Microscopic and Macroscopic Clustering in Granular Matter

Inelastic particles are used to model the behavior of granular media. On the level of individual grains, \inelastic collapse" plays an essential role for clustering in a variety of situations. On a macroscopic level, few-body dynamics can still be relevant. Orientation, form, and compactness of clusters in shear ow depend on microscopic features of the collision dynamics. Under the same external conditions, the scattering behavior of grains can lead to one or the other of two clustering mechanisms. Inelastic collapse has no notable eeect in some cases and is important in others. The practically important case when collisions at small speed are almost elastic is treated with particular emphasis.