Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session P43: Granular Materials 
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Sponsoring Units: GSNP GSOFT Chair: Eric DeGiuli, École Polytechnique Fédérale de Lausanne Room: 346 
Wednesday, March 16, 2016 2:30PM  2:42PM 
P43.00001: A Phase Diagram Unifies Energy Dissipation, Kinetics, and Rheology in Inertial Granular Flows Eric DeGiuli, Jim McElwaine, Matthieu Wyart Flows of hard granular materials depend strongly on the interparticle friction coefficient $\mu_p$ and on the inertial number I, which characterizes proximity to the jamming transition where flow stops. Guided by numerical simulations, we derive the phase diagram of dense inertial flow of spherical particles, finding three regimes for $10^{4} < I < 0.1$: frictionless, frictional sliding, and rolling. These are distinguished by the dominant means of energy dissipation, changing from collisional to sliding friction, and back to collisional, as $\mu_p$ increases from zero at constant I. The three regimes differ in their kinetics and rheology; in particular, the velocity fluctuations and the stress anisotropy both display nonmonotonic behavior with $\mu_p$, corresponding to transitions between the three regimes of flow. We characterize the scaling properties of these regimes, show that energy balance yields scaling relations for each of them, and explain why friction qualitatively affects flow. [Preview Abstract] 
Wednesday, March 16, 2016 2:42PM  2:54PM 
P43.00002: Enhanced Flow of Granular Material George McMurdy, Scott Franklin, Charles Bachmann We study a peculiar, anomalous weakening in wet sand brought about by the addition of small amounts of fine silt. The effect has been observed in uncontrolled field experiments, which we reproduce in the lab. Samples consist of sand from a local state park with a broad grainsize distribution between 300600 microns, to which we add controlled amounts of silt, with a size distribution between 2575 microns. Moisture contents range from 015\% (by mass); we find our samples unable to hold much more than 15\%. Samples are formed into freestanding cylinders and loaded from above until collapse. Mass fraction of silt varies from 020\%, spanning the range observed in coastal sands. Results are compared with dynamic deflection moduli found in the field, and possible mechanisms are discussed. [Preview Abstract] 
Wednesday, March 16, 2016 2:54PM  3:06PM 
P43.00003: Solidliquid like phase transition in a confined granular suspension Nariaki Sakai, Frederic Lechenault, Mokhtar Adda Bedia We present an experimental study of a liquidsolid like phase transition in a twodimensional granular media. Particles are placed in a vertical HeleShow cell filled with a denser solution of cesiumchloride. Thus, when the cell is rotated around its axis, hydrostatic pressure exerts a centripetal force on the particles which confines them towards the center. This force is in competition with gravity, thus by modifying the rotation rate, it is possible to transform continuously and reversibly the sample from a disordered loose state to an ordered packed state. The system presents many similarities with thermal systems at equilibrium like density and interface fluctuations, and the transition between the two phases goes through a coexistence state, where there is nucleation and growth of locally ordered domains which are captured by the correlation function of the hexatic order parameter. We discuss the possibility to extend the grandcanonical formalism to outof equilibrium systems, in order to uncover a state equation between the density and the pressure in the medium. [Preview Abstract] 
Wednesday, March 16, 2016 3:06PM  3:18PM 
P43.00004: Forces and Flows in NonNewtonian Suspensions Melody Lim, Jonathan Bares, Robert Behringer Above a certain solid mass fraction, suspensions of dense granular particles in water exhibit nonNewtonian behavior, including impactactivated solidification. Although it has been suggested that solidification depends on interactions with the suspension boundary, quantitative experiments on the boundary forces have not been reported. In the present experiments, we determine the magnitude and timings of impactordriven events in both the boundaries and body of the suspension using highspeed video, tracer particles, and photoelastic container boundaries. We observe a shocklike propagation in the cornstarch suspension during impact. The dynamics of the shockfront are strongly correlated to those of the intruder. We also observe a second extremely fast shockfront, associated with the propagation of forces to the boundaries of the suspension. The dynamics of this shockfront do not depend on the intruder dynamics, but are correlated to the volume fraction of cornstarch particles in the suspension. The observed shockfront propagates at a speed which is faster than the sound speed in the experiment container. [Preview Abstract] 
Wednesday, March 16, 2016 3:18PM  3:30PM 
