Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Y12: Disordered and Glassy Systems I |
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Sponsoring Units: DFD Chair: Katharina Vollmayer-Lee, Bucknell University Room: B110-B111 |
Friday, March 19, 2010 8:00AM - 8:12AM |
Y12.00001: Fragile granular jamming Mahesh Bandi, Michael Rivera, Florent Krzakala, Robert Ecke We demonstrate experimentally that the route to a jammed state for a set of bi-dispersed frictional disks, subjected to uni-axial compression from a random initial unjammed state, consists of a consolidation state, a fragile jammed state, and finally a rigid jammed state. In the consolidation regime, the pressure on the sides increases very slowly with the packing fraction $\phi$, and there are no detectable stress chains. In the fragile jammed state, stress chains are visible, the pressure increases exponentially with $\phi$, and the fraction of moving disks drops exponentially. Eventually, a final regime where particle displacements are below our resolution and the pressure varies approximately linearly with $\phi$ is reached. We argue that this scenario is generic for athermal frictional compressed particles. [Preview Abstract] |
Friday, March 19, 2010 8:12AM - 8:24AM |
Y12.00002: Jamming under shear Jie Zhang, Jie Ren, Somayeh Farhadi, Robert Behringer We describe experiments in which we consider the jamming of 2D granular materials under shear. We consider experiments involving both pure and simple shear. The particles making up the material are either disks or ellipses, and in both cases, they are fabricated from a photoelastic material. It is then possible to obtain quantitative data for contact forces, and all other relevant grain-scale information. A key observation from these experiments is that initial states with densities below isotropic jamming can be jammed under applied shear in a range of packing fractions between $\phi_{min} \leq \phi \leq \phi_J$, where $\phi_J$ corresponds to the isotropic (zero shear stress) jamming point. We explore the behaviour of the above systems for $\phi$'s in and near this regime. Specifically, we determine particle contacts and the mean contact number per particle, $Z$, the number of nearest neighbors, the shear and normal stresses, $\tau$ and $P$, and kinematic properties such as particle rotation and displacement. We find that the states of the system lie on a surface in a space consisting of $\phi$, P, and $\tau$. As time permits, we will explore the affine and non-affine motion of particles. [Preview Abstract] |
Friday, March 19, 2010 8:24AM - 8:36AM |
Y12.00003: Clogging Tranistion in a Tilted Silo Charles Thomas, Douglas Durian Granular media flow freely from large horizontal holes at the bottom of a container. However, if the hole is too small, or tilted too far from horizontal, a clog will eventually form at the exit and halt the flow. The number of beads which exit before a clog forms follows an exponential distribution. The average of this distribution increases with increasing hole size and with decreasing angle from horizontal, diverging above a critical hole size. We measure these hole sizes at different angles. The critical hole size as a function of angle constitutes the system's phase transition on a clogging phase diagram. In comparison, the hole sizes where the Beverloo equation predicts the flux to vanish are less than half these critical hole sizes. [Preview Abstract] |
Friday, March 19, 2010 8:36AM - 8:48AM |
Y12.00004: Kinetic Heterogeneities at Dynamical Crossovers Thomas Haxton, Andrea Liu We perform molecular dynamics simulations of a model glass-forming liquid to measure the spatial and temporal extent of kinetic heterogeneities as functions of distance $a$ and time $t$. We locate local maxima $\chi^\star$ of the dynamic susceptibility $\chi_{\rm ss}(a, t)$ at distances $a^\star$ and times $t^\star$. We find two types of maxima, both correlated with crossovers in the dynamical behavior: a smaller, early-time maximum corresponding to the crossover from ballistic to sub-diffusive motion, and a larger, late-time maximum corresponding to the crossover from sub-diffusive to diffusive motion. Our results indicate that dynamic heterogeneities are not necessarily signatures of an impending glass or jamming transition. To quantify the lifetime of heterogeneities, we measure the decay of the time correlation function of the overlap parameter. We find that the ratio of the lifetime to the relaxation time increases as temperature is decreased towards the glass transition. [Preview Abstract] |
Friday, March 19, 2010 8:48AM - 9:00AM |
Y12.00005: Elasticity near jamming probed in bidisperse foams Alexander Siemens, Martin van Hecke One of the hallmarks of the jamming transition is the difference in scaling of the shear and bulk modulus of frictionless soft particles near jamming. Here we probe this scaling by compressing and shearing a bidisperse foam monolayer sandwiched between a glass plate and a fluid surface. [Preview Abstract] |
