Spheres on a vibrating plate: Clustering and collapse

A large number (similar to 10,000) of uniform stainless steel balls comprising less than one layer coverage on a vertically shaken plate provides a simple but striking demonstration of some of the unusual phenomena associated with excited granular media. When the oscillation amplitude is large enough, the spheres are in constant motion, colliding randomly, like a two dimensional gas in equilibrium. Inter-particle collisions couple the horizontal motion of the gas to the essentially chaotic vertical motion of the spheres driven by the plate vibration. Viewed from above, the horizontal motion in the layer demonstrates 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 amplitude of the plate vibration. The increase in local dissipation leads to the nucleation of a collapse: a close-packed crystal of particles that come to rest on the shaking plate and with respect to one another. The collapse is a motionless condensate surrounded by the remaining particles that continue in motion as a less dense gas. The two phase co-existence is hysteretic, and can be related to the chaotic trajectories of an individual inelastic sphere on an oscillating plate.