Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session E29: Jamming of Frictional and Non-spherical ParticlesInvited
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Sponsoring Units: GSNP Chair: Corey O'Hern, Yale University Room: 292 |
Tuesday, March 14, 2017 8:00AM - 8:36AM |
E29.00001: Shear jamming: where does it come from and how is it affected by particle properties? Invited Speaker: Dong Wang Granular systems have been shown to be able to behave like solids, under shear, even when their densities are below the critical packing fraction for frictionless isotropic jamming. To understand such a phenomena, called shear jamming, the questions we address here is: how does shear bring a system from a unjammed state to a jammed state and how do particle properties, such as inter-particle friction and particle shape, affect shear jamming? Since $Z$ can be used to distinguish jammed states from unjammed ones ($Z=3$ is the isotropic jamming point for $2D$ frictional disks), it is vital to understand how shear increases $Z$. In the first part of this talk, we propose a set of three particles in contact, denoted as a trimer, as the basic unit to microscopically characterize the deformation of the system. Trimers, stabilized by inter-grain friction, are then expected to bend in response to shear to make extra contacts to regain stability. By defining a projection operator of the opening angle of the trimer to the compression direction in the shear, $O$, we see a systematically linear decrease of this quantity with respect to shear strain, demonstrating the bending of trimers as expected. In the second part of this talk, we look into the effect of particle properties on shear jamming. Photoelastic disks either wrapped with Teflon to reduce friction or with fine teeth on the edge to increase friction are used to study the effect of friction. In addition, disks are replaced with ellipses to introduce anisotropy into the particle shape. Shear jamming is observed for all the cases. For the disk system, the lowest packing fraction that can reach a shear jammed state increases with friction. For the ellipse system, shear brings the system to a more ordered state and particles tend to align to a certain angle relative to the principal directions of shear, regardless of packing fraction. [Preview Abstract] |
Tuesday, March 14, 2017 8:36AM - 9:12AM |
E29.00002: Simulations of Shear Jamming in Packings of Frictionless and Frictional Particles Invited Speaker: Thibault Bertrand We recently proposed a theoretical framework for predicting the protocol dependence of the jamming transition for frictionless spherical particles that interact via repulsive contact forces. We studied isostatic jammed disk packings obtained via two protocols: isotropic compression and simple shear. We showed that for frictionless systems, all jammed packings can be obtained via either protocol. We predicted the average shear strain required to jam initially unjammed isotropically compressed packings from the density of jammed packings, shape of their basins of attraction, and path traversed in configuration space. We compared our predictions to simulations of shear strain-induced jamming and found quantitative agreement. Finally, we showed that the packing fraction range, over which shear strain-induced jamming occurs, tends to zero in the large system limit for frictionless packings with overdamped dynamics. Here, we extend this theoretical framework to packings of frictional disks using two models for friction: the Cundall-Strack and geometric asperity models. We measure the applied shear strain required to jam originally unjammed packings as a function of the static friction coefficient and system size. In addition, we compare the stress and fabric anisotropies of packings obtained from the isotropic compression and shear protocols to identify macroscale properties that distinguish the packings. [Preview Abstract] |
Tuesday, March 14, 2017 9:12AM - 9:48AM |
E29.00003: Stress Transmission in Granular Packings: Localization and Cooperative Response Invited Speaker: Kabir Ramola We develop a framework for stress transmission in two dimensional granular media that respects vector force balance at the microscopic level. For a packing of grains interacting via pairwise contact forces, we introduce local gauge degrees of freedom that determine the response of the system to external perturbations. This allows us to construct unique force-balanced solutions that determine the change in contact forces as a response to external stress. By mapping this response to diffusion in the underlying contact network, we show that this naturally leads to spatial localization of forces. We present numerical evidence for stress localization using exact diagonalization studies of network Laplacians associated with soft disk packings. We use this formalism to characterize the deviation from elastic behaviour as the amount of disorder in the underlying network is varied. We discuss generalizations to systems with large friction between grains and other networks that display topological disorder. [Preview Abstract] |
Tuesday, March 14, 2017 9:48AM - 10:24AM |
E29.00004: Precisely cyclic sand: self-organization of periodically sheared frictional grains Invited Speaker: John Royer Using molecular dynamics (MD) simulations, we show that cyclic shear of a granular material leads to dynamic self-organization into several phases with different spatial and temporal order. We present a phase diagram in strain $-$ friction space which shows chaotic dispersion, crystal formation, vortex patterns and most unusually a disordered phase in which each particle precisely retraces its unique path. However the system is not reversible. Rather the trajectory of each particle, and the entire frictional, many-degree-of-freedom system, organizes itself into a limit cycle absorbing state. Surprisingly, the cyclic states remain spatially disordered while the ordered states are chaotic. [Preview Abstract] |
Tuesday, March 14, 2017 10:24AM - 11:00AM |
E29.00005: Jammed packings of deformable and rigid 2D spherocylinders and spheropolygons Invited Speaker: Mark Shattuck We study mechanically stable packings of deformable and rigid 2D spheropolygons using computer simulation. A 2D sphereopolygon is a particle shape formed by the collection of all points within a perpendicular distance $r$ from the edge of a polygon. It is a generalization of the 2D spherocylinder and a circle, which are the collection of all points within a distance $r$ from a line and a point. In our model, the spheropolygon can be deformable. The lengths of the sides are fixed, but the angles are only constrained by the requirement that the shape factor, $S=$4$\pi A$/$p^{\mathrm{2}}$ is fixed, where $A$ is the area of the polygon and $p$ is the perimeter. The particles can be made rigid by requiring that the shape factor is the maximum possible for the edge length ratios. For example, the maximum for a square is $S=\pi $/4. We present densities and average contact numbers for collections of mono- and bi-disperse packings of spheropolygons for a range of shape factors, edge numbers, and system sizes. We find mechically stable packings with fewer than isostatic contacts. [Preview Abstract] |
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