P43.00005: Cooling of 3D Granular Gases: Experiments in Microgravity Kirsten Harth, Sandra Wegner, Torsten Trittel, Ralf Stannarius Granular gases are ensembles of macroscopic grains, which move randomly and interact through inelastic collisions. This nonequilibrium statistical system is easy to picture, but still insufficiently understood. Numerous theoretical treatments have been performed, favorably with spherical grains and periodic boundaries, starting from a homogeneous state. Experimentally, such a gas in 3D can only be realized with strong external forcing or in microgravity. We have recently demonstrated that the use of elongated grains facilitates the realization of 3D experiments beyond the Knudsen regime (1). Main findings in a sounding rocket experiment were nonGaussian velocity distributions and a violation of the equipartition of kinetic energy in the steady state. Rotational degrees of freedom are underexcited. When the excitation is stopped, energy is dissipated, the granular gas is "cooling". We present the first quantitative study of the cooling of a granular gas, based on a 3D data evaluation, from drop tower experiments. The evolution of the kinetic energy in translational and rotational degrees of freedom is compared to Haff's law and recent numerical studies. Additionally, we analyze velocity and density distributions.\\ (1) K. Harth et al., Phys. Rev. Lett. 110 144102 (2013) [Preview Abstract] 

P43.00006: ABSTRACT WITHDRAWN 
Wednesday, March 16, 2016 3:42PM  3:54PM 
P43.00007: Flying in a sandstorm: granular flow dynamics around an intruder Yasin Karim, Eric Corwin Using highspeed imaging and direct force measurements, we study the flow dynamics around an intruder in a quasitwo dimensional granular gas. We also vary the geometry of the intruder and explore how changing the curvature, for instance, affects the lift force. For a given angle of attack, an intruder with a straighter side facing the flow experiences higher lift than one with a more convex side. We use particle image velocimetry to measure flow fields and correlate them with our direct force measurements to elaborate on how granular gas flows respond to changes in intruder geometry. [Preview Abstract] 
Wednesday, March 16, 2016 3:54PM  4:06PM 
P43.00008: Collisional Model of the Stopping Force of 3D Granular Impact Cacey Stevens, Jonathan Bares, Robert Behringer A dense granular packing can cause a freefalling intruding object to come to an abrupt stop as its momentum is dissipated to the grains. An empirical force law has been widely accepted to describe this process; it characterizes the stopping force as the sum of depthdependent friction and velocitydependent inertial drag.\footnote{H. Katsuragi et al, Nat. Phys. {\bf 3}, 6 (2007)} However, a complete interpretation of this force, incorporating grainscale interactions during impact, remains unresolved. Here, the momentum transfer is proposed to occur through sporadic collisions with clusters of high forcecarrying grains at the intruder's surface.\footnote{Y. Takehara et al EPL {\bf 92}, 44003 (2010), A. Clark et al, PRE {\bf 89}, 012201 (2014)} To test this model in 3D impact experiments, we determine the forces acting on an intruder decelerating through a dense granular medium using highspeed video of its trajectory. By attaching a rod to the intruder and observing its motion from perpendicular angles, we obtain all translational and rotational dynamics. We vary the shape of the impeding object to infer intrudergrain interactions from its consequent path. As a result, we connect the inertial drag to the effect of intruder shape and rotation based on the collisional model. [Preview Abstract] 
Wednesday, March 16, 2016 4:06PM  4:18PM 
P43.00009: Realtime magnetic resonance imaging of highly dynamic granular phenomena Alexander Penn, Klaas P. Pruessmann, Christoph Müller Probing nonintrusively the interior of threedimensional granular systems is a challenging task for which a number of imaging techniques have been applied including positron emission particle tracking, Xray tomography and magnetic resonance imaging (MRI). A particular advantage of MRI is its versatility allowing quantitative velocimetry through phase contrast encoding and tagging, arbitrary slice orientations and the flexibility to trade spatial for temporal resolution and vice versa during image reconstruction. However, previous attempts to image granular systems using MRI were often limited to (pseudo) steady state systems due to the poor temporal resolution of conventional imaging methodology. Here we present an experimental approach that overcomes previous limitations in temporal resolution by implementing a variety of methodological advances, viz. parallel data acquisition through tailored multiple receiver coils, fast gradient readouts for timeefficient data sampling and engineered granular materials that contain signal sources of high proton density. Achieving a spatial and temporal resolution of, respectively, 2 mm x 2 mm and 50 ms, we were able to image highly dynamic phenomena in granular media such as bubble coalescence and granular compaction waves. [Preview Abstract] 
Wednesday, March 16, 2016 4:18PM  4:30PM 
P43.00010: Investigation of the effect of wall friction on the flow rate in 2D and 3D Granular Flow Brenda CarballoRamirez, Mollie Pleau, Nalini Easwar, Sumit Birwa, Neil Shah, Shubha Tewari We have measured the mass flow rate of spherical steel spheres under gravity in vertical, straightwalled 2 and 3dimensional hoppers, where the flow velocity is controlled by the opening size. Our measurements focus on the role of friction and its placement along the walls of the hopper. In the 2D case, an increase in the coefficient of static friction from $\mu =$0.2 to 0.6 is seen to decrease the flow rate significantly. We have changed the placement of frictional boundaries/regions from the front and back walls of the 2D hopper to the side walls and floor to investigate the relative importance of the different regions in determining the flow rate. Fits to the Beverloo equation show significant departure from the expected exponent of 1.5 in the case of 2D flow. In contrast, 3D flow rates do not show much dependence on wall friction and its placement. We compare the experimental data to numerical simulations of gravity driven hopper granular flow with varying frictional walls constructed using LAMMPS*. $^{\mathrm{\ast }}$http://lammps.sandia.gov [Preview Abstract] 
