老化屈服应力流体中的球形沉降物引起的流动
We have studied the flow induced by a macroscopic spherical particle settling in a Laponitesuspension that exhibits a yield stress, thixotropy, and shear thinning. We show that the fluidthixotropy or aging induces an increase with time of both the apparent yield stress andshear-thinning properties but also a breaking of the flow fore-aft symmetry predicted inHershel-Bulkley fluids yield-stress, shear-thinning fluids with no thixotropy. We have also variedthe stress exerted by the particles on the fluid by using particles of different densities. Although thestresses exerted by the particles are of the same order of magnitude, the velocity field presentsutterly different features: whereas the flow around the lighter particle shows a confinement similarto the one observed in shear-thinning fluids, the wake of the heavier particle is characterized by anupward motion of the fluid “negative wake”, whatever the fluid’s age. We compare the features ofthis negative wake to the one observed in viscoelastic shear-thinning fluids polymeric or micellesolutions. Although the flows around the two particles strongly differ, their settling behaviorsdisplay no apparent difference which constitutes an intriguing result and evidences the complexityof the dependence of the drag factor on flow field.