INHOMOGENEOUS FLOWS OF COMPLEX FLUIDS - MECHANICAL INSTABILITY VERSUSNONEQUILIBRIUM PHASE-TRANSITION

We briefly review and re-interpret recent experimental results on shea r-induced inhomogeneous flows in complex fluids. These are usually exp lained in terms of a mechanical instability of the non-linear flow reg ime. However, in at least two respects, the robustness of the two-stat e banded-flow regime and the kinetics of band formation under shear, t hey present striking analogies with ordinary held-induced phase transi tions in equilibrium systems. We propose a phenomenological explanatio n of these effects in terms of an effective non-equilibrium potential that accounts for the free energy stored in a viscoelastic material un der steady-shear conditions. Within this picture, a shear-thinning non -homogeneous flow can be shown to he a manifestation of some underlyin g hidden structural transition.