Shear-induced sedimentation in yield stress fluids

TL;DR

This study investigates how shear flow induces sedimentation in yield stress fluids containing particles, revealing dependencies on shear rate, particle size, and fluid yield stress, and proposing a model for sedimentation in the plastic regime.

Contribution

It provides the first detailed analysis of shear-induced sedimentation in yield stress fluids using MRI, and introduces a model for sedimentation velocity in the plastic regime.

Findings

Sedimentation velocity increases with shear rate and particle size.

Sedimentation velocity decreases with higher yield stress.

A viscous fluid model explains sedimentation in the plastic regime.

Abstract

Stability of coarse particles against gravity is an important issue in dense suspensions (fresh concrete, foodstuff, etc.). On the one hand, it is known that they are stable at rest when the interstitial paste has a high enough yield stress; on the other hand, it is not yet possible to predict if a given material will remain homogeneous during a flow. Using MRI techniques, we study the time evolution of the particle volume fraction during the flows in a Couette geometry of model density-mismatched suspensions of noncolloidal particles in yield stress fluids. We observe that shear induces sedimentation of the particles in all systems, which are stable at rest. The sedimentation velocity is observed to increase with increasing shear rate and particle diameter, and to decrease with increasing yield stress of the interstitial fluid. At low shear rate ('plastic regime'), we show that this phenomenon can be modelled by considering that the interstitial fluid behaves like a viscous fluid -- of viscosity equal to the apparent viscosity of the sheared fluid -- in the direction orthogonal to shear. The behavior at higher shear rates, when viscous effects start to be important, is also discussed. We finally study the dependence of the sedimentation velocity on the particle volume fraction, and show that its modelling requires estimating the local shear rate in the interstitial fluid.