Chain structure of head-on collisions in boundary driven granular gases

TL;DR

This paper discovers chain structures of head-on collisions in boundary-driven granular gases, revealing a new order property that affects collision dynamics and challenges the molecular chaos assumption.

Contribution

It introduces the observation of collision chain structures and develops a model to incorporate their effects into classical mean field theories.

Findings

Chain structures form networks in boundary-driven granular gases.

Position-velocity correlation exists at both small and large relative velocities.

The observed collision chains influence collision frequency and mean field results.

Abstract

We report a peculiar dynamic phenomenon in granular gases, chain structures of head-on collisions caused by the boundary heated mechanism form a network in an Airbus micro-gravity experiment and horizontal vibrated one in the laboratory, which differ markedly from the grazing-collision-dominant in randomly driven granular fluid. This new order property is an orientation correlation between the relative position and the relative velocity of any particle pair, which weakens the collision frequency and leads a long range boundary effect. By the histogram of the relative position and the relative velocity, we find this position-velocity correlation is not only at limits of very small relative velocities but also large ones, which means the breakdown of molecular chaos assumption is not limited to a small portion of the phase space. Through a simple anisotropic angular distribution model of the relative position and the relative velocity, we could modify classical uniform angular integration results of mean field values taking the effect of the observed collision chain structure explicitly into account.