Experimental and computational studies of jamming

TL;DR

This paper explores jamming in granular materials through experiments and simulations, focusing on measuring an effective temperature near jamming and testing its thermodynamic significance.

Contribution

It provides experimental measurement of effective temperature in sheared granular materials and tests its consistency with a thermodynamic approach using simulations.

Findings

Effective temperature equilibrates across probes in experiments.

Effective temperature correlates with packing density.

Thermodynamic approach is supported by simulation results.

Abstract

Jamming is a common feature of out of equilibrium systems showing slow relaxation dynamics. Here we review our efforts in understanding jamming in granular materials using experiments and computer simulations. We first obtain an estimation of an effective temperature for a slowly sheared granular material very close to jamming. 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, independent of their characteristic features, equilibrate at the same temperature, given by the packing density of the system. This suggests that the effective temperature is a state variable for the nearly jammed system. Then we investigate numerically whether the effective temperature can be obtained from a flat average over the jammed configuration at a given energy in the granular packing, as postulated by the thermodynamic approach to grains.