), and larger for stiffer gels (indicative of aging). For single fractures, the characteristic delay time is primarily set by the elastocapillary length of the system, calculated from the differential in surface tension between the droplet and the substrate, rather than the elastic modulus as for stiffer systems. For multiple fractures, all fractures appear simultaneously and long delay times are suppressed. The experimental protocol provides a new technique for probing the energy landscape and fracture toughness of ultrasoft materials through measurement of the delay time distribution.}, number={16}, journal={SOFT MATTER}, publisher={Royal Society of Chemistry (RSC)}, author={Grzelka, Marion and Bostwick, Joshua B. and Daniels, Karen E.}, year={2017}, month={Apr}, pages={2962–2966} }
 @article{bardall_daniels_shearer_2017, title={Deformation of an elastic substrate due to a resting sessile droplet}, volume={29}, ISSN={0956-7925 1469-4425}, url={http://dx.doi.org/10.1017/S0956792517000134}, DOI={10.1017/s0956792517000134}, abstractNote={On a sufficiently soft substrate, a resting fluid droplet will cause significant deformation of the substrate. This deformation is driven by a combination of capillary forces at the contact line and the fluid pressure at the solid surface. These forces are balanced at the surface by the solid traction stress induced by the substrate deformation. Young's Law, which predicts the equilibrium contact angle of the droplet, also indicates an a priori radial force balance for rigid substrates, but not necessarily for soft substrates that deform under loading. It remains an open question whether the contact line transmits a non-zero force tangent to the substrate surface in addition to the conventional normal (vertical) force. We present an analytic Fourier transform solution technique that includes general interfacial energy conditions, which govern the contact angle of a 2D droplet. This includes evaluating the effect of gravity on the droplet shape in order to determine the correct fluid pressure at the substrate surface for larger droplets. Importantly, we find that in order to avoid a strain singularity at the contact line under a non-zero tangential contact line force, it is necessary to include a previously neglected horizontal traction boundary condition. To quantify the effects of the contact line and identify key quantities that will be experimentally accessible for testing the model, we evaluate solutions for the substrate surface displacement field as a function of Poisson's ratio and zero/non-zero tangential contact line forces.}, number={2}, journal={European Journal of Applied Mathematics}, publisher={Cambridge University Press (CUP)}, author={Bardall, Aaron and Daniels, Karen E. and Shearer, Michael}, year={2017}, month={Jun}, pages={281–300} }
 @article{tang_brzinski_daniels_2017, title={Granular rheology: measuring boundary forces with laser-cut leaf springs}, volume={140}, ISSN={2100-014X}, url={http://dx.doi.org/10.1051/EPJCONF/201714003035}, DOI={10.1051/EPJCONF/201714003035}, abstractNote={In granular physics experiments, it is a persistent challenge to obtain the boundary stress measurements necessary to provide full a rheological characterization of the dynamics. Here, we describe a new technique by which the outer boundary of a 2D Couette cell both confines the granular material and provides spatially- and temporally- resolved stress measurements. This key advance is enabled by desktop laser-cutting technology, which allows us to design and cut linearly-deformable walls with a specified spring constant. By tracking the position of each segment of the wall, we measure both the normal and tangential stress throughout the experiment. This permits us to calculate the amount of shear stress provided by basal friction, and thereby determine accurate values of $\mu(I)$.}, journal={EPJ Web of Conferences}, publisher={EDP Sciences}, author={Tang, Zhu and Brzinski, Theodore A. and Daniels, Karen E.}, editor={Radjai, F. and Nezamabadi, S. and Luding, S. and Delenne, J.Y.Editors}, year={2017}, pages={03035} }
 @article{eaker_hight_john d. o'regan_dickey_daniels_2017, title={Oxidation-Mediated Fingering in Liquid Metals}, volume={119}, ISSN={["1079-7114"]}, DOI={10.1103/physrevlett.119.174502}, abstractNote={We identify and characterize a new class of fingering instabilities in liquid metals; these instabilities are unexpected due to the large interfacial tension of metals. Electrochemical oxidation lowers the effective interfacial tension of a gallium-based liquid metal alloy to values approaching zero, thereby inducing drastic shape changes, including the formation of fractals. The measured fractal dimension (D=1.3±0.05) places the instability in a different universality class than other fingering instabilities. By characterizing changes in morphology and dynamics as a function of droplet volume and applied electric potential, we identify the three main forces involved in this process: interfacial tension, gravity, and oxidative stress. Importantly, we find that electrochemical oxidation can generate compressive interfacial forces that oppose the tensile forces at a liquid interface. The surface oxide layer ultimately provides a physical and electrochemical barrier that halts the instabilities at larger positive potentials. Controlling the competition between interfacial tension and oxidative (compressive) stresses at the interface is important for the development of reconfigurable electronic, electromagnetic, and optical devices that take advantage of the metallic properties of liquid metals.}, number={17}, journal={PHYSICAL REVIEW LETTERS}, publisher={American Physical Society (APS)}, author={Eaker, Collin B. and Hight, David C. and John D. O'Regan and Dickey, Michael D. and Daniels, Karen E.}, year={2017}, month={Oct} }
 @article{daniels_kollmer_puckett_2017, title={Photoelastic force measurements in granular materials}, volume={88}, ISSN={["1089-7623"]}, url={https://doi.org/10.1063/1.4983049}, DOI={10.1063/1.4983049}, abstractNote={Photoelastic techniques are used to make both qualitative and quantitative measurements of the forces within idealized granular materials. The method is based on placing a birefringent granular material between a pair of polarizing filters, so that each region of the material rotates the polarization of light according to the amount of local stress. In this review paper, we summarize the past work using the technique, describe the optics underlying the technique, and illustrate how it can be used to quantitatively determine the vector contact forces between particles in a 2D granular system. We provide a description of software resources available to perform this task, as well as key techniques and resources for building an experimental apparatus.}, number={5}, journal={REVIEW OF SCIENTIFIC INSTRUMENTS}, publisher={AIP Publishing}, author={Daniels, Karen E. and Kollmer, Jonathan E. and Puckett, James G.}, year={2017}, month={May} }
 @article{amon_born_daniels_dijksman_huang_parker_schroeter_stannarius_wierschem_2017, title={Preface: Focus on imaging methods in granular physics}, volume={88}, ISSN={["1089-7623"]}, url={https://doi.org/10.1063/1.4983052}, DOI={10.1063/1.4983052}, abstractNote={Granular materials are complex multi-particle ensembles in which macroscopic properties are largely determined by inter-particle interactions between their numerous constituents. In order to understand and to predict their macroscopic physical behavior, it is necessary to analyze the composition and interactions at the level of individual contacts and grains. To do so requires the ability to image individual particles and their local configurations to high precision. A variety of competing and complementary imaging techniques have been developed for that task. In this introductory paper accompanying the Focus Issue, we provide an overview of these imaging methods and discuss their advantages and drawbacks, as well as their limits of application.}, number={5}, journal={REVIEW OF SCIENTIFIC INSTRUMENTS}, publisher={AIP Publishing}, author={Amon, Axelle and Born, Philip and Daniels, Karen E. and Dijksman, Joshua A. and Huang, Kai and Parker, David and Schroeter, Matthias and Stannarius, Ralf and Wierschem, Andreas}, year={2017}, month={May} }
 @article{amon_born_daniels_dijksman_huang_parker_schröter_stannarius_wierschem_2017, title={Publisher’s Note: “Preface: Focus on imaging methods in granular physics” [Rev. Sci. Instrum. 88, 051701 (2017)]}, volume={88}, ISSN={0034-6748 1089-7623}, url={http://dx.doi.org/10.1063/1.4993149}, DOI={10.1063/1.4993149}, abstractNote={First Page}, number={7}, journal={Review of Scientific Instruments}, publisher={AIP Publishing}, author={Amon, Axelle and Born, Philip and Daniels, Karen E. and Dijksman, Joshua A. and Huang, Kai and Parker, David and Schröter, Matthias and Stannarius, Ralf and Wierschem, Andreas}, year={2017}, month={Jul}, pages={079901} }
 @article{sinclair_levy_daniels_2017, title={Simulating surfactant spreading: Influence of a physically motivated equation of state}, volume={29}, ISSN={0956-7925 1469-4425}, url={http://dx.doi.org/10.1017/s095679251700002x}, DOI={10.1017/s095679251700002x}, abstractNote={In this paper, we present numerical simulations that demonstrate the effect of the particular choice of the equation of state (EoS) relating the surfactant concentration to the surface tension in surfactant-driven thin liquid films. Previous choices of the model EoS have been an ad-hoc decreasing function. Here, we instead propose an empirically motivated EoS; this provides a route to resolve some discrepancies and raises new issues to be pursued in future experiments. In addition, we test the influence of the choice of initial conditions and values for the non-dimensional groups. We demonstrate that the choice of EoS improves the agreement in surfactant distribution morphology between simulations and experiments, and influences the dynamics of the simulations. Because an empirically motivated EoS has regions with distinct gradients, future mathematical models may be improved by considering more than one timescale. We observe that the non-dimensional number controlling the relative importance of gravitational versus capillary forces has a larger influence on the dynamics than the other non-dimensional groups, but is nonetheless not a likely cause of discrepancy between simulations and experiments. Finally, we observe that the experimental approach using a ring to contain the surfactant could affect the surfactant and fluid dynamics if it disrupts the intended initial surfactant distribution. However, the fluid meniscus itself does not significantly affect the dynamics.}, number={1}, journal={European Journal of Applied Mathematics}, publisher={Cambridge University Press (CUP)}, author={Sinclair, Dina and Levy, Rachel and Daniels, Karen E.}, year={2017}, month={Mar}, pages={30–54} }
 @article{daniels_2017, title={The role of force networks in granular materials}, volume={140}, ISSN={2100-014X}, url={http://dx.doi.org/10.1051/epjconf/201714001006}, DOI={10.1051/epjconf/201714001006}, abstractNote={One of the most visually striking features of granular materials is their heterogeneous pattern of force transmission, commonly known as force chains. In this paper and its associated talk, we will review several experiments on two-dimensional photoelastic granular materials which highlight methods for making quantitative interparticle forces measurements, theoretical frameworks for interpreting such data, and the specific findings which result from these methods.}, journal={EPJ Web of Conferences}, publisher={EDP Sciences}, author={Daniels, Karen E.}, editor={Radjai, F. and Nezamabadi, S. and Luding, S. and Delenne, J.Y.Editors}, year={2017}, pages={01006} }
 @inproceedings{kollmer_lindauer_daniels_2016, title={Digging on Asteroids: A Laboratory Model of Granular Dynamics in Microgravity}, url={http://dx.doi.org/10.1061/9780784479971.021}, DOI={10.1061/9780784479971.021}, abstractNote={As NASA prepares to visit asteroids and other poorly-consolidated near-earth-objects (NEOs), it will be important to safely interact with the granular materials at the surface of these objects. A particular concern is the low elastic modulus of granular materials: rubble-pile asteroids are only held together by weak gravitational and van der Waals forces. This means that both the escape velocity and the sound velocity are low compared to their values on earth. To better predict the dynamics of the granular flows resulting from surface explorations such as digging, sample-collection, anchoring, or lift-off, we develop microgravity experiments which are able to predict the circumstances under which the NEO material will remain intact or become unstable. In our experiments, we insert a flexible probe into a granular material under simulated conditions of low gravity. We show that low-speed interactions reduce the effects of shock wave creation and observe that thinner diggers allow the grains to rearrange and minimize the possibility of ejecta.}, booktitle={Earth and Space 2016}, publisher={American Society of Civil Engineers}, author={Kollmer, Jonathan E. and Lindauer, Scott M. and Daniels, Karen E.}, year={2016}, month={Jan} }
