Direct Measurement of Force Configurational Entropy in Jamming

Abstract

Thermal fluctuations are not large enough to lead to state changes in granular materials. However, such materials do achieve reproducible bulk properties, suggesting that they are controlled by an underlying statistical mechanics analogous to thermodynamics. We make this connection concrete by providing a first principles derivation of the multiplicity and thus the entropy of the force networks of such granular packings. We directly measure the multiplicity of force networks using a newly proposed protocol based on the phase space volume of allowed force configurations. Analogous to Planck's constant, we find a scale factor, $h_f$, that discretizes this phase space volume into a multiplicity. To determine this constant, we measure angoricity over a wide range of pressures using the method of overlapping histograms. By combining these measurements, we concretely link thermodynamic approaches of angoricity with the microscopic multiplicity of the Force Network Ensemble.