Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session L12: Granular Materials I 
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Sponsoring Units: DFD Chair: Mark Shattuck, CUNYCCNY Room: B110B111 
Tuesday, March 16, 2010 2:30PM  2:42PM 
L12.00001: Experimental determination of frictional families in small 2D granular Mark D. Shattuck , Jerzy Blawzdziewicz , Corey O'Hern We have developed a new experimental technique to explore the effects of friction in mechanically stable twodimensional disk packings. The technique uses high frequency low amplitude vibration to relax tangential forces (friction) without significantly changing the normal forces. If short bursts ($<$10 ms) of vibration are used, the friction can be relaxed in steps. At each step the system reaches mechanical stability but from step to step the stability evolves from frictiondominated to an eventual final state that is stablized only by normal forces (frictionless). Using this protocol on a large random sample of frictional states we can determine the properties of frictional states. Previous experiments and simulations found that the {\em frictionless states} form a finite set of dilute points in configuration space. Our new study indicates that the {\em fictional states} form a finite number of families or low dimensional (usually one dimesional) branches that are connected to the frictionless points in configuration space. The branches and branch points are determined by the connectivity of the particle assembly, and the probability of the system being on a particular branch is not uniformly distributed for physical packinggeneration protocols. [Preview Abstract] 
Tuesday, March 16, 2010 2:42PM  2:54PM 
L12.00002: Bimodal behavior and compressioninduced crystallization of weakly bidisperse granular packings Kamran Karimi , Criag Maloney We perform computer simulations of 2D bidisperse frictionless granular packings for two different particle mixtures. One which is strongly bidisperse, the other which is only weakly bidisperse. Despite pronounced crystalline order in the weak system, both share common features near the jamming transition. Near jamming, the probability distribution of particlewise hydrostatic pressure has a long exponential tail. Force chains are also apparent and have similar spatial structure. At densities further above the jamming transition, the behavior of the two mixtures diverges. The strongly bidisperse system develops an essentially Gaussian pressure distribution (in agreement with previous results on monodisperse, amorphous, 3D packings), while the weakly bidisperse system shows bimodal behavior in which the large particles have larger average pressure than the small particles. Furthermore, we show that the ratio of average large particle pressure to small particle pressure is a nontrivial function of density, and we speculate that this effect is related to an observed compressioninduced increase in crystalline order. [Preview Abstract] 
Tuesday, March 16, 2010 2:54PM  3:06PM 
L12.00003: Spatiotemporally Resolved Acoustics in a Photoelastic Granular Material Eli Owens , Karen Daniels In granular materials, stress transmission is manifested as force chains that propagate through the material in a branching structure. We send acoustic pulses into a two dimensional photoelastic granular material in which force chains are visible and investigate how the force chains influence the amplitude, speed, and dispersion of the sound waves. We observe particle scale dynamics using two methods, movies which provide spatiotemporally resolved measurements and accelerometers within individual grains. The movies allow us to visualize the sound's path through the material, revealing that the sound travels primarily along the force chains. Using the brightness of the photoelastic particles as a measure of the force chain strength, we observe that the sound travels both faster and at higher amplitude along the strong force chains. An exception to this trend is seen in transient force chains that only exist while the sound is closing particle contacts. We also measure the frequency dependence of the amplitude, speed, and dispersion of the sound wave. [Preview Abstract] 
Tuesday, March 16, 2010 3:06PM  3:18PM 
L12.00004: New Kinematic Model in comparing with Langevin equation and Fokker Planck Equation Kyoung Lee , Zhijian Wang , Robin Gardner An analytic approximate solution of New Kinematic Model with the boundary conditions is developed for the incompressible packing condition in Pebble Bed Reactors. It is based on velocity description of the packing density in the hopper. The packing structure can be presented with a jamming phenomenon from flow types. The gravitydriven macroscopic motions are governed not only by the geometry and external boundary conditions of silos and hoppers, but by flow prosperities of granular materials, such as friction, viscosity and porosity. The analytical formulas for the quasilinear diffusion and convection coefficients of the velocity profile are obtained. Since it was found that the New Kinematic Model is dependent upon the granular packing density distribution, we are motivated to study the Langevin equation with friction under the influence of the Gravitational field. We also discuss the relation with the Fokker Planck Equation using Detailed balance and MetropolisHastings Algorithm. Markov chain Monte Carlo methods are shown to be a nonMaxwellian distribution function with the mean velocity of the field particles having an effective temperature. [Preview Abstract] 
