Experimental observations of non-equilibrium distributions and transitions in a 2D granular gas

A large number (~10,000) of uniform stainless steel balls comprising less than one layer coverage on a vertically shaken plate provides a rich system for the study of excited granular media. Viewed from above, the horizontal motion in the layer shows interesting collective behavior as a result of inelastic particle-particle collisions. Clusters appear as localized fluctuations from purely random density distributions, as demonstrated by increased particle correlations. The clusters grow as the medium is "cooled" by reducing the rate of energy input. Further reduction of the energy input leads to the nucleation of a collapse: a close-packed crystal of particles at rest. High speed photography allows for measurement of particle velocities between collisions. The velocity distributions deviate strongly from a Maxwell distribution at low accelerations, and show approximately exponential tails, possibly due to an observed cross-correlation between density and velocity fluctuations. When the layer is confined with a lid, the velocity distributions at higher accelerations are non-Maxwellian and independent of the granular temperature.