Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session V22: Focus Session: Jamming II |
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Sponsoring Units: GSNP Chair: David Wu, Colorado School of Mines Room: Colorado Convention Center 108 |
Thursday, March 8, 2007 11:15AM - 11:51AM |
V22.00001: A Local, Geometrical Probe for Jamming Invited Speaker: When jammed disordered materials such as Lennard-Jones systems, granulates and foams are forced externally, the resulting deformation fields exhibit large scale vortical patterns and are strongly non-affine. Here we introduce the distribution P(alpha) as a local probe of the non-affine nature of this response. Alpha denotes the angle between the bonds and the local deformations of pairs of particles in contact - hence alpha = pi/2 corresponds to particles sliding past each other, while particles squeezed together or moving apart correspond to alpha = 0 or pi. We find that near jamming, P(alpha) becomes strongly peaked around pi/2, with the width and height of the peak scaling with the distance to the jamming point. Grains then predominantly slide past each other, which signals an increasingly non-affine response of the material caused by the proximity of floppy modes. We relate this local measure to the global, anomalous scaling of the elastic moduli and contact numbers near jamming, and show the first experimental determination of P(alpha) in sheared, 2D foams. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V22.00002: A critical length scale in jammed granular media Wouter Ellenbroek, Ellak Somfai, Martin van Hecke, Wim van Saarloos Granular media consist of macroscopic, athermal particles that ``jam'' into a solid-like state when subjected to a confining pressure. Recent studies of this jamming transition in systems of frictionless particles have shown, quite remarkably, that the jamming point has many features of a critical point, exhibiting power law scalings of various quantities nearby. We study the linear response of these jammed systems to a localized mechanical perturbation. The response fluctuates over a length scale that diverges at the jamming transition, providing a direct numerical observation of a critical length scale in jammed granular media. [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V22.00003: Critical Scaling at the Jamming Transition for Zero and Finite Applied Shear Stress Peter Olsson, Stephen Teitel We carry out numerical simulations to study the jamming transition of a model granular material in two dimensions at zero temperature. Behavior is simulated as a function of particle density and applied shear stress. We find a collapse of our data to scaling curves that provides evidence for a sharp 2nd order jamming transition in non-equilibrium steady states at finite shear, that ends at a critical point (point ``J'') as the shear stress vanishes. We estimate the values of the critical exponents at both zero and finite shear stress. [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V22.00004: The fluid-glass transition for hard spheres John Drozd, Colin Denniston A gravity-driven hard sphere simulation is used to study the phenomena of disorder-order transitions, or simply the glass transition from a granular hard sphere fluid to a jammed glass. We find a diverging length scale and a diverging viscosity at this transition and compare our simulation to experiment on the connection between local velocity fluctuations and shear rate. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V22.00005: Correlated Percolation Models of Jamming and Glass Transitions Monwhea Jeng, Jennifer Schwarz Toninelli, Biroli, and Fisher recently introduced a model of correlated percolation called the Knight model, which they claimed to prove underwent a dynamical glass transition. This transition had novel properties, with a discontinuous jump in the order parameter, but with diverging time scales and correlation lengths. We show that their proof misidentified the critical point, so that these properties are currently unproven for this model. However, we show that these novel properties can in fact be proven for suitably modified models of correlated percolation, with qualitatively similar culling rules. We discuss the features of the models necessary for a rigorous proof to be possible. We also discuss properties of models such as the force balance model and the original Knight model, which appear to undergo novel transitions despite the lack of a rigorous proof of such a transition. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V22.00006: Fluctuations in the crossover from aging to equilibrium of a structural glass Azita Parsaeian, Horacio E. Castillo We investigate the fluctuations in the aging regime, the equilibrium regime and the crossover between them, for a simple structural glass with purely repulsive Weeks-Chandler-Anderson interactions. We characterize how the fluctuations evolve by studying the probability distributions of local observables such as individual particle displacements $\Delta x$ and intermediate scattering functions $C_r$ associated with small regions. We compare the results for probability distributions in the different regimes, and we also compare with results obtained previously for the aging regime of a glass with both repulsive and attractive interactions. We discuss the fitting of the probability distributions of local intermediate scattering functions $C_r$ by generalized Gumbel distributions, and the tails of the probability distributions of particle displacements by non-linear exponential forms. