Statistical properties of granular materials near jamming

This paper describes a series of experiments that probe the behavior of dense granular materials. We first establish a broad context for these studies that identifies several key properties: spatial inhomogeneity for forces represented by force networks and force chains, dilation, temporal fluctuations, and the general idea of jamming. Most of the experiments described here involve the use of photoelastic particles, and we give a discussion of some of the basic features of photoelasticity and its application to granular experiments. We then discuss experiments that probe first isotropic jamming, which occurs for a packing fraction of phi = phi(J) similar or equal to 0.84, and then shear jamming, which occurs for phi's less than fJ for frictional particles (and at least in some circumstances, for frictionless particles). Shear jamming involves force networks and stresses that are inherently anisotropic. They are not contained in the Liu-Nagel jamming scenario, which has been extensively studied in the context of frictionless sytems. In a third set of experiments we explore the idea that slow cyclic shear can provide an activation mechanism which is manifested in slow relaxation that appears to be consistent with a force ensemble picture. The last set of experiments involves impacts of a heavy intruder on a granular bed consisting of photoelastic particles. The impactor, whose speed is well below sonic, generates propagating force pulses along a more slowly evolving force network. This mechanism is sufficient to account for the stopping force that acts on the intruder.