Asymmetric response of a jammed plastic bead raft

TL;DR

This study investigates how the jamming transition affects fluctuation-dissipation relations in a driven plastic bead system, revealing asymmetric response behaviors depending on stress direction.

Contribution

It provides new insights into the anisotropic response near jamming, highlighting the limitations of effective temperature concepts in such systems.

Findings

Response time depends on stress direction

Linear strain response observed at certain stress amplitudes

Asymmetry in response indicates complex jamming dynamics

Abstract

Fluctuation-dissipation relations have received significant attention as a potential method for defining an effective temperature in nonequilibrium systems. The successful development of an effective temperature would be an important step in the application of statistical mechanics principles to systems driven far from equilibrium. Many of the systems of interest are sufficiently dense that they are close to the jamming transition, a point at which interesting correlations develop. Here we study the response function in a driven system of plastic beads as a function of the density in order to elucidate the impact of the jamming transition on the use of fluctuation-dissipation relations. The focus is on measuring the response function for applied shear stress. We find that even when the amplitude of the applied stress leads to a linear response in the strain, the time scale of the response is dependent on the direction of the applied stress.