Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session X9: Focus Session: Jamming: Theory and Experiment II 
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Sponsoring Units: GSNP Chair: Bulbul Chakraborty, Brandeis University Room: 303 
Thursday, March 19, 2009 2:30PM  3:06PM 
X9.00001: Jamming in Vibrated Granular Systems Invited Speaker: Brian Utter Granular materials exist all around us, from avalanches in nature to the mixing of pharmaceuticals, yet the behavior of these``fluids'' is poorly understood. Their flow can be characterized by the continuous forming and breaking of a strong force network resisting flow. This jamming/unjamming behavior is typical of a variety of systems, including granular flows, and is influenced by factors such as grain packing fraction, applied shear stress, and the random kinetic energy of the particles. I'll present experiments on quasistatic shear and freesurface granular flows under the influence of external vibrations. By using photoelastic grains, we are able to measure both particle trajectories and the local force network in these 2D flows. We find through particle tracking that dense granular flow is composed of comparable contributions from the mean flow, affine, and nonaffine deformations. During shear, sufficient external vibration weakens the strong force network and reduces the amount of flow driven by sidewalls. In a rotating drum geometry, large vibrations induce failure as might be expected, while small vibration leads to strengthening of the pile. The avalanching behavior is also strongly history dependent, as evident when the rotating drum is driven in an oscillatory motion, and we find that sufficient vibration erases the memory of the pile. These results point to the central role of the mobilization of friction in quasistatic granular flow. [Preview Abstract] 
Thursday, March 19, 2009 3:06PM  3:18PM 
X9.00002: Scaling of Foam Flows near Jamming Martin van Hecke, Erik Woldhuis, Brian Tighe, Joris Remmers, Wim van Saarloos We probe the scaling behavior of flows near the jamming transition of soft, viscous discs in a variant of the well known bubblemodel for foams, where we assume that the viscous forces between contacting bubbles scale with the relative velocity with an adjustable exponent $\alpha$. This allows us to explore the nontrivial dependence of global flow exponents on the local exponent $\alpha$. Even though we find that elastic stresses dominate the global stresses in the system, the exponent $\alpha$ which governs the subdominant viscous interactions still sets the global scaling exponents. [Preview Abstract] 

X9.00003: ABSTRACT WITHDRAWN 
Thursday, March 19, 2009 3:30PM  3:42PM 
X9.00004: Rheology of Soft Suspensions near Jamming Kerstin Nordstrom, Emilie Verneuil, Paulo Arratia, Jerry Gollub, Douglas Durian The rheology of a suspension of soft colloidal particles is investigated using a pressuredriven flow in a deep 25 $\mu $m wide microchannel. The system is composed of Nisopropylacrylamide (NIPA) colloidal microgel particles, suspended in aqueous solution. NIPA is temperaturesensitive in that the hydrodynamic radius decreases as temperature increases [1]. Therefore, colloidal suspensions of different packing fraction can be obtained simply by varying the temperature using a temperaturecontrolled stage. We determine the velocity profile and the local shear rate of the suspension using particle image velocimetry (PIV). We have developed methods to accurately infer the suspension shear viscosity and shear stress as a function of shear rate. The dynamical range of shear rates probed is approximately 5 orders of magnitude, ranging from 10$^{4}$ to 10$^{1}$ s$^{1}$. Results show that as the packing fraction is increased towards the jamming point, the velocity profiles are markedly nonNewtonian. Further, above the jamming point, the stress versus shear rate curves show yield stress behavior. [1] Alsayed, A.M.;Islam, M.F.;Zhang, J.;Collings, P.J.;Yodh, A.J., \textit{Science} \textbf{2005}. [Preview Abstract] 
Thursday, March 19, 2009 3:42PM  3:54PM 
