  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.
