Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session T44: Focus Session: Granular Materials and Continuum Descriptions of Discrete Media II |
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Sponsoring Units: GSNP GSOFT Chair: Jonathan Bares, Duke University Room: 214D |
Thursday, March 5, 2015 11:15AM - 11:27AM |
T44.00001: ABSTRACT WITHDRAWN |
Thursday, March 5, 2015 11:27AM - 11:39AM |
T44.00002: ABSTRACT WITHDRAWN |
Thursday, March 5, 2015 11:39AM - 11:51AM |
T44.00003: Shear of ordinary and elongated granular mixtures Alexander Hensley, Matthew Kern, Theodore Marschall, Stephen Teitel, Scott Franklin We present an experimental and computational study of a mixture of discs and moderate aspect-ratio ellipses under two-dimensional annular planar Couette shear. Experimental particles are cut from acrylic sheet, are essentially incompressible, and constrained in the thin gap between two concentric cylinders. The annular radius of curvature is much larger than the particles, and so the experiment is quasi-2d and allows for arbitrarily large pure-shear strains. Synchronized video cameras and software identify all particles and track them as they move from the field of view of one camera to another. We are particularly interested in the global and local properties as the mixture ratio of discs to ellipses varies. Global quantities include average shear rate and distribution of particle species as functions of height, while locally we investigate the orientation of the ellipses and non-affine events that can be characterized as shear transformational zones or possess a quadrupole signature observed previously in systems of purely circular particles. Discrete Element Method simulations on mixtures of circles and spherocylinders extend the study to the dynamics of the force network and energy dissipated as the system evolves. [Preview Abstract] |
Thursday, March 5, 2015 11:51AM - 12:03PM |
T44.00004: Maxwell Construction for a Nonequilibrium Steady-State Phase Separation in Granular Matter Marco G. Mazza, James Clewett, Jack Wade, Roger Bowley, Stephan Herminghaus, Michael Swift Experiments and computer simulations are carried out to investigate phase separation in a granular gas under external vibration in a large sample cell. The densities of the dilute and the dense phase are found to follow a lever rule, suggesting an equation of state. We show that this equation of state, which exhibits a non-monotonic pressure-volume characteristic, $P(v)$, can be obtained from simulations of a small cell. A Maxwell construction is found to predict both the coexisting pressure and binodal densities remarkably well, despite the fact that $P(v)$ is not an isotherm. Although the system is far from equilibrium and energy conservation is strongly violated, we can derive this finding from an energy minimization argument of uctuating currents. [Preview Abstract] |
Thursday, March 5, 2015 12:03PM - 12:15PM |
T44.00005: Electrical charging in shaken granular media Freja Nordsiek, Daniel Lathrop Collisional electrification of granular particles and the resulting electric fields are seen but poorly understood in sand storms, volcanic ash clouds, thunderstorms, and thundersnow. We present results on the electrical charging of granular media (100 micron to 1 mm in size) shaken between two conducting plates. The voltage between the plates was measured. We saw particle electrification through capacitive coupling with the plates and electrical discharges for a diverse class of materials: polystyrene (polymer), soda-lime glass (glass), 69\%:31\% ZrO$_2$:SiO$_2$ (ceramic), and aluminum (metal). We found 1) a monotonic increase in charging with shaking strength, 2) a threshold in the number of particles to see charging of about the number of particles needed to form a monolayer on the plate, 3) material and diameter differences causing an order of magnitude spread in measured signal but little difference between mono-material sets with one size range and bi-material and/or bi-size range set combinations, and 4) long time scale transients. We argue that while two-body collisions and the physical properties of the particles (material and size) are relevant, collective phenomena are a necessary part of explaining natural charging of granular flows. [Preview Abstract] |
Thursday, March 5, 2015 12:15PM - 12:27PM |
T44.00006: Complex Kepler Orbits and Particle Aggregation in Charged Microscopic Grains Victor Lee, Scott Waitukaitis, Marc Miskin, Heinrich Jaeger Kepler orbits are usually associated with the motion of astronomical objects such as planets or comets. Here we observe such orbits at the microscale in a system of charged, insulating grains. By letting the grains fall freely under vacuum, we eliminate the effects of air drag and gravity, and by imaging them with a co-falling high-speed camera we track the relative positions of individual particles with high spatial and temporal precision. This makes it possible to investigate the behaviors caused by the combination of long-range electrostatic interactions and short-range, dissipative, contact interactions in unprecedented detail. We make the first direct observations of microscopic elliptical and hyperbolic Kepler orbits, collide-and-capture events between pairs of charged grains, and particle-by-particle aggregation into larger clusters. Our findings provide experimental evidence for electrostatic mechanisms that have been suspected, but not previously observed at the single-event level, as driving the early stages of particle aggregation in systems ranging from fluidized particle bed reactors to interstellar protoplanetary disks. Furthermore, since particles of different net charge and size are seen to aggregate into characteristic spatial configurations, our results suggest new possibilities for the formation of charge-stabilized ``granular molecules''. We can reproduce the observed molecule configurations by taking many-body, dielectric polarization effects into account. [Preview Abstract] |