Tuesday, March 16, 2010 3:18PM  3:30PM 
L12.00005: Structure of granular clusters formed by capillary aggregation analyzed with Voronoi diagrams Michael Berhanu , Arshad Kudrolli We investigate the spatial structure of particle aggregates floating at an airliquid interface as a model system to understand heterogeneity of cohesive granular matter. The meniscus around identical floating particles introduces short range capillary attraction between the particles. In our experimental system, we increase slowly and continuously the particle number density to observe significant structural transformations. After imaging and tracking all the particles, the structure is characterized quantitatively by using Voronoi diagrams which allow us to elucidate small and large scale properties. We show that the system is organized by attraction for low and intermediate densities, which creates a short range order and gives it heterogeneity with pores of various sizes. As the free pore space are filled at high density, the role of attraction becomes less important compared with steric effects and aggregates show characteristics similar to noncohesive granular media. Mechanical properties of aggregates will be also discussed in light of jamming transition for attractive athermal particles. [Preview Abstract] 
Tuesday, March 16, 2010 3:30PM  3:42PM 
L12.00006: Random Packing Density of Platonic Solids Jessica Baker , Arshad Kudrolli Motivated by the relation between particle shape and packing, we investigate the volume fraction occupied by faceted particles as a function of number of particle sides. Such particles are arguably better representative of natural sand than spheres. For simplicity, we focus on the highly symmetric five Platonic solids which are polygons with congruent sides, vertices and angles, and experimentally measure their packing densities. Plastic dice with 4, 6, 8, 12, and 20 sides are fluidized or shaken randomly to find configurations corresponding to the loosest stable packing and densest packing, respectively. We find that the packing fraction obtained by both protocols peak at the cube and then monotonically decrease below the corresponding values obtained for spheres. Interestingly, the overall trend is similar but systematically lower that the maximum volume fractions reported for frictionless platonic solid particles. The effect of friction of the particles and the shape of the boundary shape on observed packing fractions is also investigated. [Preview Abstract] 
Tuesday, March 16, 2010 3:42PM  3:54PM 
L12.00007: Generalized Hertz Law for Grains with Nonelliptic Contacts Diankang Sun , Chiara Daraio , Surajit Sen Consider two elastic grains of radii of curvature $R_1 ,R_2 $, which are in intimate contact. The contact region between the grains is assumed to be elliptical (along the contact plane). It turns out that the repulsive potential between the compressed elastic grains then behave as the overlap $\delta ^{5/2}$ (Hertz law), where $\delta \equiv R_1 +R_2 z_{12} $, $z_{12} $ being the distance between the centers of the grains when compressed. Here we show that for paraboloidal shaped grains, by modifying the contact region from elliptical to a nonelliptical geometry, we are able to modify the repulsive potential to being dependent on $\delta ^n$, where $n>2$. Energy transport in granular chains with different contact potential laws will be briefly discussed. (Research Supported by US ARO) [Preview Abstract] 
Tuesday, March 16, 2010 3:54PM  4:06PM 
L12.00008: Frictional effects on pressure in a column of granular material Randy Back Frictional effects in granular materials are not well understood. In Janssen's original paper he ignored internal friction completely. More recent theories that treat the granular materials as a continuum assume a slip condition for the internal friction. We report on measurements of the pressure at the bottom a column of granular material for several different frictional coefficients. [Preview Abstract] 
Tuesday, March 16, 2010 4:06PM  4:18PM 
L12.00009: Nonlinear Breathing in Compressed Granular Chains Robert Simion , Adam Sokolow , Surajit Sen When a compressive applied force at the zero frequency limit is applied on confined granular alignments it is shown to result in tunable and higher frequency \textit{nonlinear granular breathing}. We use extensive dynamical simulations and simple arguments to probe the origins of these breathing processes. In the presence of dissipation, the breathing has a lifetime that is inversely proportional to the dissipation constant. The possible use of the concept of nonlinear granular breathing in recovering the energy released at the beaches by surface gravity waves using a system made largely of nonmoving parts is mentioned as a possible application. In closing, studies on the effects of time dependent applied forces will be summarized. [Preview Abstract] 
Tuesday, March 16, 2010 4:18PM  4:30PM 
