Orientational correlations and the effect of spatial gradients in the equilibrium steady state of hard rods in 2D : A study using deposition-evaporation kinetics

TL;DR

This study investigates how deposition-evaporation kinetics influence the orientational order and correlations of hard rods in 2D, revealing a transition to nematic phases with power-law decay of correlations and effects of spatial inhomogeneity.

Contribution

It provides a detailed analysis of the equilibrium steady state of 2D hard rods under deposition-evaporation dynamics, including the effects of spatial gradients and inhomogeneities.

Findings

Power-law decay of orientational correlations in nematic phase.

Spatial gradients induce alignment along the gradient direction.

Random inhomogeneities lead to glassy, frustrated states.

Abstract

Deposition and evaporation of infinitely thin hard rods (needles) is studied in two dimensions using Monte Carlo simulations. The ratio of deposition to evaporation rates controls the equilibrium density of rods, and increasing it leads to an entropy-driven transition to a nematic phase in which both static and dynamical orientational correlation functions decay as power laws, with exponents varying continuously with deposition-evaporation rate ratio. Our results for the onset of the power-law phase agree with those for a conserved number of rods. At a coarse-grained level, the dynamics of the non-conserved angle field is described by the Edwards-Wilkinson equation. Predicted relations between the exponents of the quadrupolar and octupolar correlation functions are borne out by our numerical results. We explore the effects of spatial inhomogeneity in the deposition-evaporation ratio by simulations, entropy-based arguments and a study of the new terms introduced in the free energy. The primary effect is that needles tend to align along the local spatial gradient of the ratio. A uniform gradient thus induces a uniformly aligned state, as does a gradient which varies randomly in magnitude and sign, but acts only in one direction. Random variations of deposition-evaporation rates in both directions induce frustration, resulting in a state with glassy characteristics.