  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.
