Protocol dependence and state variables in the force-moment ensemble

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 and a new temperature-like quantity associated with the force-tile area which we name keramicity. Each quantity is observed to be inversely proportional to the global confining pressure; however only keramicity exhibits the protocol-independence expected of a state variable, while tensorial angoricity behaves as a variable of process.