Clusters and collective motions in Brownian vibrators

Abstract

Using Brownian vibrators, where single particles can undergo Brownian motion under vibration, we experimentally investigated self-organized structures and dynamics of quasi-two-dimensional (quasi-2d) granular materials with volume fractions 0.111 ≤ φ ≤ 0.832. We show rich structures and dynamics in hard-disk systems of inelastic particle collisions, with four phases corresponding to cluster fluid, collective fluid, poly-crystal, and crystal. While poly-crystal and crystal are strikingly similar to the equilibrium hard disks, the first two phases differ substantially from the equilibrium ones and the previous quasi-2d experiments of uniformly driven spheres. Our investigation provides single-particle-scale evidence that granular materials subject to uniform random forcing are weakly cohesive with complex internal structures and dynamics. Moreover, our experiment shows that large-scale collective motion can arise in a purely repulsive hard-disk system. The collective motion emerges near φ = 0.317, where the most significant clusters span half of the system, and disappears near φ = 0.713, around which the system crystallizes and the melting transition occurs in the equilibrium hard disks.