Thursday, March 5, 2015 12:27PM - 12:39PM |
T44.00007: A nonlinear feedback model for granular and surface charging Troy Shinbrot, Leo Kozachkov, Theo Siu Independent laboratories have experimentally demonstrated that identical materials brought into symmetric contact generate contact charges. Even the most basic features of this odd behavior remain to be explained. In this talk, we provide a simple, Ising-like, model that appears to account for many of the observed phenomena. We calculate the electric field acting on surface molecules in a lattice, and we show that if the molecules are polarizable, then infinitesimal random polarizations typically build exponentially rapidly in time. These polarizations self-assemble to produce surface patterns that come in two types, and we find that one of these types accounts for strong localized charging, while the other produces a weaker persistent surface charge pattern. We summarize predictions for both ideal surfaces and for defects in granular beds. [Preview Abstract] |
Thursday, March 5, 2015 12:39PM - 12:51PM |
T44.00008: Normal and Tangential Coefficient of Restitution Measurements in an Inelastic Billiard Experiment Jeffrey Olafsen, Martin Martinez Driven granular media generate a large amount of dissipation in their particle-particle and particle-boundary interactions. As such, our understanding of the fundamental dynamics in these systems is complicated by the velocity-dependent nature of the coefficient of restitution of these interactions. Indeed, how a driven granular flow jams also necessitates a better understanding of the details of this dissipative mechanism. A large number of very sophisticated experiments have sought to better understand and predict the velocity dependence of the coefficient of restitution by trying to constrain and control aspects of the particle-particle or particle-boundary collisions. Here, a careful and in-depth analysis from previously published results [1] for an inelastic billiard moving within a confining boundary allows the velocity-dependence to be measured as the dynamics freely evolve over multiple collisions in the driven system. The large amount of data generated in this experiment allows the contributions from both the normal and tangential velocity components in the particle-boundary interactions to be examined. Two derivative experiments, one for particle-boundary and the other for particle-particle collisions will also be discussed. \\[4pt] [1] S. Feldt and J.S. Olafsen. ``Inelastic Gravitational Billiards.'' \textit{Physical Review Letters} (2005): 224102. [Preview Abstract] |
Thursday, March 5, 2015 12:51PM - 1:03PM |
T44.00009: Onset of silo collapse under gravity-driven granular discharges Claudia Colonnello, Gustavo Guti\'errez, Leonardo Reyes, Fabian Brau, Eric Cl\'ement Thin walled silos exhibit a critical filling height, $L_c$, above which the lateral wall buckles as a consequence of the frictional forces exerted by the grains during the granular discharge, producing a catastrophic failure of the structure. We use laboratory scale silos made of paper to study this process experimentally. Based on the observation of the deformation pattern that develops on the silo wall during the discharge, we have proposed a criterion for determining the time of onset of collapse, allowing us to study the conditions under which the collapse is triggered. In particular, we study the behavior of the grains in contact with the wall during the time interval before failure occurs and find that, according to this criterion, the collapse is triggered before a maximal mobilization of the grain-to-wall effective friction force is reached. This can be related to a theoretical model which treats the silo as a thin cylindrical shell subjected to an axial stress with the profile predicted by Janssen's model for the stresses in a silo filled with a granular material. This model predicts correctly the experimental scaling of $L_c$ with various parameters of the system. [Preview Abstract] |
(Author Not Attending)
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T44.00010: Effect of system and particle properties on closure relations for granular segregation models Abhinendra Singh, D. R. Tunuguntla, A.R. Thornton In recent years, much effort has been made on developing valid constitutive laws for continuum models to describe kinetic sieving driven segregation in granular flows over inclined channels. Surprisingly, the existing closure relations for such continuum models have not considered factors such as particle contact stiffness, coefficient of restitution etc. Using Discrete Element method simulations, we investigate the effects of these factors on particle segregation and thereby formulate a constitutive law which takes particle properties into account. Additionally, apart from studying the effects of particle properties on segregating flows, we investigate the effects of gravity on our granular system. We consider a varied range of gravity and find that rate of segregation, for bidisperse mixtures varying in size alone, is proportional to the square root of gravity which is often assumed but was never validated. To be more precise concerning the effects of varying gravity on the steady states of bidisperse flows, varying in size alone, we investigate how the Peclet number (ratio of the segregation rate to diffusion) is affected. [Preview Abstract] |
Thursday, March 5, 2015 1:15PM - 1:27PM |
