Critical evaluations of different implementations of Edwards volume ensemble

TL;DR

This paper critically evaluates various implementations of Edwards' volume ensemble in granular systems, comparing experimental data and theoretical models to clarify the effective temperature concept and its practical applications.

Contribution

It systematically compares different methods for determining compactivity, clarifies discrepancies, and introduces the effective temperature Chi_f for topological excitations in granular packings.

Findings

Heat capacity and histogram methods yield consistent Chi values.

Discrepancies mainly due to reference states and microstate definitions.

Chi_f better describes topological excitations but is a quasi-equilibrium measure.

Abstract

Granular systems can display reproducible microscopic distributions governed by a few macroscopic parameters, parallel to equilibrium statistical mechanics. Building on this analogy, Edwards' s pioneering framework proposes a volume ensemble of equiprobable jammed states, introducing compactivity chi as an effective temperature. Despite its promise, debates persist regarding the framework' s formulation and validity, with practical implementation often exposing inconsistencies. This study systematically examines different implementations using experimental data from spherical particle packings with varying friction coefficients, subjected to tapping and shearing. Our findings show that the heat capacity and overlapping histogram methods yield consistent Chi values. Discrepancies in other approaches, such as the free volume model, primarily stem from differing reference-state compactivities and microstate definitions, whereas the mean-field theory, which effectively accounts for mechanical stability within a geometric coordination-number phase space, is constrained by its isostatic assumption and inherent approximations. Additionally, an effective temperature Chi_f defined on Delaunay tetrahedra better describes topological excitations, but represents a quasi-equilibrium state, with true equilibrium achieved only under Chi. This study refines and expands our understanding of the Edwards volume ensemble in line with Edwards' s original ideas, highlighting the combined contributions of friction and inherently disordered packing structures.