ABSTRACT
We discuss two experiments that explore microscopic behavior in dense granular materials. In the first, we measured the response of a 2D granular material to point forces. These responses are complex; to obtain the mean behavior, we ensemble-averaged responses of multiple realizations of each material. The particles, made of a photoelastic polymer, allow us to determine the forces at the grain scale. We used disks and pentagons, so that we could span the range from ordered to disordered packings. A primary finding is that spatial ordering of the particles is a key factor in the force response. Ordered packings have a propagative component that does not occur in disordered packings. In a second experiments, we characterize the fluctuations of a slowly sheared 2D granular material. We focus here on a continuous transition that occurs as the packing fraction γ passes through γ c. As γ → γ c from above: 1) the compressibility diverges; 2) slowing down of the mean velocity occurs; 3) the force distributions change; 4) the mean stress chain size grows as the network of chains becomes less tangled; 5) the distribution of avalanche events, defined below, changes from exponential above γ c to approximately power-law like as γ → γc .
