Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Z44: Focus Session: Jamming in Granular Media III |
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Sponsoring Units: GSNP GSOFT Chair: Justin Burton, Emory University Room: 214D |
Friday, March 6, 2015 11:15AM - 11:27AM |
Z44.00001: Short time intermittency and long time plastic correlations in jammed systems Arka Roy, Craig Maloney Using numerical simulations of model amorphous, frictionless, soft discs, we study the effect of strain rates ($\dot{\gamma}$) and volume fractions ($\phi$) on the microscopic dynamics and plastic activity near, but above, the jamming point ($\phi_J$). Well above $\phi_J$, at slow shear rate, the system responds in a highly intermittent way, reminiscent of other dynamically critical systems with a power law distribution of energy dissipation rates. With increasing $\dot{\gamma}$ at fixed $\phi$ or letting $\phi\rightarrow\phi_J$ from above at a fixed rate, the intermittent behavior vanishes. All displacement distributions show non-Fickian behavior at short time crossing to Fickian behavior at longer times. Very surprisingly, the characteristic strain for that crossover is independent of $\phi$. We also find that, despite the dramatic differences in the short time dynamics, the long time plastic rearrangements are essentially identical. Long-range spatial correlations in strain are cut off only by the size of the simulation cell. [Preview Abstract] |
Friday, March 6, 2015 11:27AM - 11:39AM |
Z44.00002: Local origin of global contact numbers in frictional ellipsoid packings Fabian Schaller, Max Neudecker, Mohammad Saadatfar, Gary Delaney, Gerd Schr\"oder-Turk, Matthias Schr\"oter In particulate soft matter systems the average number of contacts $Z$ of a particle is an important predictor of the mechanical properties of the system. Using X-ray tomography, we analyze packings of frictional, oblate ellipsoids of various aspect ratios $\alpha$, prepared at different global volume fractions $\phi_g$. We find that $Z$ is a monotonously increasing function of $\phi_g$ for all $\alpha$. We demonstrate that this functional dependence can be explained by a local analysis where each particle is described by its local volume fraction $\phi_l$ computed from a Voronoi tessellation. $Z$ can be expressed as an integral over all values of $\phi_l$: $Z(\phi_g, \alpha, X) = \int Z_l (\phi_l, \alpha, X) \; P(\phi_l | \phi_g) \; d\phi_l$. The local contact number function $ Z_l (\phi_l, \alpha, X)$ describes the relevant physics in term of locally defined variables only, including possible higher order terms $X$. The conditional probability $P(\phi_l | \phi_g)$ to find a specific value of $\phi_l$ given a global packing fraction $\phi_g$ is found to be independent of $\alpha$ and $X$. Our results demonstrate that for frictional particles a local approach is not only a theoretical requirement but also feasible. [Preview Abstract] |
Friday, March 6, 2015 11:39AM - 11:51AM |
Z44.00003: Mechanical Response in Particulate Media Niranjan Warnakulasooriya, Leo Silbert We study the mechanical behavior of granular particle system in two dimensions in response to a dynamical intruder using computer simulations. We created mechanically stable granular packings of bidisperse discs with various coefficients of friction spanning several orders of magnitude and packing fractions in the vicinity above the critical packing fraction $\phi_c$. For each packing, we find the critical force $F_c$, the minimum force required to induce motion of a probe particle that we are trying to drag through the packing. Below the critical force the probe particle does not sustain continued motion. Just at the critical force, the probe particle moves through the system strongly intermittently. When the force is slightly larger than the $F_c$, the probe moves with well-defined average velocity. We find how the critical force and the average probe velocity depend on the packing pressure and particle frictions. [Preview Abstract] |
Friday, March 6, 2015 11:51AM - 12:03PM |
Z44.00004: Rigidity percolation in generic and regular isostatic lattices Leyou Zhang, D. Zeb Rocklin, Bryan Chen, Xiaoming Mao Rigidity percolation, the emergence of rigidity as bonds are randomly added to a structure, has been studied using various models, yielding a rich variety of behaviors including continuous/discontinuous transitions as well as mean field/anomalous scalings. Here we present our study of rigidity percolation in isostatic lattices, which are at the verge of mechanical instability and thus adding a vanishing fraction of next-nearest-neighbor bonds ("braces") can rigidify the lattice. However, we find that how the lattice rigidifies as braces are added depends on the lattice architecture in interesting ways. We study this problem in both regular (periodic, with bonds following straight lines) and generic (sites are randomly moved, keeping only the topology of the connectivity) versions of isostatic square and kagome lattices via simulation. We discover that (1) rigidity percolation in generic isostatic lattices is discontinuous, with a sudden emergence of a rigid bulk, before which no stress can appear, sharing intriguing similarities with jamming, and (2) regular isostatic lattices, in contrast, show mixed features of continuous and discontinuous transitions. We propose analytic theories to explain our observations. [Preview Abstract] |
Friday, March 6, 2015 12:03PM - 12:15PM |