L12.00010: Spiral patterns in wet granular matter under vertical vibrations Kai Huang , Frank Gollwitzer , Ingo Rehberg From the evolution of galaxy to hurricane, from the inner structure of sea shell to the cochlea of our inner ears, spirals are widely existing in nature. In the past decades, spiral patterns have been discovered and extensively studied in model systems such as RayleighB\'{e}rnard convection, BelousovZhabotinksy reactions and various biological systems. Here we report spiral patterns observed in a thin layer of wet granular matter driven by vertical vibrations. In the phase diagram of driven wet granular matter, spirals appear close to a fluidgas coexistence phase and show hysteresis. The trajectory and rotation velocity of the threearmed spirals are studied as a function of the driving parameters and compared with other model systems. [Preview Abstract] 
Tuesday, March 16, 2010 4:30PM  4:42PM 
L12.00011: Kinetic Energy Fluctuations in Loaded, Confined NonDissipative Granular Chains Yoichi Takato , Surajit Sen We consider confined granular alignments of sizes $N=50,~100$ and $500$. The grains repel according to the nonlinear Hertz potential. The effect of loading is to introduce a harmonic term in addition to the Hertz term in the graingrain potential. We show that in the absence of dissipation, a perturbed granular alignment at zero loading asymptotically relaxes into an equilibriumlike state where the kinetic energy fluctuations can be quite significant. Introducing the harmonic term in the potential tends to eventually suppress fluctuations. The talk shall focus on why this fluctuation suppression occurs. [Preview Abstract] 
Tuesday, March 16, 2010 4:42PM  4:54PM 
L12.00012: Monte Carlo Library Least Square (MCLLS) Method for Multiple Radioactive Particle Tracking in BPR Zhijian Wang , Kyoung Lee , Robin Gardner In This work, a new method of radioactive particles tracking is proposed. An accurate Detector Response Functions (DRF's) was developed from MCNP5 to generate library for NaI detectors with a significant speedup factor of 200. This just make possible for the idea of MCLLS method which is used for locating and tracking the radioactive particle in a modular Pebble Bed Reactor (PBR) by searching minimum Chisquare values. The method was tested to work pretty good in our lab condition with a six 2" X 2" NaI detectors array only. This method was introduced in both forward and inverse ways. A single radioactive particle tracking system with three collimated 2" X 2" NaI detectors is used for benchmark purpose. [Preview Abstract] 
Tuesday, March 16, 2010 4:54PM  5:06PM 
L12.00013: Column Collapse of Rodlike Granular Materials Melissa Trepanier , Scott Franklin We study the collapse of piles of rodlike granular materials, in particular how the particle aspect ratio (length/width) and coefficient of friction affect the runoff. Rod particles can maintain the shape of their container, something round particles cannot, and we find transitional pile heights that determine the onset of collapse. For low aspect ratios, pile heights of less than a particle length do not collapse, implying that vertically oriented rods are anchoring the pile and providing stability. There is a broad transition range of pile heights in which the probability of collapse grows linearly from 0 to 1. The scaling of the runoff distance in and above this region is independent of aspect ratio and friction, depending only on the initial pile geometry. This work could have significant implications for construction of stable structures and understanding avalanches of needlelike snow crystals (hoar). [Preview Abstract] 

L12.00014: ABSTRACT WITHDRAWN 
Tuesday, March 16, 2010 5:18PM  5:30PM 
L12.00015: Molasses Tail in Dense Hard Core Fluids Masaharu Isobe , Berni Alder The long slow decaying potential part of the shearstress autocorrelation function has been called the ``molasses tail'' to differentiate it from the hydrodynamic origin of the long time tail in the velocity autocorrelation function and to emphasize its relation to the highly viscous glassy state [1]. Some twenty years ago, the molasses tail in dense liquids near the solidfluid transition point was speculated to be due to transient crystal nuclei formation [2].This slow decaying process of the OACF and its decomposition (pair, triplet, and quadruplet) is a key factor in understanding the onset of the glass transition. With additional computer power, we are now investigating the origin of the molasses tail using a modern fast algorithm based on eventdriven Molecular Dynamics (MD) simulation.We confirm the nonalgebraic decay (stretched exponential) at intermediate times corresponding to the existence of various cluster sizes a solid cluster at high densities. The decay in dense systems thus consists of a three stage relaxation process, which are the kinetic regime, the molasses regime and the diffusional power regime[3]. [1] B. J. Alder, in Molecular Dynamics Simulation of Statisticalmechanical Systems, G. Ciccotti and W. G. Hoover, eds.(NorthHolland, Amsterdam, 1986) 66. [2] A. J. C. Ladd, and B. J. Alder, J. Stat. Phys. 57, 473 (1989). [3] M. Isobe and B. J. Alder, Mol. Phys., 107, 609 (2009). [Preview Abstract] 
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