Fluid Induced Particle Size Segregation in Sheared Granular Assemblies

TL;DR

This study uses molecular dynamics to explore how fluid viscosity and shear influence particle size segregation in granular flows, revealing conditions for band formation and flow-dependent segregation suppression.

Contribution

It introduces a mechanism linking viscous length scale to size segregation and compares simplified and full Navier-Stokes models for flow behavior.

Findings

Size segregation occurs when viscous length scale is smaller than particle size.

Particles form flow-aligned bands of different sizes under shear and Poiseuille flow.

Flow zones with low shear rate can suppress segregation, depending on flow conditions.

Abstract

We perform a two-dimensional molecular-dynamics study of a model for sheared bidisperse granular systems under conditions of simple shear and Poiseuille flow. We propose a mechanism for particle-size segregation based on the observation that segregation occurs if the viscous length scale introduced by a liquid in the system is smaller than of the order of the particle size. We show that the ratio of shear rate to viscosity must be small if one wants to find size segregation. In this case the particles in the system arrange themselves in bands of big and small particles oriented along the direction of the flow. Similarly, in Poiseuille flow we find the formation of particle bands. Here, in addition, the variety of time scales in the flow leads to an aggregation of particles in the zones of low shear rate and can suppress size segregation in these regions. The results have been verified against simulations using a full Navier-Stokes description for the liquid.