Protocol-Dependence and State Variables in the Force-Moment Ensemble

TL;DR

This study tests stress-based statistical mechanics in granular materials, finding that the force-tile area variable is protocol-independent and acts as a true state variable, unlike the angoricity.

Contribution

The paper introduces keramicity as a new temperature-like state variable that remains protocol-independent, advancing the understanding of stress ensembles in granular systems.

Findings

Force-tile area distribution is exponential-tailed.

Keramicity is protocol-independent and inversely proportional to pressure.

Angoricity is protocol-dependent and varies with process.

Abstract

Stress-based ensembles incorporating temperature-like variables have been proposed as a route to an equation of state for granular materials. To test the efficacy of this approach, we perform experiments on a two-dimensional photoelastic granular system under three loading conditions: uniaxial compression, biaxial compression, and simple shear. From the interparticle forces, we find that the distributions of the normal component of the coarse-grained force-moment tensor are exponential-tailed, while the deviatoric component is Gaussian-distributed. This implies that the correct stress-based statistical mechanics conserves both the force-moment tensor and the Maxwell-Cremona force-tiling area. As such, two variables of state arise: the tensorial angoricity ($\hat{\alpha}$) and a new temperature-like quantity associated with the force-tile area which we name {\it keramicity} ($\kappa$). Each quantity is observed to be inversely proportional to the global confining pressure; however only $\kappa$ exhibits the protocol-independence expected of a state variable, while $\hat{\alpha}$ behaves as a variable of process.