Flow induced by a sphere settling in an aging yield-stress fluid

TL;DR

This study investigates how a sphere's settling in an aging, yield-stress fluid affects flow patterns, revealing effects of thixotropy and particle density on flow symmetry and wake structures, with implications for complex fluid dynamics.

Contribution

It demonstrates how thixotropy and particle density influence flow symmetry and wake formation in yield-stress fluids, highlighting differences from classical models.

Findings

Flow symmetry breaks due to thixotropy and aging.

Heavier particles create a negative wake, unlike lighter ones.

Settling velocity remains similar despite flow differences.

Abstract

We have studied the flow induced by a macroscopic spherical particle settling in a Laponite suspension that exhibits a yield-stress, thixotropy and shear-thinning. We show that the fluid thixotropy (or aging) induces an increase with time of both the apparent yield stress and shear-thinning properties but also a breaking of the flow fore-aft symmetry predicted in Hershel-Bulkley fluids (yield-stress, shear-thinning fluids with no thixotropy). We have also varied the stress exerted by the particles on the fluid by using particles of different densities. Although the stresses exerted by the particles are of the same order of magnitude, the velocity field presents utterly different features: whereas the flow around the lighter particle shows a confinement similar to the one observed in shear-thinning fluids, the wake of the heavier particle is characterized by an upward motion of the fluid ("negative wake"), whatever the fluid's age. We compare the features of this negative wake to the one observed in viscoelastic shear-thinning fluids (polymeric or micelle solutions). Although the flows around the two particles strongly differ, their settling behaviors display no apparent difference which constitutes an intriguing result and evidences the complexity of the dependence of the drag factor on flow field.