We report on systematic measurements of the distribution of normal forces exerted by granular material under uniaxial compression onto the interior surfaces of a confining vessel. Our experiments on three-dimensional, random packings of monodisperse glass beads show that this distribution is nearly uniform for forces below the mean force and decays exponentially for forces greater than the mean. The shape of the distribution and the value of the exponential decay constant are unaffected by changes in the system preparation history or in the boundary conditions. An empirical functional form for the distribution is proposed that provides an excellent fit over the whole force range measured and is also consistent with recent computer simulation data.

Granular materials and ordinary fluids react differently to shear stresses. Rather than deforming uniformly, materials such as dry sand or cohesionless powders develop shear bands--narrow zones of large relative particle motion, with essentially rigid adjacent regions. Because shear bands mark areas of flow, material failure and energy dissipation, they are important in many industrial, civil engineering and geophysical processes. They are also relevant to lubricating fluids confined to ultrathin molecular layers. However, detailed three-dimensional information on motion within a shear band, including the degree of particle rotation and interparticle slip, is lacking. Similarly, very little is known about how the microstructure of individual grains affects movement in densely packed material. Here we combine magnetic resonance imaging, X-ray tomography and high-speed-video particle tracking to obtain the local steady-state particle velocity, rotation and packing density for shear flow in a three-dimensional Couette geometry. We find that key characteristics of the granular microstructure determine the shape of the velocity profile.

We present a combined experimental and theoretical study of memory effects in vibration-induced compaction of granular materials. In particular, the response of the system to an abrupt change in shaking intensity is measured. At short times after the perturbation a granular analog of aging in glasses is observed. Using a simple two-state model, we are able to explain this short-time response. We also discuss the possibility for the system to obey an approximate pseudo-fluctuation-dissipation theorem relationship and relate our work to earlier experimental and theoretical studies of the problem.

We report on experiments to measure the temporal and spatial evolution of packing arrangements of anisotropic, cylindrical granular material, using high-resolution capacitive monitoring. In these experiments, the particle configurations start from an initially disordered, low-packing-fraction state and under vertical vibrations evolve to a dense, highly ordered, nematic state in which the long particle axes align with the vertical tube walls. We find that the orientational ordering process is reflected in a characteristic, steep rise in the local packing fraction. At any given height inside the packing, the ordering is initiated at the container walls and proceeds inward. We explore the evolution of the local as well as the height-averaged packing fraction as a function of vibration parameters and compare our results to relaxation experiments conducted on spherically shaped granular materials.

Experimental measurements of particle dynamics on the lower surface of a 3D Couette cell containing monodisperse spheres are reported. The average radial density and velocity profiles are similar to those previously measured within the bulk and on the lower surface of the 3D cell filled with mustard seeds. Observations of the evolution of particle velocities over time reveal distinct motion events, intervals where previously stationary particles move for a short duration before jamming again. The cross-correlation between the velocities of two particles at a given distance r from the moving wall reveals a characteristic lengthscale over which the particles are correlated. The autocorrelation of a single particle's velocity reveals a characteristic timescale τ which decreases with distance from the inner moving wall. This may be attributed to the increasing rarity at which the discrete motion events occur and the reduced duration of those events at large r. The relationship between the RMS azimuthal velocity fluctuations, δv θ (r), and average shear rate,γ(r), was found to be δv θ ∝γ α with α = 0.52 ± 0.04. These observations are compared with other recent experiments and with the modified hydrodynamic model recently introduced by Bocquet et al. INTRODUCTIONThe detailed understanding of slow flow in dense granular systems has remained one of the central challenges in the field of granular materials [1]. While fast dilute granular flows are fluid-like and can be described well by granular kinetic theory [2][3][4], slow flows at high packing fraction preserve many of the complex properties of static granular packs and may be more accurately described as slow, plastic deformation of a metastable granular solid than flow of a fluid. Recent work by Howell and Behringer [5,6] showed that many of the intriguing properties of granular solids, such as the broad distribution of stresses and the presence of "force chains" which focus stresses along paths of many connected particles, play a crucial role. Direct visualization revealed that the flow is intermittant in time and that correlations, both in time and in space, exist [7]. These are seen as intervals over which one or more particles in a region become unjammed, move for a short time, and then become jammed again. Such properties of dense granular flow are reminiscent of behavior seen in glasses, dense colloidal suspensions, and foams [8]. Recent studies have been successful at relating stresses in stationary bead packs with those in glassy fluids [9], suggesting the two systems may also have similar flow properties for high packing fractions and low shearing rates. It is hoped that an understanding of dense granular flow will provide insight into the properties of static granular packs as well as the broader class of jammed systems.From previous work a number of unresolved fundamental questions about slow, dense granular shear flow emerge. Although videos and plots of particle trajectories suggest that particle velocities are correlated both in space and in time, the...