Z44.00005: Experimental avalanches in a two-dimensional rotating drum: Universality or a first-order phase transition? Aline Hubard, Corey O'Hern, Mark Shattuck We study experimentally the dynamics of steel spheres in a quasi-two dimensional rotating drum to investigate whether avalanches occur as continuous or first-order transitions. In our experiments, monodisperse steel spheres are confined within a cylindrical region between the glass walls of the drum, and the drum rotation axis is perpendicular to the direction of gravity. The drum and spheres first rotate as a solid body, and the slope of the sphere packing increases until the packing becomes unstable. The avalanche proceeds until the system finds another stable packing. Using high-speed video, we track the particle displacements during each avalanche to quantify the statistics of the avalanche sizes and durations as a function of the rotation rate and particle size distribution. We find that the avalanche size and duration distributions display power-law scaling over several decades, which suggests universal behavior in this system. [Preview Abstract] |
Friday, March 6, 2015 12:15PM - 12:27PM |
Z44.00006: Mean-field approach for random close packings of non-spherical and adhesive particles Adrian Baule, Romain Mari, Lin Bo, Louis Portal, Wenwei Liu, Shuiqing Li, Hernan Makse Random packings of objects of a particular shape are ubiquitous in science and engineering. However, such jammed matter states have eluded any systematic theoretical treatment due to the strong positional and orientational correlations involved. Here, a mean field theory based on a statistical treatment of the Voronoi volume is presented, which allows for the calculation of the random close packing of spherical as well as non-spherical hard particles. The extension of the framework to packings of adhesive particles is discussed. A phase diagram is presented that describes non-spherical and adhesive particles in terms of analytic continuations from the spherical random close packing. [Preview Abstract] |
Friday, March 6, 2015 12:27PM - 12:39PM |
Z44.00007: Jamming aids jumping in granular media Jeffrey Aguilar, Andras Karsai, Daniel I. Goldman Little is known about the impulsive force and flow fields generated during jumping on granular media. We use a simple robot jumping on poppy seeds to explore maneuvers that induce jammed (non-yielding) states, and find sensitive dependence of jumping performance to movement strategy. On loose packings (volume fraction $\phi = 0.57$), a preliminary hop followed by a delay (``delayed stutter jump'') improves the height of a push-off maneuver (``single jump''). Constant speed intrusion force measurements suggest that reentry of the foot during the preliminary hop reintroduces high surface resistance. An optimal delay time ($t = 50$ ms) leads to maximal jump heights, while a short delay time ($t \approx 0$ ms) produces the lowest jumps. Velocimetry of grain flow reveals that non-delay stutters induce fluid-like granular states into which the robot sinks before jamming occurs, lowering jump heights. While simulations of single and delayed stutter jumps are well described using a frictional (depth dependent) plus drag (velocity dependent) penetration resistance, this model does not capture stutter jump performance at low $\phi$. However, addition of an added mass term improves agreement, signaling the need for a more complex reactive force theory in impulsively forced granular media. [Preview Abstract] |
Friday, March 6, 2015 12:39PM - 12:51PM |
Z44.00008: Exploring the Bernoulli effect on airfoils in a granular flow Yasin Karim, Eric Corwin The Bernoulli effect describes the decrease in pressure that results from a fluid accelerating over an airfoil. While granular materials lack many of the features of fluids (i.e. they are compressible, do not have a well-defined viscosity, and are non-cohesive) they nonetheless can be made to flow. We report on experiments carried out to study Bernoulli lift in granular flows as a function of flow speed, density, airfoil shape. Using velocimetry and force sensors we probe the existence of a Bernoulli lift on an airfoil as glass beads flow around it in a quasi-two dimensional system. [Preview Abstract] |
Friday, March 6, 2015 12:51PM - 1:03PM |
Z44.00009: Non-crystalline states in a 2D dusty plasma Juan-Jose Lietor-Santos, Cao Cong, Justin Burton When suspended in a plasma, colloidal particles become negatively charged due to a preponderance of collisions with free electrons. If the plasma is weakly-ionized, the resulting repulsive electrostatic forces cause the particles to self-organize into a single 2D layer in the plasma sheath near a surface. At high concentrations and particle charging, a hexagonal crystalline lattice is formed which supports the propagation of underdamped, phonon-like waves. This ``dusty plasma'' is an ideal model system to study low-temperature dynamics in solids, where the individual particle motions can be visualized and tracked [1]. Here we report the creation of non-crystalline states in a dusty plasma by combining two particle species of different size and material density. By finely-tuning these variables, we show that both particle populations lie in the same plane, leading to a 2D amorphous structure which can be used to study the dynamics of glassy and jammed systems at low temperatures and frequencies. \\[4pt] [1] Chaudhuri et al., Soft Matter 7, 1287-1298, (2011). [Preview Abstract] |
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