Mapping the limits of equilibrium in sheared granular liquid crystals

TL;DR

This study investigates how elongated granular particles under shear exhibit equilibrium-like orientational behavior similar to liquid crystals, and identifies the limits where this analogy breaks down due to factors like aspect ratio and friction.

Contribution

It demonstrates that dense sheared granular rods can exhibit quasi-equilibrium orientational statistics described by liquid crystal theory, and maps the transition to far-from-equilibrium states.

Findings

Granular rods follow Jeffery-like orbits in viscous fluids.

Quasi-equilibrium orientational statistics match liquid crystal theory.

Friction and aspect ratio cause deviations from equilibrium behavior.

Abstract

Athermal elongated particles are well-known to follow Jeffery orbits when sheared in viscous fluids. It is less clear if similar orbits appear in dense granular flows. We show that when sheared for long enough, sufficiently elongated frictionless granular rods, rather than following noisy Jeffery-like orbits, exist in a quasi-equilibrium state, whose orientational statistics are quantitatively described by classical liquid crystal theory, where the noise is provided by collisions due to shear. At the same time, we demonstrate a systematic breakdown of this equilibrium analogy at two distinct limits: at low aspect ratios, where the equilibrium theory incorrectly predicts an isotropic state, and as inter-particle friction is introduced, where the system moves from steric screening to frictional gearing. Even within this frictionally geared state, the rotational dynamics remain distinct from classical Jeffery orbits. We link this frictional breakdown directly to the system being driven far from equilibrium, as quantified by an effective Ericksen number that compares non-equilibrium rotational driving to steric ordering. Our results provide a quantitative map of the transition from a quasi-equilibrium to a far-from-equilibrium steady state in a dense, driven system, defining the limits of applicability for thermal liquid crystal theory in athermal matter.