Friday, March 19, 2010 9:00AM - 9:12AM |
Y12.00006: Structural signature of jamming transition Ning Xu In thermal amorphous systems, the first peak of the pair correlation function $g(r)$ reaches the maximum height $g_1^ {\rm max}$ at a crossover volume fraction $\phi_v$ when the volume fraction $\phi$ is varied. In the $T=0$ limit, $\phi_v$ approaches $\phi_c$, the critical volume fraction of the $T=0$ jamming transition, accompanied by a diverging $g_1^{\rm max} $. The occurrence of $g_1^{\rm max}$ at $T>0$ thus reminisces the $T=0$ jamming transition. By measuring typical quantities such as the pressure, bulk modulus, shear modulus, and characteristic frequency of the boson peak, which all show power law scalings with $\phi-\phi_c$ in marginally jammed solids at $T=0$, we observe that $\phi=\phi_v$ separates the thermal amorphous systems into two regimes with distinct material properties: these quantities show similar power law scalings with $\phi-\phi_c$ to marginally jammed solids when $\phi>\phi_v$, which break down when $\phi<\phi_v$. Therefore, the occurrence of $g_1^{\rm max}$ signifies the jamming transition at $T>0$. Because the scalings are manipulated by $\phi_c$, the $T=0$ jamming transition should be the only critical point that controls the jamming transition and properties of jammed solids at $T>0$. [Preview Abstract] |
Friday, March 19, 2010 9:12AM - 9:24AM |
Y12.00007: Investigating Jamming percolation using renormalization group methods Samuel Schoenholz, Amy Bug, Andrea Liu We develop renormalization group-based methods to determine the percolation threshold and exponents for jamming-percolation models. Such models exhibit mixed phase transitions in finite dimensions, with a discontinuous jump in the order parameter and an exponentially diverging length scale: $\xi \approx \exp(|p-p_c^\infty|^\mu)$, where $p_c^\infty$ marks the percolation transition for the infinite system. To extract $p_c^\infty$ we use a Monte-Carlo scheme to find $p_c(L)$ for increasing $L$ and extrapolate to $L\to\infty$. We investigate several models in two dimensions to test for universality of the exponent $\mu$. [Preview Abstract] |
Friday, March 19, 2010 9:24AM - 9:36AM |
Y12.00008: Yielding of colloidal glasses and gels George Petekidis, Nick Koumakis, John Brady Simple hard sphere glasses exhibit a single step yielding under oscillatory or steady shear that is related with the entropic elasticity and breaking of the near neighbours cage. However, when attractions are added, for example in the form of a short range depletion, the system yields in a two step manner. It has been proposed that these processes reflect an initial particle bond breaking and a subsequent breaking of an attractive cage [1]. Here we attempt to elucidate the origin of the two step yielding by examining the rheological response of a series of samples with the same interparticle attraction ranging from high volume fraction attractive glasses to the low volume fraction colloidal gels (0.1$<\phi <$0.6). We examine the linear and non-linear properties with both oscillatory and steady shear rheology. We find that the transition from a highly concentrated attractive glass to a low volume fraction colloid-polymer gel takes place gradually with the cage breaking process being substituted by a cluster dominated process as the volume fraction is decreased. Rheological measurements are complemented by Brownian Dynamics simulations in order to gain insight on the microscopic rearrangements and structural changes that occur during yielding. Different ranges of attraction are implemented both experimentally and in simulations to validate the main mechanisms involved. \\[4pt] [1] K. Pham et al. J Rheology (2008). [Preview Abstract] |
Friday, March 19, 2010 9:36AM - 9:48AM |
Y12.00009: Dynamics of a colloidal glass during stress-mediated structural arrest Ajay Negi, Chinedum Osuji We employ parallel superposition rheology to study the dynamics of an aging colloidal glass in the presence of a mean field stress. Over a range of intermediate stresses, the loss modulus exceeds the storage modulus at short times but develops a maximum concomitant with a crossover between the two as the system ages. This is attended by a narrowing of the loss peak on increasing stress. We show that this feature is characteristic of the structural arrest in these materials, which is made observable on reasonable timescales by the activating influence of the stress. The arrest time displays an exponential dependence on inverse stress. These results provide experimental validation of the role of stress as an effective temperature in soft glassy systems as has been advanced in recent theoretical frameworks. [Preview Abstract] |
Friday, March 19, 2010 9:48AM - 10:00AM |
