Active dry granular flows: Rheology and rigidity transitions

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 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 A. At a finite value of the activity At, 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 A > A(t), we provide evidence for a highly intermittent dynamics of this "active fluid"