Inhomogeneous Flows of Complex Fluids: Mechanical Instability Versus Non-Equilibrium Phase Transition

We briefly review and re-interpret recent experimental results on shear-induced inhomogeneous flows in complex fluids. These are usually explained in terms of a mechanical instability of the non-linear flow regime. However, in at least two respects, the robustness of the two-state banded-flow regime and the kinetics of band formation under shear, they present striking analogies with ordinary field-induced phase transitions in equilibrium systems. We propose a phenomenological explanation of these effects in terms of an effective non-equilibrium potential that accounts for the free energy stored in a viscoelastic material under steady-shear conditions. Within this picture, a shear-thinning non-homogeneous flow can be shown to be a manifestation of some underlying hidden structural transition.