@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{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{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{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} }