 @article{papadopoulos_puckett_daniels_bassett_2016, title={Evolution of network architecture in a granular material under compression}, volume={94}, ISSN={["2470-0053"]}, DOI={10.1103/physreve.94.032908}, abstractNote={As a granular material is compressed, the particles and forces within the system arrange to form complex and heterogeneous collective structures. Force chains are a prime example of such structures, and are thought to constrain bulk properties such as mechanical stability and acoustic transmission. However, capturing and characterizing the evolving nature of the intrinsic inhomogeneity and mesoscale architecture of granular systems can be challenging. A growing body of work has shown that graph theoretic approaches may provide a useful foundation for tackling these problems. Here, we extend the current approaches by utilizing multilayer networks as a framework for directly quantifying the progression of mesoscale architecture in a compressed granular system. We examine a quasi-two-dimensional aggregate of photoelastic disks, subject to biaxial compressions through a series of small, quasistatic steps. Treating particles as network nodes and interparticle forces as network edges, we construct a multilayer network for the system by linking together the series of static force networks that exist at each strain step. We then extract the inherent mesoscale structure from the system by using a generalization of community detection methods to multilayer networks, and we define quantitative measures to characterize the changes in this structure throughout the compression process. We separately consider the network of normal and tangential forces, and find that they display a different progression throughout compression. To test the sensitivity of the network model to particle properties, we examine whether the method can distinguish a subsystem of low-friction particles within a bath of higher-friction particles. We find that this can be achieved by considering the network of tangential forces, and that the community structure is better able to separate the subsystem than a purely local measure of interparticle forces alone. The results discussed throughout this study suggest that these network science techniques may provide a direct way to compare and classify data from systems under different external conditions or with different physical makeup.}, number={3}, journal={PHYSICAL REVIEW E}, publisher={American Physical Society (APS)}, author={Papadopoulos, Lia and Puckett, James G. and Daniels, Karen E. and Bassett, Danielle S.}, year={2016}, month={Sep} }
 @article{huang_daniels_2016, title={Friction and pressure-dependence of force chain communities in granular materials}, volume={18}, ISSN={["1434-7636"]}, url={https://doi.org/10.1007/s10035-016-0681-6}, DOI={10.1007/s10035-016-0681-6}, abstractNote={Granular materials transmit stress via a network of force chains. Despite the importance of these chains in characterizing the stress state and dynamics of the system, there is no common framework for quantifying their properties. Recently, attention has turned to the tools of network science as a promising route to such a description. In this paper, we apply a common network-science technique, community detection, to the force network of numerically-generated packings of spheres over a range of interparticle friction coefficients and confining pressures. In order to extract chain-like features, we use a modularity maximization with a recently-developed geographical null model (Bassett et al. in Soft Matter 11:2731–2744, 2015), and optimize the technique to detect sparse structures by minimizing the normalized convex hull ratio of the detected communities. We characterize the force chain communities by their size (number of particles), network force (interparticle forces), and normalized convex hull ratio (sparseness). We find that the first two are highly correlated and are therefore largely redundant. For both pressure P and interparticle friction $$\mu $$ , we observe two distinct transitions in behavior. One, for $$\mu \lesssim 0.1$$ the packings exhibit more distinguishability to pressure than at higher $$\mu $$ . Two, we identify a transition pressure $$P^*$$ at which the frictional dependence switches behaviors. Below $$P^*$$ there are more large/strong communities at low $$\mu $$ , while above $$P^*$$ there are more large/strong communities at high $$\mu $$ . We explain these phenomena by comparison to the spatial distribution of communities along the vertical axis of the system. These results provide new tools for considering the mesoscale structure of a granular system and pave the way for reduced descriptions based on the force chain structure.}, number={4}, journal={GRANULAR MATTER}, publisher={Springer Science and Business Media LLC}, author={Huang, Yuming and Daniels, Karen E.}, year={2016}, month={Nov} }
 @article{laskar_kumar_herminghaus_daniels_schroeter_2016, title={High refractive index immersion liquid for superresolution 3D imaging using sapphire-based aplanatic numerical aperture increasing lens optics}, volume={55}, ISSN={["2155-3165"]}, DOI={10.1364/ao.55.003165}, abstractNote={Optically transparent immersion liquids with refractive index (n∼1.77) to match the sapphire-based aplanatic numerical aperture increasing lens (aNAIL) are necessary for achieving deep 3D imaging with high spatial resolution. We report that antimony tribromide (SbBr3) salt dissolved in liquid diiodomethane (CH2I2) provides a new high refractive index immersion liquid for optics applications. The refractive index is tunable from n=1.74 (pure) to n=1.873 (saturated), by adjusting either salt concentration or temperature; this allows it to match (or even exceed) the refractive index of sapphire. Importantly, the solution gives excellent light transmittance in the ultraviolet to near-infrared range, an improvement over commercially available immersion liquids. This refractive-index-matched immersion liquid formulation has enabled us to develop a sapphire-based aNAIL objective that has both high numerical aperture (NA=1.17) and long working distance (WD=12  mm). This opens up new possibilities for deep 3D imaging with high spatial resolution.}, number={12}, journal={APPLIED OPTICS}, publisher={The Optical Society}, author={Laskar, Junaid M. and Kumar, P. Shravan and Herminghaus, Stephan and Daniels, Karen E. and Schroeter, Matthias}, year={2016}, month={Apr}, pages={3165–3169} }
 @article{ortiz_riehn_daniels_2016, title={Nonaffine deformation under compression and decompression of a flow-stabilized solid}, volume={2016}, ISSN={["1742-5468"]}, DOI={10.1088/1742-5468/2016/08/084003}, abstractNote={Understanding the particle-scale transition from elastic deformation to plastic flow is central to making predictions about the bulk material properties and response of disordered materials. To address this issue, we perform experiments on flow-stabilized solids composed of micron-scale spheres within a microfluidic channel, in a regime where particle inertia is negligible. Each solid heap exists within a stress field imposed by the flow, and we track the positions of particles in response to single impulses of fluid-driven compression or decompression. We find that the resulting deformation field is well-decomposed into an affine field, with a constant strain profile throughout the solid, and a non-affine field. The magnitude of this non-affine response decays with the distance from the free surface in the long-time limit, suggesting that the distance from jamming plays a significant role in controlling the length scale of plastic flow. Finally, we observe that compressive pulses create more rearrangements than decompressive pulses, an effect that we quantify using the Dmin2 statistic for non-affine motion. Unexpectedly, the time scale for the compression response is shorter than for decompression at the same strain (but unequal pressure), providing insight into the coupling between deformation and cage-breaking.}, number={8}, journal={JOURNAL OF STATISTICAL MECHANICS-THEORY AND EXPERIMENT}, publisher={IOP Publishing}, author={Ortiz, Carlos P. and Riehn, Robert and Daniels, Karen E.}, year={2016}, month={Aug} }
 @article{andreotti_baeumchen_boulogne_daniels_dufresne_perrin_salez_snoeijer_style_2016, title={Solid capillarity: when and how does surface tension deform soft solids?}, volume={12}, ISSN={["1744-6848"]}, DOI={10.1039/c5sm03140k}, abstractNote={Soft solids differ from stiff solids in an important way: their surface stresses can drive large deformations. Based on a topical workshop held in the Lorentz Center in Leiden, this Opinion highlights some recent advances in the growing field of solid capillarity and poses key questions for its advancement.}, number={12}, journal={SOFT MATTER}, author={Andreotti, Bruno and Baeumchen, Oliver and Boulogne, Francois and Daniels, Karen E. and Dufresne, Eric R. and Perrin, Hugo and Salez, Thomas and Snoeijer, Jacco H. and Style, Robert W.}, year={2016}, pages={2993–2996} }
 @article{subramanian_brausch_daniels_bodenschatz_schneider_pesch_2016, title={Spatio-temporal patterns in inclined layer convection}, volume={794}, ISSN={["1469-7645"]}, DOI={10.1017/jfm.2016.186}, abstractNote={This paper reports on a theoretical analysis of the rich variety of spatio-temporal patterns observed recently in inclined layer convection at medium Prandtl number when varying the inclination angle ${\it\gamma}$ and the Rayleigh number $R$. The present numerical investigation of the inclined layer convection system is based on the standard Oberbeck–Boussinesq equations. The patterns are shown to originate from a complicated competition of buoyancy driven and shear-flow driven pattern forming mechanisms. The former are expressed as longitudinal convection rolls with their axes oriented parallel to the incline, the latter as perpendicular transverse rolls. Along with conventional methods to study roll patterns and their stability, we employ direct numerical simulations in large spatial domains, comparable with the experimental ones. As a result, we determine the phase diagram of the characteristic complex 3-D convection patterns above onset of convection in the ${\it\gamma}{-}R$ plane, and find that it compares very well with the experiments. In particular we demonstrate that interactions of specific Fourier modes, characterized by a resonant interaction of their wavevectors in the layer plane, are key to understanding the pattern morphologies.}, journal={JOURNAL OF FLUID MECHANICS}, publisher={Cambridge University Press (CUP)}, author={Subramanian, Priya and Brausch, Oliver and Daniels, Karen E. and Bodenschatz, Eberhard and Schneider, Tobias M. and Pesch, Werner}, year={2016}, month={May}, pages={719–745} }
 @article{giusti_papadopoulos_owens_daniels_bassett_2016, title={Topological and geometric measurements of force-chain structure}, volume={94}, ISSN={["2470-0053"]}, DOI={10.1103/physreve.94.032909}, abstractNote={Developing quantitative methods for characterizing structural properties of force chains in densely packed granular media is an important step toward understanding or predicting large-scale physical properties of a packing. A promising framework in which to develop such methods is network science, which can be used to translate particle locations and force contacts into a graph in which particles are represented by nodes and forces between particles are represented by weighted edges. Recent work applying network-based community-detection techniques to extract force chains opens the door to developing statistics of force-chain structure, with the goal of identifying geometric and topological differences across packings, and providing a foundation on which to build predictions of bulk material properties from mesoscale network features. Here we discuss a trio of related but fundamentally distinct measurements of the mesoscale structure of force chains in two-dimensional (2D) packings, including a statistic derived using tools from algebraic topology, which together provide a tool set for the analysis of force chain architecture. We demonstrate the utility of this tool set by detecting variations in force-chain architecture with pressure. Collectively, these techniques can be generalized to 3D packings, and to the assessment of continuous deformations of packings under stress or strain.}, number={3}, journal={PHYSICAL REVIEW E}, publisher={American Physical Society (APS)}, author={Giusti, Chad and Papadopoulos, Lia and Owens, Eli T. and Daniels, Karen E. and Bassett, Danielle S.}, year={2016}, month={Sep} }
 @article{bassett_owens_daniels_porter_2015, title={Erratum: Influence of network topology on sound propagation in granular materials [Phys. Rev. E86, 041306 (2012)]}, volume={92}, DOI={10.1103/physreve.92.039905}, number={3}, journal={Physical Review E}, publisher={American Physical Society (APS)}, author={Bassett, Danielle S. and Owens, Eli T. and Daniels, Karen E. and Porter, Mason A.}, year={2015}, month={Sep} }
