Merging fluid and solid granular behavior

TL;DR

This paper develops unified constitutive relations for frictionless, deformable granular particles under shear, bridging fluid and solid regimes, and highlighting the influence of particle stiffness and proximity to jamming on stress and temperature behaviors.

Contribution

It introduces a novel approach to merge fluid and solid granular behaviors into continuous, differentiable constitutive relations accounting for particle stiffness and jamming proximity.

Findings

Scaling laws for pressure, shear stress, and temperature are derived.

Unified relations accurately describe transition near jamming.

Shear stress depends on shear rate and particle stiffness, unlike pressure.

Abstract

Simple homogeneous shear flows of frictionless, deformable particles are studied by particle simulations at large shear rates and for differently soft, deformable particles. The particle stiffness sets a time-scale that can be used to scale the physical quantities; thus the dimensionless shear rate, i.e. the inertial number (inversely proportional to pressure), can alternatively be expressed as inversely proportional to the square root of the particle stiffness. Asymptotic scaling relations for the field variables pressure, shear stress and granular temperature are inferred from simulations in both fluid and solid regimes, corresponding to unjammed and jammed conditions. Then the limit cases are merged to unique constitutive relations that cover also the transition zone in proximity of jamming. By exploiting the diverging behavior of the scaling laws at the jamming density, we arrive at continuous and differentiable phenomenological constitutive relations for the stresses and the granular temperature as functions of the volume fraction, shear rate, particle stiffness and distance from jamming. In contrast to steady shear flows of hard particles the (shear) stress ratio does not collapse as a function of the inertial number, indicating the need for an additional control parameter. In the range of particle stiffnesses investigated, in the solid regime, only the pressure is rate independent, whereas the shear stress exhibits a slight shear rate- and stiffness-dependency.