Memory-induced motion reversal in Brownian liquids

TL;DR

This paper investigates how dense Brownian liquids exhibit motion reversal after external shear is removed, combining simulations and power functional theory to reveal the role of memory-induced forces in this viscoelastic behavior.

Contribution

It introduces a theoretical framework using power functional theory with a nonlocal memory kernel to explain motion reversal in sheared Brownian liquids, validated by simulations.

Findings

Dense hard sphere fluid reverses motion after shear removal

Superadiabatic forces oppose the current due to memory effects

Theory quantitatively matches simulation results

Abstract

We study the Brownian dynamics of hard spheres under spatially inhomogeneous shear, using event-driven Brownian dynamics simulations and power functional theory. We examine density and current profiles both for steady states and for the transient dynamics after switching on and switching off an external square wave shear force field. We find that a dense hard sphere fluid (volume fraction $\approx$0.35) undergoes global motion reversal after switching off the shear force field. We use power functional theory with a spatially nonlocal memory kernel to describe the superadiabatic force contributions and obtain good quantitative agreement of the theoretical results with simulation data. The theory provides an explanation for the motion reversal: Internal superadiabatic nonequilibrium forces that oppose the externally driven current arise due to memory after switching off. The effect is genuinely viscoelastic: in steady state, viscous forces oppose the current, but they elastically generate an opposing current after switch-off.