X9.00005: Inertia and the Distribution of Avalanches in Sheared Glasses Kenneth Salerno, Mark Robbins, Craig Maloney Many slowly driven condensed matter systems show highly intermittent and spatially organized dynamics where relaxation proceeds via ``avalanches''. This talk describes the role of inertia in determining the distribution of avalanches in sheared glassy systems. Simulations were performed on binary mixtures of LennardJones particles in two dimensions. The distribution of events was evaluated during steadystate quasistatic deformation in pure shear. The temperature was maintained at zero by damping the relative velocity of interacting particles. The damping rate was increased to transition from an inertial to an overdamped limit. The magnitude of avalanches E is defined as the energy dissipated. In the inertial limit the distribution P(E) of events follows power law scaling over more than four decades. As for earthquake fault systems, P(E) $\sim $ 1/E. The largest event and number of events increase roughly linearly with the system width L for L varying from 25 to 400 particle diameters. Increasing the damping or damping the total velocity removes the large events and leads to qualitative changes in the distribution. [Preview Abstract] 
Thursday, March 19, 2009 3:54PM  4:06PM 
X9.00006: Density of states in twodimensional colloidal system Ke Chen, Zexin Zhang, Peter Yunker, Arjun Yodh The vibrational density of states (VDOS) of particles in a twodimensional binary colloidal system was investigated using video microscopy. Our ultimate goal is to explore how the VDOS varies near the jamming transition [1].~ Various distributions of NIPA particles, whose diameters can be tuned by small temperature variations, were loaded into parallelplate microscope cells, and their motions tracked with video microscopy. This approach permits insitu observation over a wide range of particle packing fractions, from colloidal fluids to colloidal glasses. A search for excess VDOS at low frequencies in colloidal glass is ongoing. 1. N. Xu, M. Wyart, A. J. Liu, and S. R. Nagel, \textit{Phys. Rev. Lett.} \textbf{98}, 175502 (2007) This work is supported by NSF DMR080488, MRSEC DMR0520020 [Preview Abstract] 
Thursday, March 19, 2009 4:06PM  4:18PM 
X9.00007: Applying the model of Soft Glassy Rheology to slowly driven dense granular matter Dapeng Bi, Bulbul Chakraborty In recent work by S. Henkes and B. Chakraborty (PRL 95, 198002 (2005)), a new statistical framework is proposed to describe static granular packings. In this framework, stress replaces energy as the conserved quantity and fluctuations in the stress are controlled by a quantity analogous to the thermodynamic temperature. We adapt this framework in the quasistatic limit and the model of Soft Glassy Rheology (P. Sollich, PRE 78, 2020 (1997)) to describe the rheological behavior of slowly driven dense granular matter. The model explains the experimental observation of R. P. Behringer et al. (Nature 421, 928 (2003)). We will describe ongoing efforts to apply this model to different categories of slowly driven granular media, and to relate the model to threshold critical dynamics in other driven random media. [Preview Abstract] 
Thursday, March 19, 2009 4:18PM  4:30PM 
X9.00008: Theory of random packings Hernan Makse, Chaoming Song, Ping Wang We present a theory of random packings to describe the statistical mechanics of jammed matter with the aim of shedding light to the longstanding problem of characterizing the random close packing (RCP) and random loose packing (RLP) of particles. We describe the jammed system with equations of state relating observables such as entropy, coordination number, volume fraction, and compactivity as well as the probability distributions of volume and contacts. We follow a systematic route to classify packings into a phase diagram of jamming, from frictionless to frictional particles, from hard to deformable particles, from monodisperse to polydisperse systems, from spherical particles to nonspherical convex particles, in an attempt to understand the packing problem from a unifying perspective. The studies of RCP and RLP includes 2d, nd, and the mean field limit of infinite dimension. [Preview Abstract] 
Thursday, March 19, 2009 4:30PM  4:42PM 
