Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session S48: Granular Materials and Flows 
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Sponsoring Units: GSNP Chair: Robert Behringer, Duke Univ Room: LACC 510 
Thursday, March 8, 2018 11:15AM  11:27AM 
S48.00001: Critical Scaling near Yield in Granular Materials Abe Clark, Mark Shattuck, Nicholas Ouellette, Corey O'Hern We show that the yielding transition in granular media displays secondorder criticalpoint scaling behavior, independent from jamming. We use discrete element simulations in the overdamped limit for frictionless spherical grains undergoing simple shear at fixed Σ, which is the ratio of the applied shear stress to normal stress. To find a mechanically stable (MS) packing that can support the applied Σ, isotropically prepared states with size L must travel over a strain γ_{ms}(Σ,L). The density of MS packings, which is inversely proportional to γ_{ms}(Σ,L), vanishes for Σ = Σ_{c} ≈ 0.11 according to a critical scaling form with a length scale ξ ~ Σ  Σ_{c}^{ν}, where ν ≈ 1.8. For Σ > Σ_{c}, no MS packings that can support Σ exist as L/ξ becomes large. For MS packings obtained by shear, the packing fraction is independent of Σ, but the anisotropy in force and contact networks increases with Σ. This suggests that Σ_{c} is associated with an upper limit in how far these networks can be deformed away from an isotropic configuration. 
Thursday, March 8, 2018 11:27AM  11:39AM 
S48.00002: Pure Shear of 2d Granular Mixtures: Simulation Elisabeth Rennert, Daniel Gysbers, Scott Franklin

Thursday, March 8, 2018 11:39AM  11:51AM 
S48.00003: Intruder in a granular packing. Scott Newlon, Niranjan Warnakulasooriya, Hideyuki Mizuno, Leo Silbert The dynamics and kinematics of a single intruder particle in two dimensional, disordered packings of frictionless bidisperse discs interacting through linear and nonlinear contact forces are investigated using computer simulations. To initiate motion of the intruder requires a threshold force to be exceeded which depends on the pressure of the packing. Beyond this critical force, for a given driving force, the average intruder velocity scales with the excess driving force. The velocity curves for different pressures are found to obey an universal scaling relation that depends on the initial packing pressure. We also find that the critical, threshold force is related to the shear modulus, thereby connecting the microrheological properties of the intruder at the grain scale to the bulk, macroscopic mechanical properties of the packing. 
Thursday, March 8, 2018 11:51AM  12:03PM 
S48.00004: Simulation of a Granular Flow Experiment Using GPU Parallelism Avi Vajpeyi, John Lindner, Susan Lehman, Denise Byrnes We describe a computer simulation of avalanches on a conical bead pile using both central processing units (CPUs) and graphical processing units (GPUs) to increase computational speed on a single desktop computer. We make detailed comparisons with real experiments involving physical beads. In the experimental system, the avalanche size distribution follows a power law for beads dropped onto the pile apex from a low drop height. The simulation models each bead independently with its own position and velocity. As each bead is independent, we can use parallelism to thread various computations on the beads to the GPU. This allows the simulation to run significantly faster than the physical experiment while still involving tens of thousands of beads. By monitoring every bead, we can extend the research beyond what is experimentally possible. The experiments are sensitive only to avalanches that exit the pile, while the simulation is also sensitive to avalanches confined to the pile. 
Thursday, March 8, 2018 12:03PM  12:15PM 
S48.00005: Selfcharging of Identical Grains in the Absence of an External Field Nuno Araujo, Ryuta Yoshimatsu, Andre Matias, Gerhard Wurm, Hans Herrmann, Troy Shinbrot Completely identical insulator grains do charge one another upon contact and the charge difference even increases with repeated contact. Several experimental and theoretical works have provided a strong analytic foundation for charging mechanisms due to geometric or material asymmetries, but the mechanism in the case of absolutely identical grains is not at all clear. Here, we investigate the electrostatic charging of an agitated bed of identical grains using particlebased simulations, mathematical modeling, and experiments. We simulate a discreteelement model including electrical multipoles and find that infinitesimally small initial charges can grow exponentially rapidly. We confirm the predicted exponential growth in experiments using vibrated grains under microgravity, and we describe novel predicted spatiotemporal states that merit further study. Finally, we discuss how the charging dynamics emerge from the competition between the polarization and relaxation time scales. 
