Ordering in granular rod monolayers driven far from thermodynamic equilibrium

TL;DR

This study investigates the orientational order of granular rod monolayers under agitation, revealing universal ordering behaviors similar to equilibrium systems and independence from energy injection details.

Contribution

It demonstrates that granular rods form ordered states analogous to equilibrium phases and that these states are unaffected by agitation parameters.

Findings

Short rods form tetratic order at high density.

Long rods form uniaxial nematic order at high density.

Order transition occurs around a length-to-width ratio of 7.3.

Abstract

The orientational order in vertically agitated granular rod monolayers is investigated experimentally and compared quantitatively with equilibrium Monte Carlo simulations and density functional theory. At sufficiently high number density, short rods form a tetratic state and long rods form a uniaxial nematic state. The length-to-width ratio at which the order changes from tetratic to uniaxial is around $7.3$ in both experiments and simulations. This agreement illustrates the universal aspects of the ordering of rod-shaped particles across equilibrium and nonequilibrium systems. Moreover, the assembly of granular rods into ordered states is found to be independent of the agitation frequency and strength, suggesting that the detailed nature of energy injection into such a nonequilibrium system does not play a crucial role.