Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session W44: Focus Session: Jamming in Granular Media I |
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Sponsoring Units: GSOFT GSNP Chair: Corey O'Hern, Yale University Room: 214D |
Thursday, March 5, 2015 2:30PM - 2:42PM |
W44.00001: Cyclically sheared foams Merlijn van Deen, Vera Janssen, Alexander Siemens, Martin van Hecke We experimentally apply a cyclic pure shear deformation to a two-dimensional foam at different densities. We probe the onset of irreversibility and relate this to local rearrangements in the system and show the behavior close to jamming is different from that in dense systems. [Preview Abstract] |
Thursday, March 5, 2015 2:42PM - 2:54PM |
W44.00002: Imaging Force Anisotropy in 3D Sheared Granular Media Joshua Dijksman, Marcel Workamp We experimentally study force distributions in three-dimensional driven granular suspensions. By suspending slightly polydisperse, soft photoelastic gel spheres in an index-matching fluid, we obtain a direct method to assess spatial stress fields in the bulk. Using a rheometer equipped with a camera and simple optics, we simultaneously probe the rheology and force structures in Couette geometry. Our experimental system gives insight in the anisotropy of force structures inside flowing suspensions, one of the mechanisms behind the emergent shear-induced rigidity in these materials. [Preview Abstract] |
Thursday, March 5, 2015 2:54PM - 3:06PM |
W44.00003: Scaling of Force Networks for Compressed Particulate Systems Lenka Kovalcinova, Arnaud Goullet, Lou Kondic We consider the distribution of cluster sizes in compressed particulate systems as a function of the force experienced by the particles. The considered systems differ by the distribution of particle sizes and by their frictional properties. To obtain good statistics we consider various systems sizes and large number of realizations. While for some of the considered systems we find consistent scaling exponents describing the behavior of the force clusters, we are also finding that this behavior is {\it not universal}. For example, monodisperse frictionless systems that crystallize under compression, show very different scaling properties compared to other systems, particularly as the systems approach jamming transition. The findings are confirmed by explicitly computing fractal dimension of the considered clusters. [Preview Abstract] |
Thursday, March 5, 2015 3:06PM - 3:18PM |
W44.00004: Steady State Shear Driven Flow of Frictionless Spherocylindrical~Particles in Two Dimensions Daniel Van Hoesen, Theodore Marschall, Scott Franklin, Stephen Teitel We carry out~simulations of a model of frictionless spherocylindrical rods in two dimensions,~under~uniform steady state shear driven flow.~~Rods repel elastically when they come into contact, and~dissipate~energy with respect to a uniformly sheared host medium. We pay~particular attention to the tumbling~motion of the rods that is induced by the~shear flow, computing the average angular velocity, and the~nematic and~tetratic orientational order parameters.~~We find non-monotonic behavior of the orientational~order parameters as~the packing fraction increases toward the jamming transition.~~Orientational and~translational~correlation functions are computed to measure cooperative behavior as the~packing fraction~increases. [Preview Abstract] |
Thursday, March 5, 2015 3:18PM - 3:30PM |
W44.00005: Fraction of clogging configurations in granular hopper flow Charles Thomas, Douglas Durian The clogging of granular media flowing from a hopper is a quintessential example of a system that spontaneously evolves from a freely flowing state to a jammed state under constant forcing. If suitably arranged, the grains at the opening are stable, initiate a jamming front, and block the flow throughout the bulk. We measure the fraction F of possible grain arrangements that lead to such a system-spanning clog for a range of experimental conditions, varying aperture shape, size, and orientation. We find for circular holes that F is a function only of the aperture size projected in the direction of the average exiting grain velocity. Furthermore, for long narrow slits F is found to be identical to the value expected for a set of independent holes. Finally, we successfully model the form of F versus aperture size by considering the accessible microstates of individual grains near the exit. The data as interpreted through this model suggest that the fraction of individual grain microstates that can lead to a clog is constant for large opening sizes. This conclusion implies that there may be no well-defined critical aperture size above which clogging is impossible. [Preview Abstract] |
Thursday, March 5, 2015 3:30PM - 3:42PM |
