Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session N36: Focus Session: Granular Liquids and Gases II |
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Sponsoring Units: GSNP DFD DCMP Chair: Arshad Kudrolli, Clark University Room: LACC 510 |
Wednesday, March 23, 2005 8:00AM - 8:12AM |
N36.00001: Structure formation in electromagnetically driven magnetic granular media Alexey Snezhko, Igor Aranson, Wai-Kwong Kwok We report structure formation in submonolayers of magnetic microparticles subjected to periodic electrostatic and magnetic excitations. Depending on the excitation parameters, rich variety of structures: clusters, rings, chains, and networks can be generated in the system. The growth dynamics and shapes of the structures are strongly dependent on the amplitude and frequency of the external magnetic field. It was found that for pure ac magnetic driving at low densities of particles, the low-frequency magnetic excitation favors clusters while high frequency excitation favors chains and net-like structures. An abrupt phase transition from chains to a network phase with the frequency of external ac magnetic filed was observed for a high density of particles. [Preview Abstract] |
Wednesday, March 23, 2005 8:12AM - 8:24AM |
N36.00002: Nematic, Smectic and Possibly Tetratic Steady States in Agitated Monolayers of Rods Vijay Narayan, Narayanan Menon, Sriram Ramaswamy We present experimental results on the nonequilibrium phase diagram and dynamics of a vertically vibrated monolayer of rodlike particles (diameter d from 0.5 to 1 mm) lying horizontally in a quasi-2d cell of height $<$ 2d. With increasing area fraction, rods with aspect ratio $\sim$12 form nematics and, possibly, translationally ordered phases. Rods with aspect ratio $\sim$5 form striped phases instead. While these results agree with thermal equilibrium simulations [Bates and Frenkel, (2000), Lagomarsino et al. (2003), Khandkar and Barma (unpublished)], some clear nonequilibrium signatures are observed, including global, systematic rotation of the ordered phase in response to weak asymmetries in the sample cell. To find nematics and smectics it was crucial to taper the tips of the rods, without which only tetratic correlations were seen. We will present comparisons with the theory of active nematics [EPL 62 (2003) 196-202], and discuss the possibility of long-ranged nematic and quasi-long-ranged smectic order in these nonequilibrium 2d systems. [Preview Abstract] |
Wednesday, March 23, 2005 8:24AM - 8:36AM |
N36.00003: Bouncing dimer Stephane Dorbolo, Dmitri Volfson, Lev Tsimring, Arshad Kudrolli A dimer composed of centimetric beads have been excited on a vibrated plate. The motion of the beads have been recorded. Four excited modes have been observed for accelerations of the plate below the gravity. By tuning the amplitude of the vibration of the plate, the dimer changes from one energetic mode to another. These transitions are discrete and depend on the initial conditions. Moreover the first excited mode has a novel horizontal drift in which one end of the dimer stays on the plate during most of the cycle, while the other end bounces in phase with the plate. The speed and direction of the drift depend on the aspect ratio of the dimer. [Preview Abstract] |
Wednesday, March 23, 2005 8:36AM - 8:48AM |
N36.00004: First-order phase transition in a 2D granular fluid Rohit Ingale, Mark Shattuck We experimentally examine first-order freezing/melting phase transition in a non-equilibrium system --- a vertically oscillated two-dimensional isobaric granular fluid. The steady state transition occurs between a gas and a crystal and is characterized by discontinuous change in both density and temperature. The phase transition also shows rate dependent hysteresis. The hysteresis disappears for sufficiently slow heating rate producing a reversible transition characterized by single curve as in a first order phase transition in ordinary fluids. We also study the effect of pressure variation and number of particles on the phase transition and hysteresis. We further probe the system in the vicinity of the transition point to study the coexistence between the low temperature crystal and a high temperature gas and subsequent transition between the two states. The results obtained provide better understanding of the specifics of phase transition in granular fluids. [Preview Abstract] |
Wednesday, March 23, 2005 8:48AM - 9:00AM |
N36.00005: Phase diagram of vibrated granular media confined between two parallel plates Paul Melby, Francisco Vega Reyes, David A. Egolf, Jeffrey S. Urbach We present the results from simulations and experiments of vibrated granular media which are confined between two parallel plates. Depending on the density and gap spacing, we see solid phases with hexagonal or square symmetry, or zig-zag `buckled' phases. This phase behavior is remarkably similar to the phase behavior of similarly confined hard-sphere colloidal suspensions in equilibrium. In the case of colloids, the phase diagram is determined through entropy maximization and depends only on the gap between the confining plates and the density of colloidal particles. For the granular system, however, there are modifications of the phase diagram which are caused by the presence of forcing and dissipation in the system. In particular, we find that the solid-liquid coexistence region in the granular system is much larger than in the equilibrium system. This difference is a direct consequence of the lack of equipartition between the coexisting phases. [Preview Abstract] |
Wednesday, March 23, 2005 9:00AM - 9:12AM |
N36.00006: Measurements of Grain Motion in a Bubbling Fluidized Granular bed Chao Huan, Kevin Facto, Donald Candela, Ronald Walsworth, Ross Mair The fluctuating motions of grains in a bubbling gas-fluidized granular bed are measured using NMR. Most experimental results, including ours, indicate that a gas-fluidized granular bed is only truly fluid in the bubbling state which occurs at high gas flow rates. In this state large grain-free voids (bubbles) rise through the bed and activate motion at smaller scales throughout the rest of the bed, which remains dense. Thus the fluidized bed is an inherently complex, multi-scale granular flow state. Using NMR we are able to measure the distributions of horizontal and vertical grain motions over time scales from one to several hundred milliseconds, and also to probe correlations of the motion over successive time intervals. As a function of observation time crossovers are observed that appear to delineate grain-scale diffusion as well as coherent and stochastic convection of the granular fluid. [Preview Abstract] |
