Experimental measurement of an effective temperature for jammed granular materials

TL;DR

This study experimentally measures an effective temperature in jammed granular materials by shearing packed beads, finding that different probes equilibrate at the same temperature related to packing density, suggesting a thermodynamic-like description.

Contribution

It provides the first laboratory measurement of an effective temperature in jammed granular systems, supporting the applicability of thermodynamic concepts to out-of-equilibrium materials.

Findings

Probes equilibrate at a common temperature.

Effective temperature correlates with packing density.

Supports thermodynamic description of jammed systems.

Abstract

A densely packed granular system is an example of an out-of-equilibrium system in the jammed state. It has been a longstanding problem to determine whether this class of systems can be described by concepts arising from equilibrium statistical mechanics, such as an ``effective temperature'' and ``compactivity''. The measurement of the effective temperature is realized in the laboratory by slowly shearing a closely-packed ensemble of spherical beads confined by an external pressure in a Couette geometry. All the probe particles considered in this study, independent of their characteristic features, equilibrate at the same temperature, given by the packing density of the system.