W44.00006: Sub-Jamming Transition in Jammed Binary Sphere Mixtures Ishan Prasad, Christian Santangelo, Gregory Grason We study influence of bi-dispersity on structural evolution of jammed binary sphere mixtures with increasing small-sphere composition, $f_s$. In binary spheres, maximally dense, random packing is achieved at infinite size ratio and unique composition ($f_s=0.2659$) where small spheres jam within interstitial volume of jammed large spheres, leading to a kink in total volume fraction, $\phi$, vs. $f_s$. Using simulations of athermally jammed packings, we explore how this critical feature influences the evolution of random binary sphere packings at finite size ratio, $\alpha$, ranging from 1 to 10. We report a clear distinction between large and small $\alpha$ behavior, separated by a critical value of $\alpha_c = 5.8$. For $\alpha < \alpha_c$ structural properties --such as total volume fraction, rattler fraction and contact statistics-- are found to crossover smoothly from small to large $f_s$, while above a critical size asymmetry these properties indicate an abrupt, first-order like transition. We correlate this sharp transition with a ``sub-jamming'' transition of small-spheres occurring at finite values of $f_s$, which becomes cooperative only for sufficiently asymmetric mixtures. We propose a heuristic geometric and mechanical argument to understand what determines $\alpha_c$. [Preview Abstract] |
Thursday, March 5, 2015 3:42PM - 4:18PM |
W44.00007: A Unified Framework to Understand Shear Induced Rigidity in Athermal Materials Invited Speaker: Sumantra Sarkar Recent studies of athermal systems such as dry grains and dense, non-Brownian suspensions have shown that shear can lead to solidification through the process of shear jamming in grains and discontinuous shear thickening in suspensions. The similarities observed between these two distinct phenomena suggest that the physical processes leading to shear-induced rigidity in athermal materials are universal. We present a unified, non-equilibrium statistical mechanics model for these shear-driven transitions, which exhibits the phenomenology of shear jamming and discontinuous shear thickening in different regions of the predicted phase diagram. Our analysis identifies the crucial physical processes underlying shear-driven rigidity transitions, and clarifies the distinct roles played by shearing forces and the density of grains. [Preview Abstract] |
Thursday, March 5, 2015 4:18PM - 4:30PM |
W44.00008: Effect of Friction on Shear Jamming Dong Wang, Jie Ren, Joshua Dijksman, Jonathan Bares, Robert Behringer Shear jamming of granular materials was first found for systems of frictional disks, with a static friction coefficient $\mu \approx 0.6$ (Bi et al. Nature (2011)). Jamming by shear is obtained by starting from a zero-stress state with a packing fraction $\phi$ between $\phi_J$ (isotropic jamming) and a lowest $\phi_S$ for shear jamming. This phenomenon is associated with strong anisotropy in stress and the contact network in the form of force chains, which are stabilized and/or enhanced by the presence of friction. Whether shear jamming occurs for frictionless particles is under debate. The issue we address experimentally is how reducing friction affects shear jamming. We put the Teflon-wrapped photoelastic disks, lowering the friction substantially from previous experiments, in a well-studied 2D shear apparatus (Ren et al. PRL (2013)), which provides a uniform simple shear. Shear jamming is still observed; however, the difference $\phi_J-\phi_S$ is smaller with lower friction. We also observe larger anisotropies in fragile states compared to experiments with higher friction particles at the same density. In ongoing work we are studying systems using photoelastic disks with fine gears on the edge to generate very large effective friction. [Preview Abstract] |
Thursday, March 5, 2015 4:30PM - 4:42PM |
W44.00009: Clogging and Jamming Transitions in Granular Matter Flowing Through Obstacles Cynthia Olson Reichhardt, Charles Reichhardt We consider a two-dimensional system of bidisperse disks driven through a landscape of fixed obstacles. In the limit of a single obstacle, the disks cease moving when the disk density is increased to the jamming density. The threshold density value decreases as the number of obstacles increases, but we also observe a change in the nature of the frozen state. At low obstacle density we find a homogeneous jammed state, but for higher obstacle density we instead find a heterogeneous clogged state containing void areas and possessing a memory of the driving direction. The transition to the clogged state is strongly stochastic and we observe large fluctuations in clogging time both for clogging in the original driving direction and for transverse clogging when the drive is suddenly rotated by 90 degrees. We find evidence for a diverging clogging transition time at a critical disk density well below the jamming density in a clean system. [Preview Abstract] |
Thursday, March 5, 2015 4:42PM - 4:54PM |
