SHEAR-INDUCED PHASE-SEPARATION OF COMPLEX FLUIDS - THE ROLE OF FLOW-CONCENTRATION COUPLING

We propose a classification scheme for the instabilities that might ar ise in sheared complex fluids. A central role is played by the couplin g between the flow and the solute concentration, which is a combinatio n of the parameters describing (a) the tendency of solute molecules to migrate to regions of small or high shear rates and (b) the variation of the viscosity with concentration. Using a mean field approach, we show that the nature and the geometry of the instability can be predic ted from the knowledge of the coupling parameter, the diffusion coeffi cient, and the derivative of the stress with respect to the shear rate . We also successfully compare a description of the variation of the s tress at and just beyond the instability threshold, with experimental results from a wormlike surfactant solution of CPClO3.