Anomalous large-scale collective motion in granular Brownian vibrators

TL;DR

This study reveals large-scale collective motion in granular materials driven by Brownian vibrators, identifying four phases and linking collective behavior to volume fraction, active forces, and inelastic collisions, with implications for active matter physics.

Contribution

It uncovers a new phase of collective motion in granular systems driven by vibrators and connects experimental observations with recent active matter theories.

Findings

Collective motion appears at $a0.317$ with density fluctuations peak.

Transition to crystalline state occurs at $a0.713$, ceasing collective motion.

The observed phenomena differ from equilibrium systems and previous experiments, indicating a non-equilibrium origin.

Abstract

Using Brownian vibrators, we conducted a study on the structures and dynamics of quasi-2d granular materials with packing fractions ($\phi$) ranging from 0.111 to 0.832. Our observations revealed a remarkable large-scale collective motion in hard granular disk systems, encompassing four distinct phases: granular fluid, collective fluid, poly-crystal, and crystal. The collective motion emerge at $\phi=$0.317, coinciding with a peak in local density fluctuations. However, this collective motion ceased to exist at $\phi=$0.713 when the system transitioned into a crystalline state. While the poly-crystal and crystal phases exhibited similarities to equilibrium hard disks, the first two phases differed significantly from the equilibrium systems and previous experiments involving uniformly driven spheres. This disparity suggests that the collective motion arises from a competition controlled by volume fraction. Specifically, it involves an active force and an effective attractive interaction resulting from inelastic particle collisions. Remarkably, these findings align with recent theoretical research on the flocking motion of spherical active particles without alignment mechanisms.