Wednesday, March 16, 2016 4:30PM  4:42PM 
P43.00011: Normal coefficient of restitution of wet particles Kai Huang, Thomas Mueller, Ingo Rehberg The normal coefficient of restitution (COR) for a spherical particle bouncing on a wet surface is investigated experimentally. The dependence of the COR on the impact velocity and various particle and liquid properties will be presented and discussed in terms of dimensionless numbers that characterize the interplay between inertial, viscous, and surface forces. At a fixed ratio of the liquid film thickness $\delta$ to the particle diameter $D$, the wet COR is found to be inverse proportional to the Stokes number $St$, which measures the inertia of the particle to the viscous force from the liquid. This relation provides a convenient way of predicting wet COR with two fit parameters. For two different types of particles, we vary systematically the dimensionless film thickness $\delta/D$ and discuss its influence on the fit parameters. Finally, we rationalize the observations with a model that considers possible sources of energy dissipation associated with a wet impact. [Preview Abstract] 
Wednesday, March 16, 2016 4:42PM  4:54PM 
P43.00012: Longterm behavior of granular chains held between walls is really equilibrium. Michelle Przedborski, Surajit Sen, Thad Harroun Granular chains have been the focus of a number of studies, in part due to their numerous applications, ranging from shock absorption and vibration reduction to energy localization. Force impulses to an unloaded granular chain result in a propagating solitary wave (SW), analogous to a soliton of the Kortewegde Vries equation. When SWs collide with a boundary or another SW, secondary solitary waves (SSWs) are produced as grains break contact. A consequence of this process is the transition from a nonergodic, SW dominant, phase to the stable ``quasiequilibrium'' (QEQ) phase, thought to be distinct from true thermodynamic equilibrium due to the absence of equipartitioning of energy. We show that, in the absence of energy dissipation, when granular systems are allowed to evolve to extremely long times, the number of SSWs becomes sufficiently large that the system actually approaches a true equilibrium phase. In this extremetime limit, energy in fact becomes equipartitioned among all grains, and we illustrate how the specific heat and kinetic energy fluctuations can be predicted by the generalized equipartition theorem, regardless of the degree of the interaction potential. This opens up the possibility that granular systems should be treated by equilibrium statistical mechanics. [Preview Abstract] 
Wednesday, March 16, 2016 4:54PM  5:06PM 
P43.00013: Experimental observations of root growth in a controlled photoelastic granular material Serge Mora, Jonathan Bares, JeanYves Delenne, Thierry Fourcaud The mechanism of root growth in soil is a key issue to understand both how to improve plant development and how to stabilize grounds. However, no experimental studies have been carried out to directly observe root development and surrounding stress while imposing specific grain configurations or mechanical loading. We present a novel setup which permits to observe the development of chickpea root networks in a 2D granular material made of bidisperse photoelastic discs while imposing the position of the grains, the intergranular spacing and the nature of the system confinement: (i) open cell, (ii) confined cell (iii) sheared cell. In the experimental apparatus several root development cells are treated in parallel to increase the statistical meaning of the observations. Evolution of the root network is followed as well as position and pressure inside each disc by mean of a camera and classical photoelastic techniques. Preliminary results will be presented. [Preview Abstract] 
Wednesday, March 16, 2016 5:06PM  5:18PM 
P43.00014: Topological interlocking provides stiffness to stochastically microcracked materials beyond the transport percolation limit. Anirban Pal, Catalin Picu, Marian V. Lupulescu We study the mechanical behavior of twodimensional, stochastically microcracked continua in the range of crack densities close to, and above the transport percolation threshold. We show that these materials retain stiffness up to crack densities much larger than the transport percolation threshold, due to topological interlocking of sample subdomains. Even with a linear constitutive law for the continuum, the mechanical behavior becomes nonlinear in the range of crack densities bounded by the transport and stiffness percolation thresholds. The effect is due to the fractal nature of the fragmentation process and is not linked to the roughness of individual cracks. We associate this behavior to that of itacolumite, a sandstone that exhibits unusual flexibility. [Preview Abstract] 
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