Bulletin of the American Physical Society
75th Annual Meeting of the Southeastern Section of APS
Volume 53, Number 13
Thursday–Saturday, October 30–November 1 2008; Raleigh, North Carolina
Session CA: Granular Physics |
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Chair: Karen Daniels, North Carolina State University Room: Holiday Inn Brownstone Roosevelt |
Thursday, October 30, 2008 10:45AM - 11:15AM |
CA.00001: Jamming in Granular Systems Invited Speaker: Granular materials exist all around us, from the formation of sand dunes and collapse of seemingly stable grain silos, to the mixing of pharmaceuticals and other industrial materials. The behavior of these ``fluids'' though poorly understood. Their flow can be characterized by the continuous forming and breaking of a strong force network resisting flow. This jamming/unjamming behavior is typical of a variety of systems, including granular flows, and is influenced by factors such as grain packing fraction, applied shear stress, and the random kinetic energy of the particles. We present experiments on quasi-static shear and free-surface granular flows under the influence of external vibrations. By using photoelastic grains, we are able to measure particle trajectories and the local force network in these 2D flows. We find that during shear, sufficient shaking weakens the strong force network and reduces the amount of flow driven by sidewalls. We vibrate either the driving wall (sidewall forcing) or the entire shearing zone (bulk forcing). For sidewall forcing, flow behavior is controlled by vibration amplitude in particular and slipping of force chains at the boundary. In a rotating drum geometry, we find that small vibration leads to strengthening of the pile while larger vibrations induce failure as might be expected. This behavior is strongly history dependent and sufficient vibration erases the memory of the pile. [Preview Abstract] |
Thursday, October 30, 2008 11:15AM - 11:45AM |
CA.00002: Dense dry granular material under pure shear and shear reversal Invited Speaker: We have performed 2D granular experiments under pure shear using bidisperse photo-elastic disks. Starting from a stress free state, a square box filled with granular particles is subject to shear with the area fixed. The forward shears involved various number of steps, leading to maximum strains between 0.1 and 0.3. Reverse shear was then applied. Depending on the packing fraction and the strain, the force chain network built prior to the shear reversal may be destroyed completely or partially destroyed. If the reverse shear is continuously applied, there is a force chain strengthening. Following each change of the system, contact forces of individual disks were measured. We also kept track of the displacement and rotation of every particle. I will present the results for the structure failure and reconstruction during shear reversals. I will also present results from a physical experiment and a DEM simulation. Particular attention is paid to the deformation and dissipation within a class of particle clusters found to be confined to the shear band, each comprising of a buckled force chain segment and its laterally supporting neighbors. The predominant mode of contact failure in a force chain undergoing buckling, and in the contacts with and within its laterally supporting neighbors, is frictional rolling. [Preview Abstract] |
Thursday, October 30, 2008 11:45AM - 12:15PM |
CA.00003: Experimental investigation of state variables in a dense granular layer Invited Speaker: Stationary states in dense granular systems lack a predictive statistical description, as kinetic theory approaches fail when the interactions significantly deviate from binary collisions. In particular, because of the degeneracy of geometric states due to friction forces, it has been argued that a comprehensive theory of such dense granular systems must incorporate additional state variables associated with constraint fluctuations. We investigate the relevance of various ensembles in a dense mixture of disks laid on a horizontal air table and driven into steady states by random kicks at the boundaries. We study how microscopically defined intensive parameters affect the macroscopic response of the system, and clarify the equilibration properties of these parameters. In collaboration with Karen Daniels, North Carolina State University. [Preview Abstract] |
Thursday, October 30, 2008 12:15PM - 12:45PM |
CA.00004: Entropy driven patterning in vibrofluidized granular materials Invited Speaker: We investigate pattern formation in vertically vibrofluidized rod and sphere granular mixtures confined to quasi-2D containers. In a pure rod system, crowding induces an isotropic-to-nematic phase transition. The inclusion of spheres destabilizes both the isotropic and nematic states. Instead, small independent rod bundles form, where the long axes of rods are closely approximated. To investigate the role of entropy maximization in the granular experiments, we performed strict-2D equilibrium Monte Carlo simulations of hard rods and spheres and found analogous patterning. Similarities and differences between the steady state experiments and equilibrium simulations will be discussed. In collaboration with Daniel Harries, The Hebrew University of Jerusalem. [Preview Abstract] |
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