W44.00010: Diffusion in jammed particle packs Dan S. Bolintineanu, Leonardo E. Silbert, Gary S. Grest, Jeremy B. Lechman Diffusive transport in jammed particle packs is of interest for a number of applications, as well as being a potential indicator of structural properties near the jamming point. To this end, we report stochastic simulations of equilibrium diffusion through monodisperse sphere packs near the jamming point in the limit of a perfectly insulating surrounding medium. The time dependence of various diffusion properties is resolved over several orders of magnitude. Two time regimes of expected Fickian diffusion are observed, separated by an intermediate regime of anomalous diffusion. This intermediate regime grows as the particle volume fraction approaches the critical jamming transition. The diffusion behavior is fully controlled by the extent of the contacts between neighboring particles, which in turn depend on proximity to the jamming point. In particular, the mean first passage time associated with the escape of random walkers between neighboring particles is shown to control both the time to recover Fickian diffusion and the long time diffusivity. Scaling laws are established that relate these quantities to the difference between the actual and critical jamming volume fractions. [Preview Abstract] |
Thursday, March 5, 2015 4:54PM - 5:06PM |
W44.00011: Statistics and Correlations of Conserved Quantities in Mechanically Stable Packings of Frictionless Disks Above Jamming Stephen Teitel, Yegang Wu We consider mechanically stable packings of soft-core, frictionless, bidisperse disks in two dimensions above the jamming transition. Using an algorithm that generates packings with an isotropic global stress tensor, we compute the distribution of various conserved quantities on compact subclusters of particles, as a function of the total system stress and the cluster size. We consider the stress on the cluster, the Maxwell-Cremona force-tile area, the Voronoi volume, and the numbers of small and big particles in the cluster, and we compute the averages, variances and correlations among these different quantities. We compare two different ensembles of clusters: (i) clusters defined by a fixed radius, and (ii) clusters defined by a fixed number of particles. We find several significant differences between these two ensembles and we comment on the implications of our findings for maximum entropy models of jammed packings. [Preview Abstract] |
Thursday, March 5, 2015 5:06PM - 5:18PM |
W44.00012: Shear Jamming in Frictionless Particulate Media Thibault Bertrand, Corey S. O'Hern, R.P. Behringer, Bulbul Chakraborty, Mark D. Shattuck We numerically study two-dimensional packings of frictionless bidisperse disks created using compresive and simple shearing protocols. To create jammed packings by compression, we start $N$ particles from random positions and grow their diameters followed by relaxation of particle overlaps using energy minimization. These compressed packings exist over a range of packing fractions $\phi$. As a result, during compression the system may reach a $\phi$ above the minimum value before jamming. If this unjammed packing is then sheared by a strain $\gamma$, it can jam. Using a combination of compression and shearing, we can define jamming protocols as trajectories in the $(\phi, \gamma)$ plane that yield jammed packings. In this plane, we can reach a particular point $(\phi_n, \gamma_n)$ in many ways. We will focus on two protocols: (1) shearing to $\gamma_n$ at $\phi=0$ followed by compression to $\phi_n$ at $\gamma= gamma_n$ and (2) compression to $\phi_n$ at $\gamma=0$ followed by shearing to $\gamma_n$ at $\phi=\phi_n$. For protocol 1, we find that the probability of finding a jammed packing at $\phi$ and $\gamma$, $P(\phi,\gamma)=Q(\phi)$ is indepependent of $\gamma$. For protocol 2, we use a simple theory to deduce $P(\phi,\gamma)$ from $Q(\phi)$. [Preview Abstract] |
Thursday, March 5, 2015 5:18PM - 5:30PM |
W44.00013: Jammed elastic shells - a 3D experimental soft frictionless granular system Jissy Jose, Gerhard A. Blab, Alfons van Blaaderen, Arnout Imhof We present a new experimental system of monodisperse, soft, frictionless, fluorescent labelled elastic shells for the characterization of structure, universal scaling laws and force networks in 3D jammed matter. The interesting fact about these elastic shells is that they can reversibly deform and therefore serve as sensors of local stress in jammed matter. Similar to other soft particles, like emulsion droplets and bubbles in foam, the shells can be packed to volume fractions close to unity, which allows us to characterize the contact force distribution and universal scaling laws as a function of volume fraction, and to compare them with theoretical predictions and numerical simulations. However, our shells, unlike other soft particles, deform rather differently at large stresses. They deform \textit{without} conserving their inner volume, by forming dimples at contact regions. At each contact one of the shells buckled with a dimple and the other remained spherical, closely resembling overlapping spheres. We conducted 3D quantitative analysis using confocal microscopy and image analysis routines specially developed for these particles. In addition, we analysed the randomness of the process of dimpling, which was found to be volume fraction dependent. [Preview Abstract] |
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