 @article{bassett_owens_porter_manning_daniels_2015, title={Extraction of force-chain network architecture in granular materials using community detection}, volume={11}, ISSN={["1744-6848"]}, DOI={10.1039/c4sm01821d}, abstractNote={Force chains form heterogeneous physical structures that can constrain the mechanical stability and acoustic transmission of granular media. However, despite their relevance for predicting bulk properties of materials, there is no agreement on a quantitative description of force chains. Consequently, it is difficult to compare the force-chain structures in different materials or experimental conditions. To address this challenge, we treat granular materials as spatially-embedded networks in which the nodes (particles) are connected by weighted edges that represent contact forces. We use techniques from community detection, which is a type of clustering, to find sets of closely connected particles. By using a geographical null model that is constrained by the particles' contact network, we extract chain-like structures that are reminiscent of force chains. We propose three diagnostics to measure these chain-like structures, and we demonstrate the utility of these diagnostics for identifying and characterizing classes of force-chain network architectures in various materials. To illustrate our methods, we describe how force-chain architecture depends on pressure for two very different types of packings: (1) ones derived from laboratory experiments and (2) ones derived from idealized, numerically-generated frictionless packings. By resolving individual force chains, we quantify statistical properties of force-chain shape and strength, which are potentially crucial diagnostics of bulk properties (including material stability). These methods facilitate quantitative comparisons between different particulate systems, regardless of whether they are measured experimentally or numerically.}, number={14}, journal={SOFT MATTER}, publisher={Royal Society of Chemistry (RSC)}, author={Bassett, Danielle S. and Owens, Eli T. and Porter, Mason A. and Manning, M. Lisa and Daniels, Karen E.}, year={2015}, pages={2731–2744} }
 @article{strickland_shearer_daniels_2015, title={Spatiotemporal measurement of surfactant distribution on gravity-capillary waves}, volume={777}, ISSN={["1469-7645"]}, DOI={10.1017/jfm.2015.352}, abstractNote={Materials adsorbed onto the surface of a fluid – for instance, crude oil, biogenic slicks or industrial/medical surfactants – will move in response to surface waves. Owing to the difficulty of non-invasive measurement of the spatial distribution of a molecular monolayer, little is known about the dynamics that couple the surface waves and the evolving density field. Here, we report measurements of the spatiotemporal dynamics of the density field of an insoluble surfactant driven by gravity–capillary waves in a shallow cylindrical container. Standing Faraday waves and travelling waves generated by the meniscus are superimposed to create a non-trivial surfactant density field. We measure both the height field of the surface using moiré imaging, and the density field of the surfactant via the fluorescence of NBD-tagged phosphatidylcholine, a lipid. Through phase averaging stroboscopically acquired images of the density field, we determine that the surfactant accumulates on the leading edge of the travelling meniscus waves and in the troughs of the standing Faraday waves. We fit the spatiotemporal variations in the two fields using an ansatz consisting of a superposition of Bessel functions, and report measurements of the wavenumbers and energy damping factors associated with the meniscus and Faraday waves, as well as the spatial and temporal phase shifts between them. While these measurements are largely consistent for both types of waves and both fields, it is notable that the damping factors for height and surfactant in the meniscus waves do not agree. This raises the possibility that there is a contribution from longitudinal waves in addition to the gravity–capillary waves.}, journal={JOURNAL OF FLUID MECHANICS}, publisher={Cambridge University Press (CUP)}, author={Strickland, Stephen L. and Shearer, Michael and Daniels, Karen E.}, year={2015}, month={Aug} }
 @article{swanson_strickland_shearer_daniels_2015, title={Surfactant spreading on a thin liquid film: reconciling models and experiments}, volume={94}, ISSN={["1573-2703"]}, DOI={10.1007/s10665-014-9735-0}, abstractNote={The spreading dynamics of surfactant molecules on a thin fluid layer is of both fundamental and practical interest. A mathematical model formulated by Gaver and Grotberg [J Fluid Mech 235:399–414, 1992] describing the spreading of a single layer of insoluble surfactant has become widely accepted, and several experiments on axisymmetric spreading have confirmed its predictions for both the height profile of the free surface and the spreading exponent (the radius of the circular area covered by surfactant grows as $$t^{1/4}$$ ). However, these prior experiments utilized primarily surfactant quantities exceeding (sometimes far exceeding) a monolayer. In this paper, we report that this regime is characterized by a mismatch between the timescales of the experiment and model and, additionally, find that the spatial distribution of surfactant molecules differs substantially from the model prediction. For experiments performed in the monolayer regime for which the model was developed, the surfactant layer is observed to have a spreading exponent of less than $$1/10$$ , far below the predicted value, and the surfactant distribution is also in disagreement. These findings suggest that the model is inadequate for describing the spreading of insoluble surfactants on thin fluid layers.}, number={1}, journal={JOURNAL OF ENGINEERING MATHEMATICS}, publisher={Springer Nature}, author={Swanson, Ellen R. and Strickland, Stephen L. and Shearer, Michael and Daniels, Karen E.}, year={2015}, month={Oct}, pages={63–79} }
 @article{bi_henkes_daniels_chakraborty_2015, title={The Statistical Physics of Athermal Materials}, volume={6}, ISSN={1947-5454 1947-5462}, url={http://dx.doi.org/10.1146/annurev-conmatphys-031214-014336}, DOI={10.1146/annurev-conmatphys-031214-014336}, abstractNote={ At the core of equilibrium statistical mechanics lies the notion of statistical ensembles: a collection of microstates, each occurring with a given a priori probability that depends on only a few macroscopic parameters, such as temperature, pressure, volume, and energy. In this review, we discuss recent advances in establishing statistical ensembles for athermal materials. The broad class of granular and particulate materials is immune to the effects of thermal fluctuations because the constituents are macroscopic. In addition, interactions between grains are frictional and dissipative, which invalidates the fundamental postulates of equilibrium statistical mechanics. However, granular materials exhibit distributions of microscopic quantities that are reproducible and often depend on only a few macroscopic parameters. We explore the history of statistical ensemble ideas in the context of granular materials, clarify the nature of such ensembles and their foundational principles, highlight advances in testing key ideas, and discuss applications of ensembles to analyze the collective behavior of granular materials. }, number={1}, journal={Annual Review of Condensed Matter Physics}, publisher={Annual Reviews}, author={Bi, Dapeng and Henkes, Silke and Daniels, Karen E. and Chakraborty, Bulbul}, year={2015}, month={Mar}, pages={63–83} }
 @article{daniels_bauer_shinbrot_2014, title={Correlations between electrical and mechanical signals during granular stick-slip events}, volume={16}, ISSN={["1434-7636"]}, DOI={10.1007/s10035-013-0471-3}, abstractNote={Powders and grains exhibit unpredictable jamming-to-flow transitions that manifest themselves on geophysical scales in catastrophic slip events such as landslides and earthquakes, and on laboratory/industrial scales in profound processing difficulties. Over the past few years, insight into these transitions has been provided by new evidence that slip events may be accompanied, or even preceded, by electrical effects. In the present work, we quantify the correlation between slip and the separation of electrical charges, using an archetypal granular material: photoelastic polymers. We measure a strong correlation between material displacement, acoustic emissions, and voltage. We find that the generation of voltage is associated with surface, rather than bulk properties of the granular materials. While voltage precursors are only occasionally observed in this system, there is some asymmetry in the cross-correlation between the slip and voltage signals that indicates differences between the pre- and post-slip dynamics.}, number={2}, journal={GRANULAR MATTER}, publisher={Springer Nature}, author={Daniels, Karen E. and Bauer, Caroline and Shinbrot, Troy}, year={2014}, month={Apr}, pages={217–222} }
 @article{bostwick_shearer_daniels_2014, title={Elastocapillary deformations on partially-wetting substrates: rival contact-line models}, volume={10}, ISSN={1744-683X 1744-6848}, url={http://dx.doi.org/10.1039/C4SM00891J}, DOI={10.1039/c4sm00891j}, abstractNote={A partially-wetting liquid can deform the underlying elastic substrate upon which it rests. This situation requires the development of theoretical models to describe the wetting forces imparted by the drop onto the solid substrate, particularly those at the contact-line. We construct a general solution using a displacement potential function for the elastic deformations within a finite elastic substrate associated with these wetting forces, and compare the results for several different contact-line models. Our work incorporates internal contributions to the surface stress from both liquid/solid Σls and Σsg solid/gas solid surface tensions (surface stress), which results in a non-standard boundary-value problem that we solve using a dual integral equation. We compare our results to relevant experiments and conclude that the generalization of solid surface tension Σls ≠ Σsg is an essential feature in any model of partial-wetting. The comparisons also allow us to systematically eliminate some proposed contact-line models.}, number={37}, journal={Soft Matter}, publisher={Royal Society of Chemistry (RSC)}, author={Bostwick, Joshua B. and Shearer, Michael and Daniels, Karen E.}, year={2014}, month={Jul}, pages={7361} }
 @article{ortiz_daniels_riehn_2014, title={Nonlinear elasticity of microsphere heaps}, volume={90}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000341301600006&KeyUID=WOS:000341301600006}, DOI={10.1103/physreve.90.022304}, abstractNote={Thermal fluctuations, geometric exclusion, and external driving all govern the mechanical response of dense particulate suspensions. Here, we measure the stress-strain response of quasi-two-dimensional flow-stabilized microsphere heaps in a regime in which all three effects are present using a microfluidic device. We observe that the elastic modulus and the mean interparticle separation of the heaps are tunable via the confining stress provided by the fluid flow. Furthermore, the measured stress-strain curves exhibit a universal nonlinear shape, which can be predicted from a thermal van der Waals equation of state with excluded volume. This analysis indicates that many-body interactions contribute a significant fraction of the stress supported by the heap.}, number={2}, journal={Physical Review. E, Statistical, Nonlinear, and Soft Matter Physics}, publisher={American Physical Society (APS)}, author={Ortiz, C. P. and Daniels, K. E. and Riehn, R.}, year={2014}, pages={9} }
