Equilibrating temperature-like variables in jammed granular subsystems

TL;DR

This study experimentally investigates temperature-like variables in jammed granular systems, finding that angoricity equilibrates between subsystems while compactivity does not, supporting the stress ensemble theory.

Contribution

The paper provides experimental evidence that angoricity equilibrates in jammed granular systems, validating theoretical predictions of the stress ensemble approach.

Findings

Angoricity equilibrates between subsystems.

Compactivity does not equilibrate.

Both components of angoricity are linearly related to pressure.

Abstract

Although jammed granular systems are athermal, several thermodynamic-like descriptions have been proposed which make quantitative predictions about the distribution of volume and stress within a system and provide a corresponding temperature-like variable. We perform experiments with an apparatus designed to generate a large number of independent, jammed, two-dimensional configurations. Each configuration consists of a single layer of photoelastic disks supported by a gentle layer of air. New configurations are generated by alternately dilating and re-compacting the system through a series of boundary displacements. Within each configuration, a bath of particles surrounds a smaller subsystem of particles with a different inter-particle friction coefficient than the bath. The use of photoelastic particles permits us to find all particle positions as well as the vector forces at each inter-particle contact. By comparing the temperature-like quantities in both systems, we find compactivity (conjugate to the volume) does not equilibrate between the systems, while the angoricity (conjugate to the stress) does. Both independent components of the angoricity are linearly dependent on the hydrostatic pressure, in agreement with predictions of the stress ensemble.