Particle Diffusion in Slow Granular Bulk Flows

TL;DR

This study investigates particle diffusion in slow, three-dimensional granular flows, revealing that diffusivity depends on local strain rate and depth, with Gaussian behavior emerging after a certain strain scale.

Contribution

It uncovers the depth dependence of particle diffusivity and the strain scale governing Gaussian displacement statistics in granular flows.

Findings

Diffusivity is proportional to local strain rate.

Diffusivity inversely depends on particle depth.

Gaussian displacement statistics emerge after a specific strain scale.

Abstract

We probe the diffusive motion of particles in slowly sheared three dimensional granular suspensions. For sufficiently large strains, the particle dynamics exhibits diffusive Gaussian statistics, with the diffusivity proportional to the local strain rate - consistent with a local, quasi static picture. Surprisingly, the diffusivity is also inversely proportional to the depth of the particles within the flow - at the free surface, diffusivity is thus ill defined. We find that the crossover to Gaussian displacement statistics is governed by the same depth dependence, evidencing a non-trivial strain scale in three dimensional granular flows.