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V22.00007: Colloidal Glass Transition Observed in Confinement Kazem Edmond, Carrie Nugent, Hetal N. Patel, Eric R. Weeks We study a colloidal suspension confined between two parallel walls as a model system for glass transitions in confined geometries. The suspension is a mixture of two particle sizes to prevent wall-induced crystallization. We use confocal microscopy to directly observe the motion of colloidal particles. This motion is slower in confinement, thus producing glassy behavior in a sample which is a liquid in an unconfined geometry. We present results from a range of volume fractions. Our results demonstrate that the maximum thickness at which confinement effects still occur defines a length scale for a given particle volume fraction. This length scale increases as the volume fraction increases toward the glass transition. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V22.00008: The Manhattan Model: A simple model for glassy dynamics Prasanta Pal, Corey O'Hern, Jerzy Blawzdziewicz We study the dynamics of 1d hard rods undergoing Brownian motion in an array of narrow, multiply intersecting channels. In the current version, the junction size equals the particle size and particles are prevented from making turns at each intersection. This simple model shares many of the important features of glassy systems including kinetic arrest, cooperative and heterogeneous dynamics, and aging behavior in the high packing fraction limit. One of our key results is that the structural relaxation time and other dynamical quantities increase super-exponentially with $\phi$ and diverge at $\phi_g <\phi_{cp}$ significantly below the close-packed density. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V22.00009: Simultaneous measurements of bulk moduli and particle dynamics in a sheared colloidal glass Michael V. Massa, Christoph Eisenmann, Chanjoong Kim, David A. Weitz We present a novel study of glassy colloidal systems, using a stress-controlled rheometer in conjunction with a confocal microscope. This experimental setup combines the measurement of bulk moduli, using conventional rheology, with the ability to track the motion of individual particles, through confocal microscopy techniques. We explore the response of the system to applied shear, by simultaneously monitoring the macroscopic relaxation and microscopic particle dynamics, under conditions from the quiescent glass to a shear-melted liquid. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V22.00010: Discontinuous shear thickening, or shear jamming, of dense suspensions of uniform non-spherical particles Ryan Larsen, Jin-Woong Kim, David Weitz Discontinuous shear thickening, or shear jamming, occurs when suspensions undergo a shear-induced transition from fluid-like behavior to solid-like behavior. Because jamming is associated with geometrical confinement of the particles, it is reasonable to expect particle shape to have an effect on the jamming of suspensions. To test this dependence, we synthesize uniform polystyrene particles of dumbbell and triangle shape and compare their jamming behavior to that of equivalent spheres. We show that the non-spherical particles display more dramatic viscosity increases during jamming, and they persist in the jammed state for longer periods of time. Moreover, as the spherical particles approach the jamming transition, they oscillate stochastically between the jammed and un-jammed states on milli-second time scales, whereas the non-spherical particles display no such behavior. We rationalize these qualitative differences in jamming behavior in terms of the higher packing efficiency of non-spheres at low shear rates relative to that at high shear rates. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V22.00011: Flow fields caused by local perturbations of colloidal glasses Lov Goel, Eric R. Weeks Colloidal suspensions can be used to model atomic and molecular systems. At relatively high densities, these particle suspensions form a metastable jammed phase known as a colloidal glass; we study a series of colloidal samples close to this glass transition point. We add a small number of superparamagnetic beads to our samples so that we can perturb the colloidal system locally with an external magnetic field. We use laser scanning confocal microscopy to observe the response of the colloidal particles to this perturbation. The trajectories of several thousand particles surrounding the magnetic bead are followed to high accuracy using particle tracking techniques. The perturbed motion of colloidal particles decays exponentially away from the moving magnetic bead, thus revealing a new length scale. We measure this length scale as a function of volume fraction and applied force. [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V22.00012: Signatures of critical phenomena of a filled elastomer under deformation Mindaugas Rackaitis, Xiaorong Wang Fluctuations and critical phenomena have drawn much attention for many years. But, no report anticipates that an elastomer containing fillers under gentle deformations will show similar effects. In this presentation, we show that a filled rubber system under about 2{\%} strain may display feature fluctuations that could be associated with a transition of the filler from an elastic solid state to a dispersed fluid state and that is reminiscent of critical phenomena. Besides, electrical conductivity fluctuations and their link to the mechanical fluctuations will also be discussed. In addition, we show that the fluctuation of macroscopic parameters can be related to the microscopic fluctuation of filler structures in the rubber compound and in principle it can act as a probe of what is happening physically at the microscopic scale. [Preview Abstract] |
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