 @article{daniels_2014, title={Pushing on a Nonlinear Material}, volume={7}, ISSN={1943-2879}, url={http://dx.doi.org/10.1103/Physics.7.113}, DOI={10.1103/Physics.7.113}, abstractNote={Understanding how materials bend, stretch, or compress under an applied force (or “load”) is essential in the design of buildings, vehicles, and machines. These deformations can be straightforwardly calculated for materials like aluminum, glass, or carbon fiber, which tend to deform little and can therefore, to a good approximation, be treated as linear: doubling the load doubles the effect, like pulling on a spring. In contrast, scientists are still looking for the right way to perform the equivalent calculations on stretchy materials like rubbers and foams, and disconnected “granular” materials, such as soil, sand, and cereals. All of these materials share a common property that they are easy to deform by large amounts, and therefore, the linear approximation no longer applies. Corentin Coulais, now at the University of Leiden, the Netherlands, and his colleagues report experiments on a model “soil” in which they carefully tracked the soil’s elastic response to a force exerted on it via a series of very gradual steps [1]. By comparing their results to predictions of an idealized theory of granular materials, they were able to identify the theory’s strengths and limitations. The work could therefore help researchers develop more reliable methods for predicting the behavior of granular materials. Physicists have been working on a number of models to understand how granular materials cross the boundary between being able to support a load to failing completely—a problem of considerable importance for engineers who need to know if a soil can serve as a foundation for a house or bridge. A promising route is to describe this boundary as a “jamming transition” [2, 3]. In this picture, particles are collectively “jammed” into place by their neighbors. However, under an applied stress, they can dislodge from those cages and give way—an “unjamming” transition that is analogous to the melting of an ordinary solid. Similar to the thermodynamic equations of state, where compressibility is a known function of state variables like pressure or temperature, the theory behind jamming can predict material properties of granular materials, such as how they stiffen as a function of particle packing density. But, so far, most of these predictions assume that the particles are frictionless, leaving it unclear which aspects of the theory apply to real situations in which friction is present. Coulais and his colleagues focused their study on two key predictions from the jamming framework [2, 3] that had not been tested for real, frictional particles. The first is the values of the critical exponents that describe how elasticity and dilatancy (the tendency to decrease packing density under shear [4]) vary as the material is sheared near the jamming transition. The second is to measure how characteristic lengths, in this case the one that characterizes the onset of nonlinearity as a function of distance from a disturbance, depend on the proximity of the material to the jamming/unjamming transition. To test these predictions, they used an experimental technique that allowed them to measure the force on each particle as they pushed grains in a jammed state outward from the center of the material. Instead of actual grains of sand, they used 5-millimeter-diameter polymer disks, laying 8000 of these disks in a single, densely packed layer. The polymer disks are photoelastic, meaning they rotate the polarization of light passing through them by an amount that depends on how much stress they experience. When properly calibrated [5, 6], these patterns of birefringence (Fig. 1) can measure the force at each individual contact. To generate shear within the layer of disks, Coulais and his colleagues slowly inflated a 26-millimeter “balloon” at the center of the packed layer. They then observed the progressive changes in the particle-scale stress, strain, dilation, and pressure resulting from each inflation step. For the tiniest steps possible, the researchers observed that the grain-scale stresses (forces between particles) were a nonlinear function of the local strain (the amount the aggregate deformed). But with each subsequent step, the packing experienced a dilatancy-associated pressure rise and a decreased in stiffness. This dilatancy softening}, journal={Physics}, publisher={American Physical Society (APS)}, author={Daniels, Karen}, year={2014}, month={Nov} }
 @article{strickland_hin_sayanagi_gaebler_daniels_levy_2014, title={Self-healing dynamics of surfactant coatings on thin viscous films}, volume={26}, ISSN={1070-6631 1089-7666}, url={http://dx.doi.org/10.1063/1.4872020}, DOI={10.1063/1.4872020}, abstractNote={We investigate the dynamics of an insoluble surfactant on the surface of a thin viscous fluid spreading inward to fill a surfactant-free region. During the initial stages of surfactant self-healing, Marangoni forces drive an axisymmetric ridge inward to coalesce into a growing central distension; this is unlike outward spreading, in which the ridge only decays. In later dynamics, the distension slowly decays and the surfactant concentration equilibrates. We present results from experiments in which we simultaneously measure the surfactant concentration (using fluorescently tagged lipids) and the fluid height profile (via laser profilometry). We compare the results to simulations of a mathematical model using parameters from our experiments. For surfactant concentrations close to but below the critical monolayer concentration, we observe agreement between the height profiles in the numerical simulations and the experiment, but disagreement in the surfactant distribution. In experiments at lower concentrations, the surfactant spreading and formation of a Marangoni ridge are no longer present, and a persistent lipid-free region remains. This observation, which is not captured by the simulations, has undesirable implications for applications where uniform coverage is advantageous. Finally, we probe the generality of the effect, and find that distensions of similar size are produced independent of initial fluid thickness, size of initial clean region, and surfactant type.}, number={4}, journal={Physics of Fluids}, publisher={AIP Publishing}, author={Strickland, Stephen L. and Hin, Matthew and Sayanagi, M. Richard and Gaebler, Cameron and Daniels, Karen E. and Levy, Rachel}, year={2014}, month={Apr}, pages={042109} }
 @inproceedings{strickland_shearer_daniels_2014, title={Video: Spatiotemporal dynamics of surfactant monolayers on Faraday waves}, DOI={10.1103/aps.dfd.2014.gfm.v0069}, booktitle={67th Annual Meeting of the APS Division of Fluid Dynamics - Gallery of Fluid Motion}, publisher={American Physical Society}, author={Strickland, Stephen and Shearer, Michael and Daniels, Karen}, year={2014}, month={Nov} }
 @article{owens_daniels_2013, title={Acoustic measurement of a granular density of modes}, volume={9}, ISSN={["1744-683X"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000312553900024&KeyUID=WOS:000312553900024}, DOI={10.1039/c2sm27122b}, abstractNote={In glasses and other disordered materials, measurements of the vibrational density of states reveal that an excess number of long-wavelength (low-frequency) modes, as compared to the Debye scaling seen in crystalline materials, is associated with a loss of mechanical rigidity. In this paper, we present a novel technique for measuring the density of modes (DOM) in a real granular material, in which we excite vibrational modes using white noise acoustic waves. The resulting vibrations are detected with piezoelectric sensors embedded inside a subset of the particles, from which we are able to compute the DOM via the spectrum of the velocity autocorrelation function, a technique previously applied in thermal systems. The velocity distribution for individual particles is observed to be Gaussian, but the ensemble distribution is non-Gaussian due to varying widths of the individual distributions. We find that the DOM exhibits several thermal-like features, including Debye scaling in a compressed hexagonally ordered packing, and an increase in low-frequency modes as the confining pressure is decreased. In disordered packings, we find that a characteristic frequency fc increases with pressure, but more weakly than has been observed in simulations of frictionless packings.}, number={4}, journal={SOFT MATTER}, publisher={Royal Society of Chemistry (RSC)}, author={Owens, Eli T. and Daniels, Karen E.}, year={2013}, pages={1214–1219} }
 @article{bostwick_daniels_2013, title={Capillary fracture of soft gels}, volume={88}, ISSN={["1550-2376"]}, DOI={10.1103/physreve.88.042410}, abstractNote={A liquid droplet resting on a soft gel substrate can deform that substrate to the point of material failure, whereby fractures develop on the gel surface that propagate outwards from the contact line in a starburst pattern. In this paper, we characterize (i) the initiation process, in which the number of arms in the starburst is controlled by the ratio of the surface tension contrast to the gel's elastic modulus, and (ii) the propagation dynamics showing that once fractures are initiated they propagate with a universal power law L[proportional]t(3/4). We develop a model for crack initiation by treating the gel as a linear elastic solid and computing the deformations within the substrate from the liquid-solid wetting forces. The elastic solution shows that both the location and the magnitude of the wetting forces are critical in providing a quantitative prediction for the number of fractures and, hence, an interpretation of the initiation of capillary fractures. This solution also reveals that the depth of the gel is an important factor in the fracture process, as it can help mitigate large surface tractions; this finding is confirmed with experiments. We then develop a model for crack propagation by considering the transport of an inviscid fluid into the fracture tip of an incompressible material and find that a simple energy-conservation argument can explain the observed material-independent power law. We compare predictions for both linear elastic and neo-Hookean solids, finding that the latter better explains the observed exponent.}, number={4}, journal={PHYSICAL REVIEW E}, publisher={American Physical Society (APS)}, author={Bostwick, Joshua B. and Daniels, Karen E.}, year={2013}, month={Oct} }
 @article{puckett_daniels_2013, title={Equilibrating Temperaturelike Variables in Jammed Granular Subsystems}, volume={110}, ISSN={["0031-9007"]}, DOI={10.1103/physrevlett.110.058001}, abstractNote={Although jammed granular systems are athermal, several thermodynamiclike descriptions have been proposed which make quantitative predictions about the distribution of volume and stress within a system and provide a corresponding temperaturelike variable. We perform experiments with an apparatus designed to generate a large number of independent, jammed, two-dimensional configurations. Each configuration consists of a single layer of photoelastic disks supported by a gentle layer of air. New configurations are generated by cyclically dilating, mixing, and then recompacting the system through a series of boundary displacements. Within each configuration, a bath of particles surrounds a smaller subsystem of particles with a different interparticle friction coefficient than the bath. The use of photoelastic particles permits us to find all particle positions as well as the vector forces at each interparticle contact. By comparing the temperaturelike quantities in both systems, we find compactivity (conjugate to the volume) does not equilibrate between the systems, while the angoricity (conjugate to the stress) does. Both independent components of the angoricity are linearly dependent on the hydrostatic pressure, in agreement with predictions of the stress ensemble.}, number={5}, journal={PHYSICAL REVIEW LETTERS}, publisher={American Physical Society (APS)}, author={Puckett, James G. and Daniels, Karen E.}, year={2013}, month={Jan} }
 @article{ortiz_riehn_daniels_2013, title={Flow-driven formation of solid-like microsphere heaps}, volume={9}, ISSN={["1744-683X"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000311964800025&KeyUID=WOS:000311964800025}, DOI={10.1039/c2sm26762d}, abstractNote={We observe the formation of heaps of repulsive microspheres, created by flowing a colloidal microsphere suspension towards a flat-topped ridge placed within a quasi two-dimensional microfluidic channel. This configuration allows for both shear and normal forces on the microspheres in contact with the ridge. The heaps, which are formed against the ridge, are characterized by two distinct phases: a solid-like bulk phase in the interior and a highly fluctuating, liquid-like state which exists along its leading edge. We observe that heaps only form above a critical flow velocity, vc, and that they are destroyed by thermal rearrangements when the flow ceases. We monitor the dynamics of heap formation using fluorescence video microscopy, measuring the heap volume and the angle of repose in response to microsphere deposition and erosion processes. We find that the steady state angle of repose, θf, increases as a function of inflow velocity, v∞, with a functional form .}, number={2}, journal={SOFT MATTER}, publisher={Royal Society of Chemistry (RSC)}, author={Ortiz, Carlos P. and Riehn, Robert and Daniels, Karen E.}, year={2013}, pages={543–549} }
 @article{daniels_schroeter_2013, title={Focus on granular segregation}, volume={15}, ISSN={["1367-2630"]}, DOI={10.1088/1367-2630/15/3/035017}, abstractNote={Ordinary fluids mix themselves through thermal motions, or can be even more efficiently mixed by stirring. In contrast, granular materials such as sand often unmix when they are stirred, shaken or sheared. This granular segregation is both a practical means to separate materials in industry, and a persistent challenge to uniformly mixing them. While segregation phenomena are ubiquitous, a large number of different mechanisms have been identified and the underlying physics remains the subject of much inquiry. Particle size, shape, density and even surface roughness can play significant roles. The aim of this focus issue is to provide a snapshot of the current state of the science, covering a wide range of packing densities and driving mechanisms, from thermal-like dilute systems to dense flows.}, number={3}, journal={NEW JOURNAL OF PHYSICS}, publisher={IOP Publishing}, author={Daniels, Karen E. and Schroeter, Matthias}, year={2013}, month={Mar} }
