Active dry granular flows: rheology and rigidity transitions

TL;DR

This study uses DEM simulations to explore how active forces influence the rheology and rigidity transitions in dense granular flows, revealing two distinct liquid/solid transitions driven by activity levels.

Contribution

It introduces a dimensionless activity parameter $\mathcal{A}$ that governs rheological behavior and identifies two rigidity transitions in active granular materials.

Findings

Rheology depends on activity level $\mathcal{A}$ and inertial number $I$.

Two rigidity transitions are observed: from solid to active fluid as activity increases.

Active solids exhibit higher contact numbers and lower friction and volume fraction with increasing activity.

Abstract

The constitutive relations of a dense granular flow composed of self-propelling frictional hard particles are investigated by means of DEM numerical simulations. We show that the rheology, which relates the dynamical friction $\mu$ and the volume fraction $\phi$ to the inertial number $I$, depends on a dimensionless number $\mathcal{A}$, which compares the active force to the confining pressure. Two liquid/solid transitions -- in the Maxwell rigidity sense -- are observed. As soon as the activity is turned on, the packing becomes an `active solid' with a mean number of particle contacts larger than the isostatic value. The quasi-static values of $\mu$ and $\phi$ decrease with $\mathcal{A}$. At a finite value of the activity $\mathcal{A}_t$, corresponding to the isostatic condition, a second `active rigidity transition' is observed beyond which the quasi-static values of the friction vanishes and the rheology becomes Newtonian. For $\mathcal{A}>\mathcal{A}_t$, we provide evidence for a highly intermittent dynamics of this 'active fluid'.