X9.00009: Dense packings of hard tetrahedra Amir Haji Akbar, Xiaoyu Zheng, Rolfe Petschek, Peter PalffyMuhoray, Sharon Glotzer The densest packing of tetrahedra remains an unsolved problem, and there has been much recent debate. We simulate dense packings of mathematically smooth, hard regular tetrahedra using NPT Monte Carlo simulations and determine the density pressure equation of state. We find disordered packings with densities that significantly exceed the hardsphere FCC packing density of 0.740480490. Our findings thus demonstrate that tetrahedra obey Ulam's conjecture that spheres pack with a lower maximum packing density than any other hard convex object, despite recent conjecture to the contrary. The dense packings that we have found do not seem to be crystalline but are instead dense random packings. We show that the system is able to achieve such high packing densities by the local ordering of tetrahedra into certain favorable motifs, forming larger structures that pack efficiently but are overall jammed. We speculate that one or several denser crystalline packings exist. [Preview Abstract] 

X9.00010: ABSTRACT WITHDRAWN 
Thursday, March 19, 2009 4:54PM  5:06PM 
X9.00011: Continuous Geometric Families of Mechanically Stable Particle Packings Guojie Gao, Jerzy Blawzdziewicz, Corey O'Hern We have performed numerical simulations of quasistatic shear flow of soft disks at zero pressure to generate mechanically stable (MS) packings as a function of applied shear stress/strain in small 2D systems ranging from 4 to 20 disks. In systems composed of frictionless disks, we find that at any given shear strain, there are a finite number of dicrete MS packings characterized by the positions of all particles. In contrast, there are an infinite number of MS packings during continuous shear flow that form a finite geometric families (characterized by the network of interparticle contacts) as a function of shear strain. We count the number of geometric families and measure their length in strain as a function of system size. In particular, we will determine whether the MS packings at finite shear have different structural and mechanical properties from those at zero shear. We also study the effects of friction on MS particle packings. In contrast to frictionless MS packings, frictional packings form continuous geometric families even at a zero shear strain. [Preview Abstract] 
Thursday, March 19, 2009 5:06PM  5:18PM 
X9.00012: Mechanically Stable Packings of Spherocylinders Timothy Green , Scott Franklin Piles of long, thin rods are substantially more stable to perturbations than those of ordinary sand or rice. We generate 3d mechanically stable packings of spherocylinders by alternately enlarging particles (with an elastic repulsive interaction) and using a conjugate gradient minimization of the total elastic energy. The minimum stable packing $\phi_c$ is defined as the least dense packing for which the minimum energy is nonzero, and we investigate the average contact number, the spectrum of vibrational modes in the dynamical matrix, and other properties of this critical packing. We also test whether spherocylinders obey the isostatic conjecture, which states that the average contact number at $\phi_c$ is twice the number of degrees of freedom (for spherocylinders, 5). Spherocylinders' straight edges, compared with the convex sides of ellipsoids, puts the isostatic conjecture in jeopardy, perhaps requiring a greater number of contacts to maintain stability. [Preview Abstract] 
Thursday, March 19, 2009 5:18PM  5:30PM 
X9.00013: On the study of forcebalance percolation J. M. Schwarz, M. Jeng We study models of correlated percolation where there are constraints on the occupation of sites that mimic forcebalance, i.e. for a site to be stable (remain occupied) requires occupied neighboring sites in all four compass directions in two dimensions. We prove rigorously that $p_c<1$ for the twodimensional models studied. Numerical data indicate that the forcebalance percolation transition is discontinuous with a growing crossover length, with perhaps the same form as the jamming percolation models, suggesting the same underlying mechanism driving the transition in both cases. In other words, forcebalance percolation and jamming percolation may indeed belong to the same universality class. We find a lower bound for the correlation length in the connected phase and that the correlation function does not appear to be a power law at the transition. Finally, we study the dynamics of the culling procedure invoked to obtain the forcebalance configurations and measure a dynamical exponent similar to that found in sandpile models. [Preview Abstract] 
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