 @article{penny_daniels_thompson_2013, title={Local properties of patterned vegetation: quantifying endogenous and exogenous effects}, volume={371}, DOI={10.1098/rsta.2012.0359}, abstractNote={Dryland ecosystems commonly exhibit periodic bands of vegetation, thought to form due to competition between individual plants for heterogeneously distributed water. In this paper, we develop a Fourier method for locally identifying the pattern wavenumber and orientation, and apply it to aerial images from a region of vegetation patterning near Fort Stockton, TX, USA. We find that the local pattern wavelength and orientation are typically coherent, but exhibit both rapid and gradual variation driven by changes in hillslope gradient and orientation, the potential for water accumulation, or soil type. Endogenous pattern dynamics, when simulated for spatially homogeneous topographic and vegetation conditions, predict pattern properties that are much less variable than the orientation and wavelength observed in natural systems. Our local pattern analysis, combined with ancillary datasets describing soil and topographic variation, highlights a largely unexplored correlation between soil depth, pattern coherence, vegetation cover and pattern wavelength. It also, surprisingly, suggests that downslope accumulation of water may play a role in changing vegetation pattern properties.}, number={2004}, journal={Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences}, publisher={The Royal Society}, author={Penny, G. G. and Daniels, K. E. and Thompson, S. E.}, year={2013}, month={Nov}, pages={20120359–20120359} }
 @inbook{daniels_2013, place={Berlin Heidelberg}, title={Rubble-Pile Near Earth Objects: Insights from Granular Physics}, ISBN={9783642392436 9783642392443}, url={http://dx.doi.org/10.1007/978-3-642-39244-3_11}, DOI={10.1007/978-3-642-39244-3_11}, abstractNote={Most Near Earth Objects (NEOs) are composed of fractured rock, sometimes highly fractured and porous, and they have come to be known as rubble piles (Britt 2001; Fujiwara et al. 2006). The constituent particles, ranging from millimeters up to tens of meters, are weakly held together as an aggregate by a combination of both gravitational and van der Waals forces, which can be of comparable strength (Scheers et al. 2010). Future missions to these rubble NEOs, whether human or robotic, will need to operate in such a way that they can safely and successfully probe a fragile object. Of key importance is the ability to predict and control the circumstances under which the NEO material will remain intact or become unstable during activities such as digging, sample-collection, anchoring, or lift-off.}, booktitle={Asteroids}, publisher={Springer}, author={Daniels, Karen E.}, year={2013}, pages={271–286} }
 @article{nichol_daniels_2012, title={Equipartition of Rotational and Translational Energy in a Dense Granular Gas}, volume={108}, ISSN={["1079-7114"]}, DOI={10.1103/physrevlett.108.018001}, abstractNote={Experiments quantifying the rotational and translational motion of particles in a dense, driven, 2D granular gas floating on an air table reveal that kinetic energy is divided equally between the two translational and one rotational degrees of freedom. This equipartition persists when the particle properties, confining pressure, packing density, or spatial ordering are changed. While the translational velocity distributions are the same for both large and small particles, the angular velocity distributions scale with the particle radius. The probability distributions of all particle velocities have approximately exponential tails. Additionally, we find that the system can be described with a granular Boyle's law with a van der Waals-like equation of state. These results demonstrate ways in which conventional statistical mechanics can unexpectedly apply to nonequilibrium systems.}, number={1}, journal={PHYSICAL REVIEW LETTERS}, publisher={American Physical Society (APS)}, author={Nichol, Kiri and Daniels, Karen E.}, year={2012}, month={Jan} }
 @article{bassett_owens_daniels_porter_2012, title={Influence of network topology on sound propagation in granular materials}, volume={86}, ISSN={["2470-0053"]}, DOI={10.1103/physreve.86.041306}, abstractNote={Granular media, whose features range from the particle scale to the force-chain scale and the bulk scale, are usually modeled as either particulate or continuum materials. In contrast with each of these approaches, network representations are natural for the simultaneous examination of microscopic, mesoscopic, and macroscopic features. In this paper, we treat granular materials as spatially embedded networks in which the nodes (particles) are connected by weighted edges obtained from contact forces. We test a variety of network measures to determine their utility in helping to describe sound propagation in granular networks and find that network diagnostics can be used to probe particle-, curve-, domain-, and system-scale structures in granular media. In particular, diagnostics of mesoscale network structure are reproducible across experiments, are correlated with sound propagation in this medium, and can be used to identify potentially interesting size scales. We also demonstrate that the sensitivity of network diagnostics depends on the phase of sound propagation. In the injection phase, the signal propagates systemically, as indicated by correlations with the network diagnostic of global efficiency. In the scattering phase, however, the signal is better predicted by mesoscale community structure, suggesting that the acoustic signal scatters over local geographic neighborhoods. Collectively, our results demonstrate how the force network of a granular system is imprinted on transmitted waves.}, number={4}, journal={PHYSICAL REVIEW E}, publisher={American Physical Society (APS)}, author={Bassett, Danielle S. and Owens, Eli T. and Daniels, Karen E. and Porter, Mason A.}, year={2012}, month={Oct} }
 @article{puckett_lechenault_daniels_thiffeault_2012, title={Trajectory entanglement in dense granular materials}, volume={2012}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000305893500010&KeyUID=WOS:000305893500010}, DOI={10.1088/1742-5468/2012/06/p06008}, abstractNote={The particle-scale dynamics of granular materials have commonly been characterized by the self-diffusion coefficient $D$. However, this measure discards the collective and topological information known to be an important characteristic of particle trajectories in dense systems. Direct measurement of the entanglement of particle space-time trajectories can be obtained via the topological braid entropy $\Sbraid$, which has previously been used to quantify mixing efficiency in fluid systems. Here, we investigate the utility of $\Sbraid$ in characterizing the dynamics of a dense, driven granular material at packing densities near the static jamming point $\phi_J$. From particle trajectories measured within a two-dimensional granular material, we typically observe that $\Sbraid$ is well-defined and extensive. However, for systems where $\phi \gtrsim 0.79$, we find that $\Sbraid$ (like $D$) is not well-defined, signifying that these systems are not ergodic on the experimental timescale. Both $\Sbraid$ and $D$ decrease with either increasing packing density or confining pressure, independent of the applied boundary condition. The related braiding factor provides a means to identify multi-particle phenomena such as collective rearrangements. We discuss possible uses for this measure in characterizing granular systems.}, number={06}, journal={Journal of Statistical Mechanics-Theory and Experiment}, publisher={IOP Publishing}, author={Puckett, James G and Lechenault, Frédéric and Daniels, Karen E and Thiffeault, Jean-Luc}, year={2012}, pages={13} }
 @article{hayman_ducloue_foco_daniels_2011, title={Granular Controls on Periodicity of Stick-Slip Events: Kinematics and Force-Chains in an Experimental Fault}, volume={168}, ISSN={["1420-9136"]}, DOI={10.1007/s00024-011-0269-3}, abstractNote={It is a long-standing question whether granular fault material such as gouge plays a major role in controlling fault dynamics such as seismicity and slip-periodicity. In both natural and experimental faults, granular materials resist shear and accommodate strain via interparticle friction, fracture toughness, fluid pressure, dilation, and interparticle rearrangements. Here, we isolate the effects of particle rearrangements on granular deformation through laboratory experiments. Within a sheared photoelastic granular aggregate at constant volume, we simultaneously visualize both particle-scale kinematics and interparticle forces, the latter taking the form of force-chains. We observe stick-slip deformation and associated force drops during an overall strengthening of the shear zone. This strengthening regime provides insight into granular rheology and conditions of stick-slip periodicity, and may be qualitatively analogous to slip that accompanies longer term interseismic strengthening of natural faults. Of particular note is the observation that increasing the packing density increases the stiffness of the granular aggregate and decreases the damping (increases time-scales) during slip events. At relatively loose packing density, the slip displacements during the events follow an approximately power-law distribution, as opposed to an exponential distribution at higher packing density. The system exhibits switching between quasi-periodic and aperiodic slip behavior at all packing densities. Higher packing densities favor quasi-periodic behavior, with a longer time interval between aperiodic events than between quasi-periodic events. This difference in the time-scale of aperiodic stick-slip deformation is reflected in both the kinematics of interparticle slip and the force-chain dynamics: all major force-chain reorganizations are associated with aperiodic events. Our experiments conceptually link observations of natural fault dynamics with current models for granular stick-slip dynamics. We find that the stick-slip dynamics are consistent with a driven harmonic oscillator model with damping provided by an effective viscosity, and that shear-transformation-zone, jamming, and crackling noise theories provide insight into the effective stiffness and patterns of shear localization during deformation.}, number={12}, journal={PURE AND APPLIED GEOPHYSICS}, publisher={Springer Nature}, author={Hayman, Nicholas W. and Ducloue, Lucie and Foco, Kate L. and Daniels, Karen E.}, year={2011}, month={Dec}, pages={2239–2257} }
 @article{puckett_lechenault_daniels_2011, title={Local origins of volume fraction fluctuations in dense granular materials}, volume={83}, ISSN={["1550-2376"]}, DOI={10.1103/physreve.83.041301}, abstractNote={Fluctuations of the local volume fraction within granular materials have previously been observed to decrease as the system approaches jamming. We experimentally examine the role of boundary conditions and interparticle friction μ on this relationship for a dense granular material of bidisperse particles driven under either constant volume or constant pressure. Using a radical Voronoï tessellation, we find the variance of the local volume fraction Φ monotonically decreases as the system becomes more dense, independent of boundary condition and μ. We examine the universality and origins of this trend using experiments and the recent granocentric model [M. Clusel, E. I. Corwin, A. O. N. Siemens, and J. Brujić, Nature (London) 460, 611 (2009); E. I. Corwin, M. Clusel, A. O. N. Siemens, and J. Brujić, Soft Matter 6, 2949 (2010)], modified to draw particle locations from an arbitrary distribution P(s) of neighbor distances s. The mean and variance of the observed P(s) are described by a single length scale controlled by ̅Φ. Through the granocentric model, we observe that diverse functional forms of P(s) all produce the trend of decreasing fluctuations, but only the experimentally observed P(s) provides quantitative agreement with the measured Φ fluctuations. Thus, we find that both P(s) and P(Φ) encode similar information about the ensemble of observed packings and are connected to each other by the local granocentric model.}, number={4}, journal={PHYSICAL REVIEW E}, author={Puckett, James G. and Lechenault, Frederic and Daniels, Karen E.}, year={2011}, month={Apr} }
