@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{kollmer_shreve_claussen_gerth_salamon_uhlmann_schroeter_poeschel_2020, title={Migrating Shear Bands in Shaken Granular Matter}, volume={125}, ISSN={["1079-7114"]}, DOI={10.1103/PhysRevLett.125.048001}, abstractNote={When dense granular matter is sheared, the strain is often localized in shear bands. After some initial transient these shear bands become stationary. Here, we introduce a setup that periodically creates horizontally aligned shear bands which then migrate upward through the sample. Using x-ray radiography we demonstrate that this effect is caused by dilatancy, the reduction in volume fraction occurring in sheared dense granular media. Further on, we argue that these migrating shear bands are responsible for the previously reported periodic inflating and collapsing of the material.}, number={4}, journal={PHYSICAL REVIEW LETTERS}, author={Kollmer, Jonathan E. and Shreve, Tara and Claussen, Joelle and Gerth, Stefan and Salamon, Michael and Uhlmann, Norman and Schroeter, Matthias and Poeschel, Thorsten}, year={2020}, month={Jul} }
 @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{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} }