Towards a relevant set of state variables to describe static granular packings

TL;DR

This study combines experiments and simulations to show that static granular packings require both volume and force moment tensor for a complete description, challenging the assumption that density alone suffices.

Contribution

It demonstrates that steady states in granular materials are characterized by both volume and force moment tensor, highlighting the need for multiple variables to describe equilibrium states.

Findings

Reversible density-acceleration curve is nonmonotonous.

Same mean volume states can be reached with different tapping intensities.

States with equal stress can have different volumes.

Abstract

We analyze, experimentally and numerically, the steady states, obtained by tapping, of a 2D granular layer. Contrary to the usual assumption, we show that the reversible (steady state branch) of the density--acceleration curve is nonmonotonous. Accordingly, steady states with the same mean volume can be reached by tapping the system with very different intensities. Simulations of dissipative frictional disks show that equal volume steady states have different values of the force moment tensor. Additionally, we find that steady states of equal stress can be obtained by changing the duration of the taps; however, these states present distinct mean volumes. These results confirm previous speculations that the volume and the force moment tensor are both needed to describe univocally equilibrium states in static granular assemblies.