Wednesday, March 23, 2005 9:12AM - 9:48AM |
N36.00007: Energy Cascades in Granular Gases Invited Speaker: A new class of stationary states in granular gases where energy is transfered from large velocity scales to small velocity scales is found. These steady-states exist for arbitrary collision rules and arbitrary dimension. Their signature is a velocity distribution $f(v)$ with an algebraic high-energy tail, $f(v)\sim v^{-\sigma}$. The exponent $\sigma$ is obtained analytically and it varies continuously with the spatial dimension, the homogeneity index characterizing the collision rate, and the restitution coefficient. These stationary states are realized in numerical simulations in which energy is injected into the system by infrequently boosting particles to high velocities. It is proposed that these states may be realized experimentally in driven granular systems. [Preview Abstract] |
Wednesday, March 23, 2005 9:48AM - 10:00AM |
N36.00008: Stress and Velocity Correlations in Granular Flow Bidita J. Tithi, Shubha Tewari, Allison Ferguson, Bulbul Chakraborty We report on studies of gravity-driven, dense, granular flow via simulations of two-dimensional, inelastic, bidisperse hard disks in a vertical tube geometry. We have previously reported (Europhys. Lett. \textbf{66}, 277 (2004)) the formation of linear chain-like structures of particles undergoing frequent collisions. In order to understand the effect of these transient structures on the long-wavelength behavior of the system, we have analyzed the flow in terms of coarse-grained velocity and stress fields, and their two-point correlation functions. We find that spatial correlation of the stress increases only modestly as the flow rate decreases, yet this leads to a marked increase in the spatial correlation of the velocity. This reinforces the idea that a small fraction of the particles can play an important role in the kinematics of flow. We present data on the change in the length scale of correlations as jamming is approached. We further analyze the flow in terms of four-point correlation functions of the stress analogous to those used to characterize dynamical heterogeneities in supercooled liquids. [Preview Abstract] |
Wednesday, March 23, 2005 10:00AM - 10:12AM |
N36.00009: Velocity Distributions of Granular Gases with Long-Range Interactions Kevin Kohlstedt, Alexei Snezhko, Maxim Sapozhnikov, Igor Aranson, Jeffrey Olafsen, Eli Ben-Naim We study velocity statistics of electrostatically driven granular gases with long-range interactions. Our experiments involve anisotropic dipole forces between particles due to either magnetic or hydrodynamic interactions. Generally, the velocity distribution is non-Maxwellian, and its high-energy tail has a stretched exponential form $P(v)\sim \exp\left(-|v|^\xi\right)$. We find a simple exponential tail, $\xi=1$, for long-range dipole interactions, whereas $\xi=3/2$ for short-range, hard-core interactions. This behavior is consistent with kinetic theory of driven dissipative particles. We conclude that velocity statistics of dissipative gases are sensitive to the form of the particle interaction. [Preview Abstract] |
Wednesday, March 23, 2005 10:12AM - 10:24AM |
N36.00010: Granular Jets: The Surprising Role of Air Pressure John Royer, Andrew Flior, Eric Corwin, Heinrich Jaeger We report on the symmetric, focused jet formed by a solid sphere impacting a loosely packed, dry granular powder. Similar jets have been observed in liquids and studied in detail. However, it is surprising that such a jet can form in sand, where there is little to no attractive potential between the individual particles and no surface tension. A model of the jet formation has been proposed \footnote{S. Thoroddsen and A. Q. Shen. Phys. Fluids {\bf 13}, 4 (1996).} \footnote{D. Lohse et. al., Phys. Rev. Lett. {\bf 93}, 198003 (2004). } in which the jet is the result of the uniform collapse of the void left by the impacting sphere due to 'hydrostatic' pressure. Experimentally, we find that there is a very strong {\it air} pressure dependance in the dynamics of the jet. Instead of a single jet, we observe two jets: a small, thin jet that is not strongly affected by the air pressure followed by a second jet whose size and velocity depend strongly on the air pressure. This air pressure dependance indicates that hydrostatic models based solely on void collapse may not completely capture nature of the phenomena. [Preview Abstract] |
Wednesday, March 23, 2005 10:24AM - 10:36AM |
N36.00011: Impact induced splash and spill in a quasi-confided granular medium S. B. Ogale, S. R. Shinde, P. A. Karve, Abhijit S. Ogale, Aparna Kulkarni, Aditi Athawale, Anagha Phadke, Rajashri Thakurdas, S. N. Kale Dissipation of the energy of impact in a granular medium and its effects has been a subject of considerable scientific for quite some time. In this work we have explored and analyzed the splash and spill effects caused by the impact of a ball dropped from a height into a granular medium in a open container. Three different granular media, namely rice, mustard seeds, and cream of wheat were used. The amount of spilled-over granular matter was measured as a function of the ball-drop height. Digital pictures of the splash process were also recorded. The quantity of spilled granular matter varies linearly with the impact energy. However additional step like structures are also noted. Specifically, a distinct and large jump is seen in the spilled quantity at a specific impact energy in the case of mustard seeds, which also exhibit obvious charging effects and repulsion. Although the parameters such as mass per grain and packing density for the case of mustard seeds are intermediate between those for rice and cream of wheat, the spill quantity for comparable impact energy is considerably higher. These data will be presented and discussed. [Preview Abstract] |
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