15 research outputs found
Transition from a simple yield stress fluid to a thixotropic material
From MRI rheometry we show that a pure emulsion can be turned from a simple
yield stress fluid to a thixotropic material by adding a small fraction of
colloidal particles. The two fluids have the same behavior in the liquid regime
but the loaded emulsion exhibits a critical shear rate below which no steady
flows can be observed. For a stress below the yield stress, the pure emulsion
abruptly stops flowing, whereas the viscosity of the loaded emulsion
continuously increases in time, which leads to an apparent flow stoppage. This
phenomenon can be very well represented by a model assuming a progressive
increase of the number of droplet links via colloidal particles.Comment: Published in Physical Review E.
http://pre.aps.org/abstract/PRE/v76/i5/e05140
Flow induced by a sphere settling in an aging yield-stress fluid
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