T44.00011: Formation and properties of a dynamic suspension Tess Homan, Val\'erie Vidal, Sylvain Joubaud We experimentally study the behaviour of an immersed granular bed in a Hele-Shaw cell when perturbed by an airflow from a single inlet at the bottom. When the particles are slightly heavier than the liquid, the competition between particles being dragged up into the liquid and particles settling due to gravity results in a dynamic suspension. In the stationary regime, part of the initial granular bed never moves, and forms a so-called ``dead zone.'' We investigate its shape and extent as a function of the parameters (air flow-rate, initial grains and liquid height). We also focus on the formation and properties of the dynamic suspension by optically recording the full particle density field in the Hele-Shaw cell. The mean density and density fluctuations of the dynamic suspension are studied as a function of the gas flow-rate and the ratio between the amount of grains and liquid. [Preview Abstract] |
Thursday, March 5, 2015 1:27PM - 1:39PM |
T44.00012: Raindrop impact on sand: dynamic and crater formation Song-Chuan Zhao, Rianne de Jong, Devaraj van der Meer Droplet impact on a granular bed is very common in nature, industry, and agriculture and extends from raindrops falling on earth to wet granulation in the production process of many pharmaceuticals. In contrast to more traditionally studied impact phenomena, such as a droplet impact on solid substrate and solid object impact on fluid-like substrate, raindrop impact on sand induces more complicated interactions. First, both the intruder and the target deform during impact; second, the liquid composing the droplet may penetrate into the substrate during the impact and may, in the end, completely merge with the grains. These complex interactions between the droplet intruder and the granular target create the very diverse crater morphologies that has been described in the literature. An appealing and natural question is how the craters are formed. To gain insight in the mechanism of crater formation, we resolve the dynamics with high-speed laser profilometry and study the dependence of the dynamics on impact speed and packing fraction of the granular substrate. Finally, we establish a dynamical model to explain the various crater morphologies. [Preview Abstract] |
Thursday, March 5, 2015 1:39PM - 1:51PM |
T44.00013: Granular impact cratering by liquid drops: Understanding raindrop imprints through an analogy to asteroid strikes Xiang Cheng, Runchen Zhao, Qianyun Zhang, Hendro Tjugito When a granular material is impacted by a sphere, its surface deforms like a liquid yet it preserves a circular crater like a solid. Although the mechanism of granular impact cratering by solid spheres is well explored, our knowledge on granular impact cratering by liquid drops is still very limited. Here, by combining high-speed photography with high-precision laser profilometry, we investigate liquid-drop impact dynamics on granular surface and monitor the morphology of resulting impact craters. Surprisingly, we find that, despite the enormous energy and length difference, granular impact cratering by liquid drops follows the same energy scaling and reproduces the same crater morphology as that of asteroid impact craters. Inspired by this similarity, we integrate the physical insight from planetary sciences, the liquid marble model from fluid mechanics and the concept of jamming transition from granular physics into a simple theoretical framework that quantitatively describes all the main features of liquid-drop imprints in granular media. Our study sheds light on the mechanisms governing raindrop impacts on granular surfaces and reveals a remarkable analogy between familiar phenomena of raining and catastrophic asteroid strikes. [Preview Abstract] |
Thursday, March 5, 2015 1:51PM - 2:03PM |
T44.00014: Kinetics of Gravity-Driven Water Channels under Steady Rain Remi Dreyfus, Cesare Cejas, Remi Barrois, Yuli Wei, Christian Fretigny, Douglas Durian We investigate the physical mechanisms that govern the formation of water channels that develop from finger instabilities at the wetting front. Using controlled experiments in a quasi-2D cell and varying physical parameters (particle size, fluid viscosity, etc.), we simulate rainfall and characterize the homogeneous wetting front as well as channel size and estimate relevant time scales associated with the instability as well as channel velocity. We validate the results by developing a model based on linear-stability analysis with the addition of another term describing the homogenization of the wetting front. This shows that the way we introduce a fluid into a granular medium affects the formation of water channels. Results permit us to calculate the ideal flow rate for maximizing water distribution and minimizing runoffs using granular and fluid properties. [Preview Abstract] |
Thursday, March 5, 2015 2:03PM - 2:15PM |
T44.00015: Splash Suppression by Solvent Viscosity in Dense Suspension Impact Wendy Zhang, Kevin Dodge, Ivo Peters, Martin Klein Schaarsberg, Heinrich Jaeger When a dense suspension droplet impacts a hard surface, it will either break apart (``splash") or remain in a compact configuration without ejecting any particles. We use experiments and discrete particle simulations in which relative particle motions are penalized by lubrication-flow drag to analyze the influence of solvent viscosity on splashing. We find that suspension splash is driven by particle inertia. It can be suppressed in 2 different ways. At low solvent viscosity, lubrication drag due to viscous flow has a negligible effect. Splash is suppressed by surface tension overcoming particle inertia. At high solvent viscosity, lubrication drag alone suppresses splashing. Because impact produces an expanding flow that stretches the suspension radially, suppression in the high-viscosity regime is largely accomplished by lubrication-flow drag preventing initially nearby particle pairs from separating fully. Energy dissipation by viscous flow during collisions plays a smaller role. [Preview Abstract] |
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