Dynamical mechanism for non-locality in dense granular flows

TL;DR

This paper investigates the non-local rheology of dense granular flows, revealing that spatial variations in contact networks and shear rate relaxation, rather than activation processes or localized plastic events, underpin the non-local behavior, supporting a jamming scenario.

Contribution

It demonstrates that non-local effects in granular flows arise from spatial shear rate relaxation and geometric constraints, challenging previous activation and plasticity models.

Findings

Exponential shear rate responses do not indicate activation processes.

Non-local behavior stems from spatial shear rate relaxation.

Jamming scenario with geometrical constraints explains non-locality.

Abstract

The dynamical mechanism at the origin of the non-local rheology of dense granular flows is investigated trough discrete element simulations. We show that the influence of a shear band on the mechanical behavior of a distant zone is contained in the spatial variations observed in the network of granular contacts. Using a micro-rheology technique, we establish that the exponential responses hence obtained, do not proof the validity of a mechanical activation process as previously suggested, but stem from the spatial relaxation of the shear rate as a direct consequence of a macroscopic non-local constitutive relation. Finally, by direct visualization of the local relaxation processes, we dismiss the kinetic elasto-plastic picture, where a flow is conceived as a quasi-static sequence of localized plastic events interacting through the stress field. We therefore conclude in favor of the jamming scenario, where geometrical constrains lead to coherent non-affine displacements along floppy modes, inherently non-local.