 @article{owens_daniels_2011, title={Sound propagation and force chains in granular materials}, volume={94}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000291033900018&KeyUID=WOS:000291033900018}, DOI={10.1209/0295-5075/94/54005}, abstractNote={Granular materials are inherently heterogeneous, leading to challenges in formulating accurate models of sound propagation. In order to quantify acoustic responses in space and time, we perform experiments in a photoelastic granular material in which the internal stress pattern (in the form of force chains) is visible. We utilize two complementary methods, high-speed imaging and piezoelectric transduction, to provide particle-scale measurements of both the amplitude and speed of an acoustic wave in the near-field regime. We observe that the wave amplitude is on average largest within particles experiencing the largest forces, particularly in those chains radiating away from the source, with the force-dependence of this amplitude in qualitative agreement with a simple Hertzian-like model of particle contact area. In addition, we are able to directly observe rare transiently strong force chains formed by the opening and closing of contacts during propagation. The speed of the leading edge of the pulse is in agreement with the speed of a one-dimensional chain, while the slower speed of the peak response suggests that it contains waves which have travelled over multiple paths even within just this near-field region. These effects highlight the importance of particle-scale behaviors in determining the acoustical properties of granular materials.}, number={5}, journal={Europhysics Letters}, author={Owens, E. T. and Daniels, K. E.}, year={2011} }
 @article{thompson_daniels_2010, title={A Porous Convection Model for Grass Patterns}, volume={175}, ISSN={["0003-0147"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000272372200017&KeyUID=WOS:000272372200017}, DOI={10.1086/648603}, abstractNote={Spatial ecological patterns are usually ascribed to Turing‐type reaction‐diffusion or scale‐dependent feedback processes, but morphologically indistinguishable patterns can be produced by instabilities in fluid flow. We present a new hypothesis that suggests that fluid convection and chill damage to plants could form vegetation patterns with wavelengths ≈1–2 times the plant height. Previous hypotheses for small‐scale vegetation pattern formation relied on a Turing process driven by competition for water, which is thought to occur in large vegetation patterns. Predictions of the new hypothesis were consistent with properties of natural grass patterns in North Carolina, contradicting the Turing hypothesis. These results indicate that similarities in pattern morphology should not be interpreted as implying similarities in the pattern‐forming processes, that small‐wavelength vegetation patterns may arise from mechanisms that are distinct from those generating long‐wavelength vegetation patterns, and that fluid instabilities should be recognized as a cause of ecological patterns.}, number={1}, journal={AMERICAN NATURALIST}, publisher={University of Chicago Press}, author={Thompson, Sally E. and Daniels, Karen E.}, year={2010}, month={Jan}, pages={E10–E15} }
 @article{lechenault_daniels_2010, title={Equilibration of granular subsystems}, volume={6}, ISSN={["1744-683X"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000279046300029&KeyUID=WOS:000279046300029}, DOI={10.1039/b926754a}, abstractNote={We experimentally investigate the steady states of two granular assemblies differing in their material properties and allowed to exchange volume with each other under external agitation in the vicinity of their jamming transition. We extract the statistics of various static and dynamic quantities, and uncover a materials-independent relationship between the average packing fraction and its fluctuations. This relationship defines an intensive parameter which decouples from the volume statistics, and remarkably takes the same value in both subsystems. We also observe that an effective diffusion coefficient also takes the same value in each subsystem, even as the structural relaxation time increases over several orders of magnitude. These observations provide strong constraints on the eventual establishment of a granular equation of state.}, number={13}, journal={SOFT MATTER}, publisher={Royal Society of Chemistry (RSC)}, author={Lechenault, F. and Daniels, Karen E.}, year={2010}, pages={3074–3081} }
 @article{fallest_lichtenberger_fox_daniels_2010, title={Fluorescent visualization of a spreading surfactant}, volume={12}, DOI={10.1088/1367-2630/12/7/073029}, abstractNote={The spreading of surfactants on thin films is an industrially and medically important phenomenon, but the dynamics are highly nonlinear and visualization of the surfactant dynamics has been a long-standing experimental challenge. We perform the first quantitative, spatiotemporally resolved measurements of the spreading of an insoluble surfactant on a thin fluid layer. During the spreading process, we directly observe both the radial height profile of the spreading droplet and the spatial distribution of the fluorescently tagged surfactant. We find that the leading edge of a spreading circular layer of surfactant forms a Marangoni ridge in the underlying fluid, with a trough trailing the ridge as expected. However, several novel features are observed using the fluorescence technique, including a peak in the surfactant concentration that trails the leading edge, and a flat, monolayer-scale spreading film that differs from concentration profiles predicted by current models. Both the Marangoni ridge and the surfactant leading edge can be described to spread as R∝tδ. We find spreading exponents δH≈0.30 and δΓ≈0.22 for the ridge peak and surfactant leading edge, respectively, which are in good agreement with theoretical predictions of δ=1/4. In addition, we observe that the surfactant leading edge initially leads the peak of the Marangoni ridge, with the peak later catching up to the leading edge.}, number={7}, journal={New Journal of Physics}, publisher={IOP Publishing}, author={Fallest, David W and Lichtenberger, Adele M and Fox, Christopher J and Daniels, Karen E}, year={2010}, month={Jul}, pages={073029} }
 @article{may_shearer_daniels_2010, title={Scalar Conservation Laws with Nonconstant Coefficients with Application to Particle Size Segregation in Granular Flow}, volume={20}, ISSN={0938-8974 1432-1467}, url={http://dx.doi.org/10.1007/s00332-010-9069-7}, DOI={10.1007/s00332-010-9069-7}, abstractNote={Granular materials will segregate by particle size when subjected to shear, as occurs, for example, in avalanches. The evolution of a bidisperse mixture of particles can be modeled by a nonlinear first order partial differential equation, provided the shear (or velocity) is a known function of position. While avalanche-driven shear is approximately uniform in depth, boundary-driven shear typically creates a shear band with a nonlinear velocity profile. In this paper, we measure a velocity profile from experimental data and solve initial value problems that mimic the segregation observed in the experiment, thereby verifying the value of the continuum model. To simplify the analysis, we consider only one-dimensional configurations, in which a layer of small particles is placed above a layer of large particles within an annular shear cell and is sheared for arbitrarily long times. We fit the measured velocity profile to both an exponential function of depth and a piecewise linear function which separates the shear band from the rest of the material. Each solution of the initial value problem is nonstandard, involving curved characteristics in the exponential case, and a material interface with a jump in characteristic speed in the piecewise linear case.}, number={6}, journal={Journal of Nonlinear Science}, publisher={Springer Science and Business Media LLC}, author={May, Lindsay B. H. and Shearer, Michael and Daniels, Karen E.}, year={2010}, month={May}, pages={689–707} }
 @article{may_golick_phillips_shearer_daniels_2010, title={Shear-driven size segregation of granular materials: Modeling and experiment}, volume={81}, ISSN={["1550-2376"]}, DOI={10.1103/physreve.81.051301}, abstractNote={Granular materials segregate by size under shear, and the ability to quantitatively predict the time required to achieve complete segregation is a key test of our understanding of the segregation process. In this paper, we apply the Gray-Thornton model of segregation (developed for linear shear profiles) to a granular flow with an exponential shear profile, and evaluate its ability to describe the observed segregation dynamics. Our experiment is conducted in an annular Couette cell with a moving lower boundary. The granular material is initially prepared in an unstable configuration with a layer of small particles above a layer of large particles. Under shear, the sample mixes and then resegregates so that the large particles are located in the top half of the system in the final state. During this segregation process, we measure the velocity profile and use the resulting exponential fit as input parameters to the model. To make a direct comparison between the continuum model and the observed segregation dynamics, we map the local concentration (from the model) to changes in packing fraction; this provides a way to make a semiquantitative comparison with the measured global dilation. We observe that the resulting model successfully captures the presence of a fast mixing process and relatively slower resegregation process, but the model predicts a finite resegregation time, while in the experiment resegregation occurs only exponentially in time.}, number={5}, journal={PHYSICAL REVIEW E}, publisher={American Physical Society (APS)}, author={May, Lindsay B. H. and Golick, Laura A. and Phillips, Katherine C. and Shearer, Michael and Daniels, Karen E.}, year={2010}, month={May} }
 @article{daniels_2010, title={Student Blogging about Physics}, volume={48}, DOI={10.1119/1.3479708}, abstractNote={In traditional introductory physics classes, there is often limited opportunity for students to contribute their own ideas, interests, and experiences as they engage with the subject matter. This situation is exacerbated in university lecture-format classes, where students may not feel comfortable speaking during class. In the last few years, Internet blogs have become a decentralized format for diarists, independent journalists, and opinion makers to both post entries and allow commentary from their readers. Below, I will describe some techniques for using student blogging about physics to engage students from two different classroom environments: a calculus-based introductory mechanics class for scientists and engineers, and an honors seminar for first-year students. These assignments required them to make their own connections between classroom knowledge and situations where it might find applications. A second goal of including blogging in the introductory physics course was to induce students to write about the physics content of the class in a more substantive way than was previously part of the class.}, number={6}, journal={The Physics Teacher}, publisher={American Association of Physics Teachers (AAPT)}, author={Daniels, Karen E.}, year={2010}, pages={366–367} }
 @misc{shearer_may_giffen_daniels_goddard_giovine_jenkins_2010, title={The Gray-Thornton Model of Granular Segregation}, ISSN={0094-243X}, url={http://dx.doi.org/10.1063/1.3435407}, DOI={10.1063/1.3435407}, abstractNote={In this paper, we explore properties of the Gray‐Thornton model for particle size segregation in granular avalanches. The model equation is a single conservation law expressing conservation of mass under shear for the concentration of the smaller of two types of particle in a bidisperse mixture. Sharp interfaces across which the concentration jumps are shock wave solutions of the partial differential equation. We show that they can form internally from smooth data, as well as propagate in from boundaries of the domain. We prove a general stability result that expresses the physically reasonable notion that an interface should be stable only if the concentration of small particles is larger below the interface than above. Once shocks form, they are sheared by the flow, leading to loss of stability when an interface becomes vertical. The subsequent evolution of a mixing zone, a two‐dimensional rarefaction solution of the equation that replaces the unstable part of the shock can be tracked explicitly for a s...}, journal={AIP Conference Proceedings}, publisher={AIP}, author={Shearer, Michael and May, Lindsay B. H. and Giffen, Nicholas and Daniels, Karen E. and Goddard, Joe and Giovine, Pasquale and Jenkins, James T.}, year={2010} }
