Power-law friction in closely-packed granular materials

TL;DR

This study uses discrete element simulations to reveal that in closely-packed granular materials, the friction coefficient and volume fraction follow power-law relations with shear rate and pressure, respectively, highlighting scale-invariant behavior.

Contribution

It introduces a power-law relationship for friction and volume fraction in granular materials, unifying behavior across different pressures using the inertial number.

Findings

Friction coefficient increases as a power of shear rate with a universal exponent.

Friction coefficients at various pressures collapse onto a single curve when scaled by the inertial number.

Volume fraction also exhibits a power-law dependence on shear conditions.

Abstract

In order to understand the nature of friction in closely-packed granular materials, a discrete element simulation on granular layers subjected to isobaric plain shear is performed. It is found that the friction coefficient increases as the power of the shear rate, the exponent of which does not depend on the material constants. Using a nondimensional parameter that is known as the inertial number, the power-law can be cast in a generalized form so that the friction coefficients at different confining pressures collapse on the same curve. We show that the volume fraction also obeys a power-law.