Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session A14: Jamming of Particulate Matter IFocus

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Sponsoring Units: GSNP GSOFT Chair: Robert Behringer, Duke University Room: 273 
Monday, March 13, 2017 8:00AM  8:36AM 
A14.00001: Standing on the shores of jamming: Structure and local rigidity in packings below the jamming transition. Invited Speaker: Eric Corwin The glass transition and the athermal jamming transition are both transitions from one disordered state to another marked by a sudden increase in rigidity. Before the onset of rigidity, thermal hard spheres and athermal soft spheres both share the same configuration space. Is there a signature of the glass transition in the topology of the allowed configuration space, and is this same signature present for athermal spheres? I will answer these questions by employing the concept of local rigidity, and in doing so, I will demonstrate the existence of a prejamming phase transition coinciding with the thermal mode coupling glass transition density. In this way I hope to show that the growth and percolation of local rigidity in an athermal system is connected to the appearance of longlived regions of correlated motion in thermal systems. [Preview Abstract] 
Monday, March 13, 2017 8:36AM  8:48AM 
A14.00002: Surface Growth of Locally Rigid Clusters Approaching Jamming and Rigidity Percolation Michael van der Naald, Eric Corwin The jamming transition is defined by rigidity: at jamming every nonrattler particle belongs to a single, system spanning, rigid cluster. ~At densities below jamming no particle is rigid but, dependent on the packing protocol, particles can still accumulate into clusters. ~To investigate these clusters we define the notion of a locally rigid cluster. ~A cluster of particles is called locally rigid if it becomes rigid when all of the clusters neighbors are held fixed. By simulating two dimensional athermal soft discs at various packing fractions we can measure the size and surface properties of these locally rigid clusters. ~We draw the analogy to surface growth processes by considering packing fraction as a timelike quantity. ~We demonstrate different growth regimes as a function of packing fraction and comment on their relation to established surface growth models. [Preview Abstract] 
Monday, March 13, 2017 8:48AM  9:00AM 
A14.00003: Experimental Measurements of the Density of States for a Granular Crystal Thibault Bertrand, Corey S. O'Hern, Mark D. Shattuck The vibrational density of states relates the structure and dynamics of materials. Several previous experimental studies have measured the density of states in colloidal glasses and identified an abundance of low energy collective excitations that give rise to anomalous behavior for the shear modulus. However, very few measurements of the density of states have been performed for driven granular materials. We report experimental investigations of the vibrational density of states of a quasi2D uniformly heated granular crystal below the jamming density. We find that over the range of packing fractions we consider particles do not escape from their cages over the timescale of the measurements. We measure the density of states in two ways: from the power spectrum of the velocity autocorrelation function and the eigenvalues of the covariance matrix of particle displacements. We compare the experimental measurements of the density of states to results obtained from similar measurements on 2D and quasi2D hard sphere simulations. [Preview Abstract] 
Monday, March 13, 2017 9:00AM  9:12AM 
A14.00004: Using particle rearrangement statistics to quantify ductility in amorphous solids Meng Fan, Minglei Wang, Yanhui Liu, Jan Schroers, Mark Shattuck, Corey O'Hern The response of amorphous solids to applied shear has several distinct regimes: quasielastic, yielding, and plastic flow regimes in the absence of fracture. Both nonaffine particle motion and particle rearrangement events give rise to the strong nonlinear behavior of the stress versus strain curve. Here, we focus on computational studies of the mechanical behavior of binary LennardJones glasses in three spatial dimensions that are prepared over a wide range of cooling rates. We apply athermal quasistatic pure shear to the glasses and uniquely identify each particle rearrangement event. We then determine the frequency of rearrangements and the energy drop after each event. We also quantify ductility by measuring the critical strain at which the material fractures during tensile tests. We find that more rapidly cooled glasses undergo more frequent particle rearrangements with larger energy drops on average. In contrast, rearrangements are much less frequent and dissipate less energy in more slowly cooled glasses, and thus are more susceptible to fracture than rapidly cooled glasses. In fact, we can predict the ductility of amorphous solids by measuring the total energy loss per strain in the putative linear stress versus strain regime before fracture occurs. [Preview Abstract] 
Monday, March 13, 2017 9:12AM  9:24AM 
A14.00005: The Sounds of Failure Ted Brzinski, Karen Daniels Disordered solids including numerical packings of spheres, colloidal glasses and granular materials have all been shown to develop excess lowfrequency vibrational modes as the jamming transition is approached from above. We report experiments on sheared granular materials in which we measure the density of excited modes in a granular system under shear via passive monitoring of acoustic emissions. We show that this quantity provides information about the changing state of the material on its approach to stickslip failure, and may hold promise as the basis for forecasting the risk of a rupture. [Preview Abstract] 
Monday, March 13, 2017 9:24AM  9:36AM 
A14.00006: Softening and Irreversibility in Jammed Solids Julia Boschan, Brian Tighe Materials like foams and emulsions display complex rhelogical behavior close to their jamming transition. When driven too hard the initial linear stressstrain response breaks down and the material softens. Using simulations of soft repulsive spheres, we characterize the softening crossover by establishing the relevant strain scale below which linear response is valid. We further perform shear reversal tests to investigate the interplay between proximity to jamming and the onset of irreversibility. [Preview Abstract] 
