Clusters and collective motions in Brownian vibrators

TL;DR

This study experimentally explores self-organized structures and collective motions in vibrated granular materials, revealing complex phases and large-scale collective behavior in a quasi-2D system of inelastic particles.

Contribution

It provides the first experimental evidence of large-scale collective motion in purely repulsive hard-disk systems under uniform random forcing.

Findings

Identification of four distinct phases: cluster fluid, collective fluid, poly-crystal, and crystal.

Observation of large-scale collective motion near a specific volume fraction.

Demonstration that granular materials under random vibration are weakly cohesive with complex internal structures.

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 $0.111\le\phi\le0.832$. 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 $\phi=0.317$, where the most significant clusters span half of the system, and disappears near $\phi=0.713$, around which the system crystallizes and the melting transition occurs in the equilibrium hard disks.