Granular phase transition as a precondition for segregation.

Experimental results are presented on the segregation of a mixture of spheres with two different sizes, rolling on a circularly vibrating table. Beyond a critical density of particles a demixing occurs leading to a clustering of the larger ones. A monodisperse layer of spheres shows a liquid-solid-like phase transition at a slightly lower critical density. These critical particle densities are both found to be independent of the driving frequency, but decrease with increasing vibration amplitude.