Thursday, March 8, 2018 12:15PM  12:27PM 
S48.00006: Electromagnetic phenomena in granular flows in the laboratory and dusty plasmas in geophysics and astrophysics Daniel Lathrop, Skylar Eiskowitz, Ruben Rojas In clouds of suspended particles (grains, droplets, ash, ice crystals, etc.), collisions electrify particles and the clouds produce significant electric potential differences over large scales. This is seen most spectacularly in the atmosphere as lightning in thunderstorms, thundersnow, dust storms, and volcanic ash plumes where multimillionvolt potential differences are produced, but it is a general phenomena in granular systems as a whole. To investigate the relative importance of particle properties (material, size, etc.) and collective phenomena (behaviors of systems at large scales not easily predicted from local dynamics) in granular and atmospheric electrification, we used several tabletop experiments that excite particleladen flows. Various electromagnetic phenomena ensue. Measured electric fields are a result of capacitive coupling and direct charge transfer between the particles and the measurement plates. These results suggest that while particle properties do matter (as previous investigations have shown), macroscopic electrification of granular flows is relatively material agnostic and largescale collective phenomena play a major role. 
Thursday, March 8, 2018 12:27PM  12:39PM 
S48.00007: Investigating the Vibrational Properties of Disordered Solids Using Random Matrix Models Ethan Stanifer, Peter Morse, Arthur Middleton, Mary Manning Glasses and jammed packings exhibit interesting lowfrequency vibrational behavior, including a region in the density of states, D(ω), that scales as ω^{4} with quasilocalized excitations important for flow and failure. However, to our knowledge, there are no constructive models that generate ω^{4} scaling and explain the mechanism for quasilocalization. Recent work indicates random matrix models can provide explanations for universal vibrational properties in glasses. Here we analyze the Laplacian Matrix of ring networks where disorder is controlled by the distribution of bond weights and adding bonds to distort the network. Simulating this model with high statistics, we find 2 crossover frequencies. The larger frequency, ω*, corresponds to the end of a plateau in D(ω) and scales with Δz, just as the boson peak in jammed systems. The second crossover frequency, ω_{c}, is dependent on the density of weak bonds. For a uniform bond distribution, we find ωc scales with Δz and D(ω) scales with ω^{3} below ω_{c}, and between ω_{c} and ω*, the spectrum scaling is consistent with ω^{4} independent of Δz and system size. This scaling disappears if we perturb away from a uniform distribution. These results suggest this simple model may be a powerful tool for understanding localization in structural glasses. 
Thursday, March 8, 2018 12:39PM  12:51PM 
S48.00008: Percolation Thresholds Through the Voids Around Randomly Oriented Faceted Barrier Particles Nicholas McGuigan, Donald Priour Realistic porous materials are often made up of randomly oriented impermeable particles with fluid transport occuring through the voids. Transport is hindered with increasing concentration of barrier particles, ultimately ceasing at the percolation transition. Using a dynamical approach involving virtual tracer particles which explore and determine the extent of the voids, we have calculated percolation thresholds for a variety of included particles, having in common random orientations and angular, faceted shapes. In this regard we consider fractured spheres (e.g. hemispheres and tabular sections), polygonal and circular plates, and polyhedra such as tetrahedra and cubes. By considering asymmetrical particles with dipole moments that could respond to an electric field in a temperature dependent situation, we explore the continuum between uniformly and randomly oriented assemblies of barriers. In particular, we determine if the anisotropy due to the field induced alignment leads to distinct percolation thresholds perpendicular and parallel to the applied field. 
Thursday, March 8, 2018 12:51PM  1:03PM 
S48.00009: Friction Games: Towards exact constraint counting in frictional packings Kuang Liu, Silke Henkes, Jennifer Schwarz To count constraints in frictional packings more exactly than generalized isostaticity, we implement a (3,3) pebble game and construct rigid clusters on a model of rigidity percolation with friction. Specifically, the model is double and single bonds randomly added to a honeycomb lattice with additional next nearest neighbor bonds. The double bonds represent frictional contacts and the single, contacts at the Coulomb threshold. We find a second order transition with a fractal spanning rigid cluster and numerically determine related exponents, which are close to rigidity percolation without friction. Given the closeness in numerical values of the exponents between rigidity (with and without friction) and connectivity percolation, we unveil a new minimal rigidity percolation model whose exponents near/at the transition are in the same universality as connectivity percolation. We also address shortcomings of the (3,3) pebble game in determining constraints and develop additional methods to accommodate these shortcomings. 
