Crystallization in a horizontally vibrated monolayer of spheres

The dynamics of a cluster consisting of a few thousand steel spheres on a h orizontally vibrated plate is studied experimentally. A transition from ran domly arranged and almost independently moving particles to a two dimension al crystallike structure occurs when the filling fraction (the number of pa rticles) is increased. This transition is quantitatively characterized by a n order parameter obtained from the pair distribution function of the spher es.