Clustering transitions in vibrofluidized magnetized granular materials.

We study the effects of long-range interactions on the phases observed in cohesive granular materials. At high vibration amplitudes, a gas of magnetized particles is observed with velocity distributions similar to nonmagnetized particles. Below a transition temperature compact clusters are observed to form and coexist with single particles. The cluster growth rate is consistent with a classical nucleation process. However, the temperature of the particles in the clusters is significantly lower than the surrounding gas, indicating a breakdown of equipartition. If the system is quenched to low temperatures, a metastable network of connected chains self-assemble due to the anisotropic nature of magnetic interactions between particles.