Thursday, March 8, 2018 1:03PM  1:15PM 
S48.00010: Crackling to periodic transition in earthquake dynamics: a granular stickslip experiment Aghil Abed Zadeh, Jonathan Bares, Robert Behringer

Thursday, March 8, 2018 1:15PM  1:27PM 
S48.00011: Effect of Cohesion on the Time between Avalanches On A SlowlyDriven Conical Bead Pile Susan Lehman, Karin Dahmen, Louis McFaul A conical bead pile subject to slow driving is used as a model critical system to experimentally investigate variations in avalanche size and time between events. The pile is composed of roughly 20 000 steel spheres, 3 mm in diameter, atop a circular base. We add one bead at a time to the apex of the pile; avalanches are measured through changes in pile mass. We investigate the dynamic response of the pile by recording avalanches off the pile over the course of tens of thousands of bead drops. The avalanching behavior is studied at different drop heights and different amounts of cohesion between the beads. At low cohesion, the statistical properties of the avalanches, including probability of particular avalanche sizes, are wellcharacterized by universal power laws and scaling functions. As cohesion increases, we observe a deviation from the power law behavior. Here we focus on the time between events, both in terms of the time interval between two events both above a threshold size, and also the waiting time after a given avalanche until the next event of any size. At both high and low cohesion, the experimental results match well with a meanfield model of slip avalanches [Dahmen, Nat Phys 7, 554 (2011)]. 
Thursday, March 8, 2018 1:27PM  1:39PM 
S48.00012: Pure Shear of 2d Granular Mixtures: Experiment Daniel Gysbers, Scott Franklin We report on the behavior of particles in an annular planar Couette shear cell. Particles are made of acrylic and constrained in a vertical stack between two concentric cylinders; the top row of particles is sheared by a serrated loading ring. The annular radius of curvature is much larger than the particle length scale, and so the experiment is effectively2D and allows for arbitrarily large strains. The confining pressure and pile height can be varied independently, and we investigate pile response in this twoparameter space. Particle shapes include binary mixtures of disks, ellipses and spherocylinders as well as Ushaped and chiral particles that can be aligned with or against the shear. When the bottom boundary is smooth, the pile exhibits solid body motion with a frequency that increases with confining pressure and, as predicted by theory, with decreasing pile height. New experiments are looking at rougher boundaries and the effect of small numbers of noncircular particles embedded in a surrounding “sea” of circular particles. 
Thursday, March 8, 2018 1:39PM  1:51PM 
S48.00013: Onset of the Pullout Motion of Spherical Object from Columns of Sand Payman Jalali, Yuchen Zhao, Yue Zhang, Robert Behringer

Thursday, March 8, 2018 1:51PM  2:03PM 
S48.00014: Friction and Flow of Granular Emulsions Marcel Workamp, Joshua Dijksman We experimentally investigate the role of friction in dense nonBrownian suspensions. We perform rheological and tribological experiments using hydrogels. Similar to the droplets in an emulsion, our hydrogel particles are soft, so we can make a dense suspension. However, as in a granular material, the particles are solid and thus interact through friction. We use hydrogels with varying chemical composition to make the particles, and measure the friction coefficient of the materials with a custom tribometer. We link the microscopic friction of the particles to the rheological behavior of the suspension, by combining rheological measurements with optical techniques in a 3D printed Couette geometry. We find that the friction coefficient of the material sets the stress ratio (shear stress/pressure) of the suspension, provided the friction coefficient is sufficiently high. This suggests two dissipation mechanisms contribute to the shear resistance of the suspension. 
Thursday, March 8, 2018 2:03PM  2:15PM 
S48.00015: Particle Scale Dynamics of Oblique Granular Impact Noah Cox, Cacey Stevens Bester, Robert Behringer When a solid projectile impacts a granular target, it experiences a drag force and abruptly comes to rest as its momentum transfers to the grains. An empirical law successfully describes the force experienced by the projectile, and the corresponding grainscale mechanisms have been deciphered for normal impacts. However there is little work exploring nonnormal impacts. Here, we extend previous studies to explore the oblique impact case, in which a significant horizontal component of the drag force is present. In our experiments, a projectile impacts a quasitwodimensional bed of bidisperse photoelastic grains. We use highspeed imaging to measure highresolution position data of the projectile's trajectory and simultaneously visualize force chain propagation in the granular medium. When the impact angle becomes important, the spatial structure of the stress response reveals relatively weak force chain propagation in the horizontal direction. This is accompanied by an increasingly curved projectile trajectory. Based on these observations, we describe the change of the granular impact drag force with impact angle. 
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