 @article{fender_lechenault_daniels_2010, title={Universal Shapes Formed by Two Interacting Cracks}, volume={105}, ISSN={["0031-9007"]}, DOI={10.1103/physrevlett.105.125505}, abstractNote={We investigate the origins of the widely observed "en passant" crack pattern, which forms through interactions between two approaching cracks. A rectangular elastic plate is notched on each long side and then subjected to quasistatic uniaxial strain from the short side. The two cracks propagate along approximately straight paths until they pass each other, after which they curve and release a lenticular fragment. We find that, for materials with diverse mechanical properties, the shape of this fragment has an aspect ratio of 2:1, with the length scale set by the initial crack offset s and the time scale set by the ratio of s to the pulling velocity. The cracks have a universal square root shape, which we understand by using a simple geometric model of the crack-crack interaction.}, number={12}, journal={PHYSICAL REVIEW LETTERS}, author={Fender, Melissa L. and Lechenault, Frederic and Daniels, Karen E.}, year={2010}, month={Sep} }
 @inproceedings{daniels_hayman_nakagawa_luding_2009, title={Boundary conditions and event scaling of granular stick-slip events}, url={http://dx.doi.org/10.1063/1.3179988}, DOI={10.1063/1.3179988}, abstractNote={We describe experiments on stick‐slip failure of a granular material within a linear shear cell. The experiments are performed in a quasi‐two‐dimensional photoelastic granular material which is sheared via a slider block and spring moving at a constant velocity. The apparatus can provide either constant‐volume or constant dP/dV boundary conditions on the aggregate, and we observe boundary‐condition dependence in various size‐characterizations (duration, force drop, maximum velocity, energy released, area) of the events. Through the use of photoelastic particles, we visualize force chains before and after stick‐slip events. We observe that patterns of slip and stress release are highly heterogeneous in their spatial extent, and associate this with scatter in the scaling relations.}, booktitle={AIP Conference Proceedings}, publisher={AIP}, author={Daniels, Karen E. and Hayman, Nicholas W. and Nakagawa, Masami and Luding, Stefan}, year={2009} }
 @inproceedings{puckett_lechenault frédéric_daniels_nakagawa_luding_2009, title={Generating ensembles and measuring mixing in a model granular system}, url={http://dx.doi.org/10.1063/1.3180016}, DOI={10.1063/1.3180016}, abstractNote={A major open question in the field of granular materials is the identification of relevant state variables which can predict macroscopic behavior. We experimentally investigate the mixing properties of an idealized granular liquid in the vicinity of its jamming transition, through the generation of ensembles of configurations under various boundary conditions. Our apparatus consists of a two‐dimensional aggregate of particles which rearrange under agitation from the outer boundaries. As expected, the system acts like a slow liquid at low pressure or low packing fraction, and jams at higher pressure or high packing fraction. We characterize mixing in the system by computing the topological entropy of the braids formed by the trajectories of the grains. This entropy is shown to be well‐defined and very sensitive to the approach to jamming, reflecting the dynamical arrest of the assembly.}, booktitle={AIP Conference Proceedings}, publisher={AIP}, author={Puckett, James G. and Lechenault Frédéric and Daniels, Karen E. and Nakagawa, Masami and Luding, Stefan}, year={2009} }
 @article{puckett_lechenault_daniels_2009, title={Generating ensembles of two-dimensional granular configurations}, volume={19}, ISSN={1054-1500 1089-7682}, url={http://dx.doi.org/10.1063/1.3207830}, DOI={10.1063/1.3207830}, number={4}, journal={Chaos: An Interdisciplinary Journal of Nonlinear Science}, publisher={AIP Publishing}, author={Puckett, James G. and Lechenault, Frédéric and Daniels, Karen E.}, year={2009}, month={Dec}, pages={041108} }
 @article{cvitanović_daniels_kudrolli_losert_redner_2009, title={Introduction: Sixth Annual Gallery of Nonlinear Images (Pittsburgh, Pennsylvania, 2009)}, volume={19}, ISSN={1054-1500 1089-7682}, url={http://dx.doi.org/10.1063/1.3257679}, DOI={10.1063/1.3257679}, abstractNote={First Page}, number={4}, journal={Chaos: An Interdisciplinary Journal of Nonlinear Science}, publisher={AIP Publishing}, author={Cvitanović, Predrag and Daniels, Karen E. and Kudrolli, Arshad and Losert, Wolfgang and Redner, Sidney}, year={2009}, month={Dec}, pages={041101} }
 @article{golick_daniels_2009, title={Mixing and segregation rates in sheared granular materials}, volume={80}, ISSN={["1550-2376"]}, DOI={10.1103/PhysRevE.80.042301}, abstractNote={The vertical size segregation of granular materials, a process commonly associated with the Brazil-nut effect, has generally been thought to proceed faster the greater the size difference of the particles. We experimentally investigate sheared dense bidisperse granular materials as a function of the size ratio of the two species and find that the mixing rate at low confining pressure behaves as expected from percolation-based arguments. However, we also observe an anomalous effect for the resegregation rates, wherein the segregation rate is a nonmonotonic function of the particle size ratio with a maximum for intermediate particle size ratio. Combined with the fact that increasing the confining pressure significantly suppresses both mixing and segregation rates of particles of sufficiently dissimilar size, we propose that the anomalous behavior may be attributed to a species-dependent distribution of forces within the system.}, number={4}, journal={PHYSICAL REVIEW E}, author={Golick, Laura A. and Daniels, Karen E.}, year={2009}, month={Oct} }
 @inproceedings{owens_couvreur stéphanie_daniels_nakagawa_luding_2009, title={Spatiotemporally Resolved Acoustics in a Photoelastic Granular Material}, volume={1145}, url={http://dx.doi.org/10.1063/1.3179958}, DOI={10.1063/1.3179958}, abstractNote={The effect of the force chain network on sound propagation in a granular material is poorly understood. To quantitatively study these effects, we perform acoustics experiments in a two dimensional photoelastic granular material in which force chains are visible. We send acoustic pulses into the material from a point source and measure the effects of this pulse via two methods: accelerometers within individual grains and movies which produce spatiotemporally resolved measurements of the acoustic propagation.}, booktitle={AIP Conference Proceedings}, publisher={AIP}, author={Owens, Eli T. and Couvreur Stéphanie and Daniels, Karen E. and Nakagawa, Masami and Luding, Stefan}, year={2009}, pages={447–450} }
 @article{daniels_brausch_pesch_bodenschatz_2008, title={Competition and bistability of ordered undulations and undulation chaos in inclined layer convection}, volume={597}, ISSN={["1469-7645"]}, DOI={10.1017/S0022112007009615}, abstractNote={Experimental and theoretical investigations of undulation patterns in high-pressure inclined layer gas convection at a Prandtl number near unity are reported. Particular focus is given to the competition between the spatiotemporal chaotic state of undulation chaos and stationary patterns of ordered undulations. In experiments, a competition and bistability between the two states is observed, with ordered undulations most prevalent at higher Rayleigh number. The spectral pattern entropy, spatial correlation lengths and defect statistics are used to characterize the competing states. The experiments are complemented by a theoretical analysis of the Oberbeck–Boussinesq equations. The stability region of the ordered undulations as a function of their wave vectors and the Rayleigh number is obtained with Galerkin techniques. In addition, direct numerical simulations are used to investigate the spatiotemporal dynamics. In the simulations, both ordered undulations and undulation chaos were observed dependent on initial conditions. Experiment and theory are found to agree well.}, journal={JOURNAL OF FLUID MECHANICS}, publisher={Cambridge University Press (CUP)}, author={Daniels, Karen E. and Brausch, Oliver and Pesch, Werner and Bodenschatz, Eberhard}, year={2008}, month={Feb}, pages={261–282} }
 @article{behringer_daniels_majmudar_sperl_2008, title={Fluctuations, correlations and transitions in granular materials: statistical mechanics for a non-conventional system}, volume={366}, ISSN={["1471-2962"]}, DOI={10.1098/rsta.2007.2106}, abstractNote={In this work, we first review some general properties of dense granular materials. We are particularly concerned with a statistical description of these materials, and it is in this light that we briefly describe results from four representative studies. These are: experiment 1: determining local force statistics, vector forces, force distributions and correlations for static granular systems; experiment 2: characterizing the jamming transition, for a static two-dimensional system; experiment 3: characterizing plastic failure in dense granular materials; and experiment 4: a dynamical transition where the material ‘freezes’ in the presence of apparent heating for a sheared and shaken system.}, number={1865}, journal={PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES}, publisher={The Royal Society}, author={Behringer, R. P. and Daniels, Karen E. and Majmudar, Trushant S. and Sperl, Matthias}, year={2008}, month={Feb}, pages={493–504} }
 @article{daniels_hayman_2008, title={Force chains in seismogenic faults visualized with photoelastic granular shear experiments}, volume={113}, ISSN={["2169-9356"]}, DOI={10.1029/2008jb005781}, abstractNote={Natural faults have many characteristics in common with granular systems, including granular fault rocks, shear localization, and stick‐slip dynamics. We present experimental results which provide insight into granular behavior in natural faults. The experiments allow us to directly image force chains within a deforming granular media through the use of photoelastic particles. The experimental apparatus consists of a spring‐pulled slider block which deforms the photoelastic granular aggregate at a constant velocity. Particles that carry more of the load appear brighter when viewed through crossed polarizers, making the internal stresses optically accessible. The resulting pattern is a branched, anisotropic force chain network inclined to the shear zone boundaries. Under both constant volume and dilational boundary conditions, deformation occurs predominantly through stick‐slip displacements and corresponding force drops. The particle motion and force chain changes associated with the deformation can either be localized to the central slip zone or span the system. The sizes of the experimental slip events are observed to have power law (Gutenberg‐Richter‐like) distributions; the minimum dimensions of events and the behavior of force chains suggest that a particle scale controls the lower limits of the power law distributions. For large drops in pulling force with slip, the shape of the size distributions is strongly affected by the choice of boundary condition, while for small to moderate drops the probability distributions are approximately independent of boundary condition. These size‐dependent variations in stick‐slip behavior are associated with different spatial patterns: on average, small events typically correspond to localized force chain or particle rearrangements, whereas large events correspond to system‐spanning changes. Such force chain behavior may be responsible for similar size‐dependent behaviors of natural faults.}, number={B11}, journal={JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH}, author={Daniels, Karen E. and Hayman, Nicholas W.}, year={2008}, month={Nov} }
 @article{daniels_mukhopadhyay_houseworth_behringer_2007, title={Instabilities in Droplets Spreading on Gels}, volume={99}, DOI={10.1103/physrevlett.99.124501}, abstractNote={We report a novel surface-tension driven instability observed for droplets spreading on a compliant substrate. When a droplet is released on the surface of an agar gel, it forms arms or cracks when the ratio of surface-tension gradient to gel strength is sufficiently large. We explore a range of gel strengths and droplet surface tensions and find that the onset of the instability and the number of arms depend on the ratio of surface tension to gel strength. However, the arm length grows with an apparently universal law L proportional t(3/4).}, number={12}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Daniels, Karen E. and Mukhopadhyay, Shomeek and Houseworth, Paul J. and Behringer, Robert P.}, year={2007}, month={Sep}, pages={123501} }
 @article{daniels_behringer_2006, title={Characterization of a freezing/melting transition in a vibrated and sheared granular medium}, volume={2006}, DOI={10.1088/1742-5468/2006/07/p07018}, abstractNote={We describe experiments on monodisperse spherical particles in an annular cell geometry, vibrated from below and sheared from above. This system shows a freezing/melting transition such that under sufficient vibration a crystallized state is observed, which can be melted by sufficient shear. We characterize the hysteretic transition between these two states, and observe features reminiscent of both a jamming transition and critical phenomena.}, number={07}, journal={Journal of Statistical Mechanics: Theory and Experiment}, publisher={IOP Publishing}, author={Daniels, Karen E and Behringer, Robert P}, year={2006}, month={Jul}, pages={P07018–P07018} }
