Giant fluctuations at a granular phase separation threshold

TL;DR

This paper studies a phase separation in granular materials driven by a thermal wall, revealing giant fluctuations near the threshold where traditional hydrostatic theory fails, supported by simulations.

Contribution

It identifies and analyzes a phase separation instability in granular gases, highlighting the role of giant fluctuations near the bifurcation threshold.

Findings

Hydrostatic theory predicts phase separation above a critical aspect ratio.

Simulations confirm the bifurcation curve matches theoretical predictions.

Giant fluctuations dominate near the threshold, causing breakdown of hydrostatic assumptions.

Abstract

We investigate a phase separation instability that occurs in a system of nearly elastically colliding hard spheres driven by a thermal wall. If the aspect ratio of the confining box exceeds a threshold value, granular hydrostatics predict phase separation: the formation of a high-density region coexisting with a low-density region along the wall that is opposite to the thermal wall. Event-driven molecular dynamic simulations confirm this prediction. The theoretical bifurcation curve agrees with the simulations quantitatively well below and well above the threshold. However, in a wide region of aspect ratios around the threshold, the system is dominated by fluctuations, and the hydrostatic theory breaks down. Two possible scenarios of the origin of the giant fluctuations are discussed.