Diffusion and velocity correlations of the phase transitions in a system of macroscopic rolling spheres

TL;DR

This study investigates phase transitions in a quasi-two-dimensional granular system of rolling spheres, revealing unexpected continuous transitions, phase coexistence, and strong velocity correlations, challenging traditional equilibrium theories.

Contribution

It uncovers two novel continuous phase transitions with phase coexistence and non-equipartition in a driven granular monolayer, highlighting behaviors unlike equilibrium predictions.

Findings

Discovery of glassy and crystalline phase coexistence.

Observation of strong anti-correlated velocities upon collision.

Identification of phase transitions at low packing fractions.

Abstract

We study an air-fluidized granular monolayer, composed of plastic spheres which roll on a metallic grid. The air current is adjusted so that the spheres never loose contact with the grid, so that the dynamics may be regarded as pseudo two-dimensional (or two-dimensional, if the effects of sphere rolling are not taken into account). We find two surprising continuous transitions, both of them displaying two coexisting phases. Moreover, in all cases, we found the coexisting phases display strong energy non-equipartition. In the first transition, at weak fludization, a glassy phase coexists with a disordered fluid-like phase. In the second transition, a hexagonal crystal coexists with the fluid phase. We analyze, for these two-phase systems, the specific diffusive properties of each phase, as well as the velocity correlations. Surprisingly, we find a glass phase at very low packing fraction and for a wide range of granular temperatures. Both phases are characterized also by a strong anti-correlated velocities upon collision. Thus, the dynamics observed for this quasi two-dimensional system unveils phase transitions with peculiar properties, very different from the predicted behavior in well know theories for their equilibrium counterparts.