Ensemble Theory for Force Networks in Hyperstatic Granular Matter

TL;DR

This paper introduces an ensemble framework for analyzing force networks in static granular packings, providing analytical and numerical insights into force distributions and their dependence on network structure.

Contribution

It develops a general ensemble formulation for force networks in granular matter, deriving force distributions and exploring effects of network perturbations.

Findings

Closed-form force distributions for small packings.

Numerical analysis of larger packings.

Perturbations significantly alter force distribution patterns.

Abstract

An ensemble approach for force networks in static granular packings is developed. The framework is based on the separation of packing and force scales, together with an a-priori flat measure in the force phase space under the constraints that the contact forces are repulsive and balance on every particle. In this paper we will give a general formulation of this force network ensemble, and derive the general expression for the force distribution $P(f)$. For small regular packings these probability densities are obtained in closed form, while for larger packings we present a systematic numerical analysis. Since technically the problem can be written as a non-invertible matrix problem (where the matrix is determined by the contact geometry), we study what happens if we perturb the packing matrix or replace it by a random matrix. The resulting $P(f)$'s differ significantly from those of normal packings, which touches upon the deep question of how network statistics is related to the underlying network structure. Overall, the ensemble formulation opens up a new perspective on force networks that is analytically accessible, and which may find applications beyond granular matter.