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The role of inelastic collisions in granular hydrodynamics is studied experimentally in a two dimensional system of spherical particles rolling on a smooth surface and driven by a moving wall. Clusters form at high particle densities; the velocity distribution is narrowed, and the collision rate is enhanced within the clusters. The clusters migrate downward in the presence of a very slight tilt, but are not dispersed. A one dimensional model captures the effect of gravitation on the observed statistical distributions.
Kudrolli et al. (1997) studied this question.