Monday, March 13, 2017 9:36AM  9:48AM 
A14.00007: Finite size effects near jamming at extreme aspect ratios Dion Koeze, Brian Tighe Many soft matter systems are confined in some but not all dimensions; examples include microfluidic channels and inclined plane flows. Hence it is important to characterize finite size effects not only as a function of volume, but also for varying aspect ratio. For soft sphere packings close to the jamming transition, finite size effects are well understood, but only in square and cubic systems. In these cases there is clear evidence for a critical volume that diverges at jamming, but it is equally clear that this picture must break down for extreme aspect ratios. We perform simulations of soft spheres near jamming in two and three dimensions for aspect ratios as large as 1024. In addition to the previously identified critical volume, we find evidence for a nontrivial length scale that diverges at the jamming point. [Preview Abstract] 
Monday, March 13, 2017 9:48AM  10:00AM 
A14.00008: Jamming in attractive soft spheres Brian Tighe, Dion Koeze, Lingtjien Hong, Abhishek Kumar While jamming is best understood in the context of purely repulsive soft spheres, emulsions and other experimental realizations of the soft sphere model commonly display weakly cohesive forces. We perform simulations of soft spheres with a finiteranged attractive tail in the pair potential. The resulting attractive soft sphere packings can be stable at volume fractions below the purely repulsive jamming point. These new jammed states have counterintuitive properties  for example, while attraction introduces tensile forces, their presence leads to an increase in the compressive stress. We use critical scaling analysis to characterize the geometry and mechanics of attractive soft sphere packings as a function of both the volume fraction and the range of the attractive interaction. [Preview Abstract] 
Monday, March 13, 2017 10:00AM  10:12AM 
A14.00009: Jamming transition of twodimensional monodisperse soft particles Wen Zheng, Ning Xu By finding local minima of a thermodynamiclike potential, we genarate jammed packing of monodisperse frictionless circular disks under constant pressure in two dimensions. Contrast to the bidisperse systems, the packing of monodisperse systems display a wide distribution of packing fractions even the pressure close to zero, so that essentially all configurations are the coexistence of amorphous solid and triangular lattice in the thermodynamic limit. Moreover, both the local packing fractions and density of vibrational states (DOVS) demonstrate that jamming physics dominated the behaviour of amorphous solid constituent in two dimensional monodisperse system, and the local packing fraction of jamming transition corresponds to the previously meaured value for random close packing $\varphi =$0.84. In fact, our results provide a welldifined meaning for "random close packing" in two dimensional monodisperse systems. We also obtain numerical estimates of the random close packing density, which provides new insights into the mathematical problem of packing spheres in large dimensions. [Preview Abstract] 
Monday, March 13, 2017 10:12AM  10:24AM 
A14.00010: Scaling behavior of the yield stress near unjamming Sean Ridout, Max Lavrentovich, Andrea Liu We study the yield strain in polydisperse, athermal packings of frictionless soft discs as the unjamming transition is approached. A recently introduced scaling theory of the jamming transition predicts that the yield stress should scale with the excess coordination number with a power $s_y \sim \Delta z^{5/2}$, and therefore with the pressure like $s_y \sim p^{5/4}$ . Instead, we find $s_y \sim \Delta z^2$. We explain how this result can be reconciled with the scaling theory. We compare to the response of the system under oscillatory shear and the strain amplitude corresponding to the associated reversible to irreversible transition. The shear amplitude at the transition does not appear to scale strongly with the pressure, suggesting that yield and the reversibleirreversible transition are not related in an obvious way. [Preview Abstract] 
Monday, March 13, 2017 10:24AM  10:36AM 
A14.00011: The Granular Pebble Game Mahesh Bandi, Sathish Akella {\it The Pebble Game} represents a class of graph theoretic problems where pebbles (representing vertices) are constrained by bonds or physical contacts (edges). The pebble game also shares close correspondence with rigidity percolation transition in a variety of materials problems, where the evolution of rigidity in solids can be mapped to growth of network rigidity in a graph. Indeed, the class of materials known as topological or network glasses arose precisely from such an analysis in the context of chalcogenides and covalent glasses. Here we report a set of ultrahigh precision experiments performed on a bidispersed set of photoelastic disks subjected to uniaxial compression. We analyse the formation of an amorphous granular solid from a loose granular pack under compression, as an exercise in the growth of rigidity in the granular contact network. We present preliminary results of our analysis to understand rigidity percolation in granular packs and their connection to the granular jamming transition. [Preview Abstract] 
Monday, March 13, 2017 10:36AM  10:48AM 
A14.00012: The central role of the effective nonlocal spring constant in disordered networks Daniel Hexner, Andrea J. Liu, Sidney R. Nagel Deforming an amorphous solid leads to an inhomogeneous stress response. To understand this behavior, we show that each bond has an inherent aptitude to carry stress, which we call its effective nonlocal spring constant. This quantity enables us to understand the response to the removal of a single bond. Here we focus on the resulting change of the bulk and shear moduli in several different ensembles of disordered networks. We find that the change in shear modulus with removal of a single bond has a universal distribution, and that the change in the bulk modulus asymptotically approaches the same distribution. Additionally, in jammed networks the change in shear and bulk modulus due to removal of a bond each have correlations that become longranged at the jamming transition. For any given bond, however, the change in the bulk and shear moduli due to its removal are virtually uncorrelated. [Preview Abstract] 
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