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

We briefly review and re-interpret recent experiInental results on shear-induced inhoInogeneous flows in coInplex fluids. These are usually explained in terIns of aInechanical instability of the non-linear flow regiIne. However, in at least two respects, the robustness of the two-state banded-flow regiIne and the kinetics of band forInation under shear, they present striking analogies with ordinary field-induced phase transitions in equilibriuIn systeIns. We propose a phenoInenological explanation of these effects in terIns of an effective non-equilibriuIn potential that accounts for the free energy stored in a viscoelastic material under steady-shear conditions. Within this picture, a shear-thinning non-hoInogeneous flow can be shown to be a Inanifestation of soIne underlying hidden structural transition.