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 $\varphi$ 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 $\varphi$ 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'.

arXiv comment7 pages, 7 figures, final version, accepted for publication in Europhys. Lett

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• Flow and rheology of frictional elongated grains
• Motile topological defects hinder dynamical arrest in dense liquids of active ellipsoids
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