Y12.00010: Visualization of shear banding in colloidal glasses Vijayakumar Chikkadi, Andrew Schofield, Bernard Nienhuis, Peter Schall Shear banding, i.e. the localization of shear flow, occurs in a manifold of systems ranging from hard materials such as metallic glasses to soft materials such as clays, shaving cream or mayonnaise. We investigate this phenomenon in a dense colloidal system using confocal microscopy that enables to track individual particles in 3D space and time. The particle motions reveal a transition from homogeneous flow to shear localization above a critical shear rate. We elucidate this transition using spatial correlations in the displacement of the particles. The diffusive motion of the particles is correlated over large length, and shows intermittent, scale-free behavior, reminiscent of crystal plasticity. Further, we associate an order parameter with the mobility of particles and demonstrate that shear banding is phase coexistence of regions differing in mobilities. [Preview Abstract] |
Friday, March 19, 2010 10:00AM - 10:12AM |
Y12.00011: Density of states and soft modes in ordered and disordered colloidal systems: Experimental observations Antina Ghosh, Vijayakumar K. Chikkadi, Peter Schall, Jorge Kurchan, Daniel Bonn Glasess are structurally disordered systems that exhibit mechanical properties of solids. At low temperature the thermodynamic properties (heat capacity) of such glassy disordered materials are found to be markedly different from the respective crystal indicating a richer microscopic dynamics. Such deviations are attributed to an ``excess'' of modes at low frequencies observed in the density of states. Further insight in this problem could be gained by understanding the nature of such anomalous modes. In the present study, we compute and compare the vibrational density of states and corresponding long wavelength modes of colloidal hard sphere glasses with its crystalline counterpart from the experimental data. This identifies the observed ``excess'' modes in glasses as transverse modes. These modes appears to be quasi-localized for glasses: the participation ratio provides a quantative measure of such localization. [Preview Abstract] |
Friday, March 19, 2010 10:12AM - 10:24AM |
Y12.00012: ABSTRACT WITHDRAWN |
Friday, March 19, 2010 10:24AM - 10:36AM |
Y12.00013: Dynamics of soft spheres beyond the hard-sphere limit Michael Schmiedeberg, Andrea J. Liu In the limit of low pressures the dynamics of model glass-forming liquids with finite-ranged repulsive interactions are universal. In that limit, where the product of the pressure and the particle volume is small compared to the interaction energy, soft sphere systems behave as hard spheres, so that the dynamics correspond to those of the hard-sphere glass transition and depend only on the ratio of temperature to the product of pressure and the particle volume. However, at higher pressures relative to the interaction energy, there are deviations from this universal behavior that depend on the inter-particle potential. We consider a bidisperse system consisting of soft spheres that repel each other according to a power law potential $\delta^{\alpha}$ where $\delta$ is the particle overlap. By using molecular-dynamics simulations, we determine relaxation times as a function of temperature and pressure. We find that the deviations from hard-sphere behavior can be collapsed onto a single curve that depends on $p^{1/\alpha}$. [Preview Abstract] |
Friday, March 19, 2010 10:36AM - 10:48AM |
Y12.00014: Effects of Shape on Diffusion and Shear Flows Robert Shaw, Norman Packard Diffusion of point particles is well-understood, likewise the motion of simple particles under shear flow. However if the particles are extended objects with shape, more complicated behavior can occur. For example, objects might enter a shaped channel in a configuration that requires them to back up a finite distance in order to proceed further. A configuration that blocks flow through the channel might be statistically preferred, an attracting metastable state of the system. In the bulk, the configuration space of a set of closely packed rigid objects can become convoluted, with many dead-end alleys. If such a system is subjected to a shear, it may naturally tend to settle in such a dead-end, and have to retrace its path in order to continue further, a configuration can become locally locked. The requirement that the system backtrack to unlock distinguishes this process from ordinary jamming, there need be no dissipation or friction per se. We have a number of computer simulations of the motions of closely packed shaped objects, under both Hamiltonian and Monte Carlo dynamics. In addition we will present a simple analytic model, describing the entry and escape of the system from the attracting locked metastable states. [Preview Abstract] |
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