Clustering, Order, and Collapse in a Driven Granular Monolayer

Steady state dynamics of clustering, long-range order, and inelastic collapse are experimentally observed in vertically shaken granular monolayers. At large vibration amplitudes, particle correlations show only short-range order like equilibrium 2D hard sphere gases. Lowering the amplitude ``cools'' the system, resulting in a dramatic increase in correlations leading to either clustering or an ordered state. Further cooling forms a collapse: a condensate of motionless balls coexisting with a less dense gas. Measured velocity distributions are non-Gaussian, showing nearly exponential tails.