 @inproceedings{daniels_2005, title={Freezing and Melting in Granular Materials}, DOI={10.1063/1.2128332}, abstractNote={From bowls of nuts to eroding soil, granular materials are all around us. In spite of the fact that granular materials are dissipative and athermal, statistical mechanics allows considerable insight into their behavior. I will present experiments on particles which are vibrated from below and sheared from above within an annular channel. The vibrations have the remarkable effect of crystallizing the material, rather than melting it as temperature would an ordinary material. This freezing/melting transition is hysteretic, with the critical line corresponding to equal kinetic energies for vibration and shear. We characterize the transition between these two states, and observe features reminiscent of both a jamming transition and critical phenomena. Another remarkable property is the increase of pressure with volume over a continuum of partially and/or intermittently melted states, in contrast to standard thermodynamic behavior.}, booktitle={AIP Conference Proceedings}, publisher={AIP}, author={Daniels, Karen E.}, year={2005} }
 @article{daniels_behringer_2005, title={Hysteresis and Competition between Disorder and Crystallization in Sheared and Vibrated Granular Flow}, volume={94}, DOI={10.1103/physrevlett.94.168001}, abstractNote={Experiments on spherical particles in a 3D annular shear cell vibrated from below and sheared from above show a hysteretic freezing or melting transition. Under sufficient vibration a crystallized state is observed, which can be melted by sufficient shear. The critical line for this transition coincides with equal kinetic energies for vibration and shear. The force distribution is double peaked in the crystalline state and single peaked with an approximately exponential tail in the disordered state. Continuous relations between pressure and volume (with dP/dV>0) exist for a continuum of partially and/or intermittently melted states over a range of parameters.}, number={16}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Daniels, Karen E. and Behringer, Robert P.}, year={2005}, month={Apr}, pages={168001} }
 @inproceedings{daniels_2005, title={Research Funding and Women in Physics}, ISSN={0094-243X}, url={http://dx.doi.org/10.1063/1.2128266}, DOI={10.1063/1.2128266}, abstractNote={A round table discussion on research funding and its relation to women in physics was held during the Second IUPAP International Conference on Women in Physics. Panelists were the director of the Office of Education, Science, and Technology of the Organization of American States; the director of Programs on Women, Science, and Technology for UNESCO; the Minister of Women for Brazil; and a professor of physics from the University of Yamanashi, Japan.}, booktitle={AIP Conference Proceedings}, publisher={AIP}, author={Daniels, Karen E.}, year={2005} }
 @article{daniels_mukhopadhyay_behringer_2005, title={Starbursts and wispy drops: Surfactants spreading on gels}, volume={15}, DOI={10.1063/1.2139968}, abstractNote={The spreading dynamics of surfactant-laden droplets have been studied for both solid [1] and thin liquid [2] substrates. In general, the presence of a surfactant modifies the contact line dynamics due to the Marangoni effect, which creates a surface tension gradient and thus generates pattern-forming instabilities. To determine the effects of varying substrate mobility from a liquid to a solid, we examine the intermediate case of a viscoelastic substrate. We observe novel instabilities of the surfactant-laden drop influenced by both the substrate fluidity and the surfactant concentration. The experimental apparatus consists of a petri dish containing a gel substrate composed of 0.04% to 0.16% agar (by weight) in deionized water. The droplets are solutions of Triton X-305 (a noninonic surfactant) in deionized water at concentrations from 5 to 1000 ppm, released from a micropipette with a droplet size of 5 mL. Figure 1 shows a phase diagram with shadowgraph images as a function of gel and surfactant concentration. For weak gels, the droplet spreads in a starburst formation, with 3-10 distinct arms (red images). For intermediate gels, the central drop remains but is decorated with thin, branching wisps (blue images). For sufficiently weak gels, the droplet spreads out as upon a liquid, and no central droplet remains (green images). On the strongest gels, the surfactant drops remain circular (not shown). For very low surfactant concentrations (i 5 ppm), the behavior resembles that of pure water droplets, and no arm structures are observed. This work has been supported under NSF Grant DMS-0244498.}, number={4}, journal={Chaos: An Interdisciplinary Journal of Nonlinear Science}, publisher={AIP Publishing}, author={Daniels, Karen E. and Mukhopadhyay, Shomeek and Behringer, Robert P.}, year={2005}, pages={041107} }
 @article{daniels_beck_bodenschatz_2004, title={Defect turbulence and generalized statistical mechanics}, volume={193}, ISSN={["1872-8022"]}, DOI={10.1016/j.physd.2004.01.033}, abstractNote={We present experimental evidence that the motion of point defects in thermal convection patterns in an inclined fluid layer is well-described by Tsallis statistics with an entropic index $q \approx 1.5$. The dynamical properties of the defects (anomalous diffusion, shape of velocity distributions, power law decay of correlations) are in good agreement with typical predictions of nonextensive models, over a range of driving parameters.}, number={1-4}, journal={PHYSICA D-NONLINEAR PHENOMENA}, publisher={Elsevier BV}, author={Daniels, KE and Beck, C and Bodenschatz, EB}, year={2004}, month={Jun}, pages={208–217} }
 @article{daniels_coppock_behringer_2004, title={Dynamics of meteor impacts}, volume={14}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000225378600003&KeyUID=WOS:000225378600003}, DOI={10.1063/1.1821711}, abstractNote={The morphology of meteor craters has historically b studied via static analysis, after the fact, of what are hi dynamic impact events. As such, there are long-stan questions about the process through which a meteor com rest and forms a crater. There has been a great deal of interest in this question, using both three-dimensional two-dimensional lab-scale analogs of meteor impacts to vide controlled experiments. 1 The experiments shown in th video directly visualize the forces generated in the im bed through the use of photoelastic particles. The experimental apparatus is shown in Fig. 1. Photo tic particles of diameter 0.74 cm(800 particles ) and 0.90 cm (125 particles ) and thickness 0.64 cm were loosely sa wiched between sheets of transparent Plexiglas in a re 33 cm wide. The bidisperse mixture is used to suppress tallization. We prepared the bed before each run by tip the apparatus and allowing the particles to rain back d When visualized through crossed circular polarizers, co forces within the photoelastic particles appear as b streaks, where larger intensity gradients correspond to l forces. We calibrate the system using known forces to d}, number={4}, journal={Chaos: An Interdisciplinary Journal of Nonlinear Science}, publisher={AIP Publishing}, author={Daniels, K. E. and Coppock, J. E. and Behringer, R. P.}, year={2004}, pages={S4} }
 @article{huepe_riecke_daniels_bodenschatz_2004, title={Statistics of defect trajectories in spatio-temporal chaos in inclined layer convection and the complex Ginzburg–Landau equation}, volume={14}, ISSN={1054-1500 1089-7682}, url={http://dx.doi.org/10.1063/1.1778495}, DOI={10.1063/1.1778495}, abstractNote={For spatio-temporal chaos observed in numerical simulations of the complex Ginzburg–Landau equation (CGL) and in experiments on inclined-layer convection (ILC) we report numerical and experimental data on the statistics of defects and of defect loops. These loops consist of defect trajectories in space–time that are connected to each other through the pairwise annihilation or creation of the associated defects. While most such loops are small and contain only a few defects, the loop distribution functions decay only slowly with the quantities associated with the loop size, consistent with power-law behavior. For the CGL, two of the three power-law exponents are found to agree, within our computational precision, with those from previous investigations of a simple lattice model. In certain parameter regimes of the CGL and ILC, our results for the single-defect statistics show significant deviations from the previously reported findings that the defect dynamics are consistent with those of random walkers that are created with fixed probability and annihilated through random collisions.}, number={3}, journal={Chaos: An Interdisciplinary Journal of Nonlinear Science}, publisher={AIP Publishing}, author={Huepe, Cristián and Riecke, Hermann and Daniels, Karen E. and Bodenschatz, Eberhard}, year={2004}, month={Sep}, pages={864–874} }
 @article{daniels_wiener_bodenschatz_2003, title={Localized Transverse Bursts in Inclined Layer Convection}, volume={91}, DOI={10.1103/physrevlett.91.114501}, abstractNote={We investigate a novel bursting state in inclined layer thermal convection in which convection rolls exhibit intermittent, localized, transverse bursts. With increasing temperature difference, the bursts increase in duration and number while exhibiting a characteristic wave number, magnitude, and size. We propose a mechanism which describes the duration of the observed bursting intervals and compare our results to bursting processes in other systems.}, number={11}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Daniels, Karen E. and Wiener, Richard J. and Bodenschatz, Eberhard}, year={2003}, month={Sep}, pages={114501} }
 @article{daniels_bodenschatz_2003, title={Statistics of defect motion in spatiotemporal chaos in inclined layer convection}, volume={13}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000181202400004&KeyUID=WOS:000181202400004}, DOI={10.1063/1.1536330}, abstractNote={We report experiments on defect-tracking in the state of undulation chaos observed in thermal convection of an inclined fluid layer. We characterize the ensemble of defect trajectories according to their velocities, relative positions, diffusion, and gain and loss rates. In particular, the defects exhibit incidents of rapid transverse motion which result in power law distributions for a number of quantitative measures. We examine connections between this behavior and Lévy flights and anomalous diffusion. In addition, we describe time-reversal and system size invariance for defect creation and annihilation rates.}, number={1}, journal={Chaos: An Interdisciplinary Journal of Nonlinear Science}, author={Daniels, K. E. and Bodenschatz, E.}, year={2003}, pages={55} }
 @article{daniels_bodenschatz_2002, title={Defect Turbulence in Inclined Layer Convection}, volume={88}, DOI={10.1103/physrevlett.88.034501}, abstractNote={We report experimental results on the defect turbulent state of undulation chaos in inclined layer convection of a fluid with Prandtl number approximately 1. By measuring defect density and undulation wave number, we find that the onset of undulation chaos coincides with the theoretically predicted onset for stable, stationary undulations. At stronger driving, we observe a competition between ordered undulations and undulation chaos, suggesting bistability between a fixed-point attractor and spatiotemporal chaos. In the defect turbulent regime, we measured the defect creation, annihilation, entering, leaving, and rates. We derive a universal probability distribution function which agrees with the experimental findings.}, number={3}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Daniels, Karen E. and Bodenschatz, Eberhard}, year={2002}, month={Jan}, pages={034501} }
 @article{daniels_plapp_bodenschatz_2000, title={Pattern formation in inclined layer convection}, volume={84}, ISSN={["0031-9007"]}, DOI={10.1103/PhysRevLett.84.5320}, abstractNote={We report experiments on thermally driven convection in an inclined layer of large aspect ratio in a fluid of Prandtl number sigma approximately 1. We observed a number of new nonlinear, mostly spatiotemporally chaotic, states. At small angles of inclination we found longitudinal rolls, subharmonic oscillations, Busse oscillations, undulation chaos, and crawling rolls. At larger angles, in the vicinity of the transition from buoyancy- to shear-driven instability, we observed drifting transverse rolls, localized bursts, and drifting bimodals. For angles past vertical, when heated from above, we found drifting transverse rolls and switching diamond panes.}, number={23}, journal={PHYSICAL REVIEW LETTERS}, publisher={American Physical Society (APS)}, author={Daniels, KE and Plapp, BB and Bodenschatz, E}, year={2000}, month={Jun}, pages={5320–5323} }