Viscous fingering and related instabilities in complex fluids

Abstract

We study the viscous fingering instability for different polymer and surfactant solutions. For the polymers, for similar non-Newtonian viscosities, the results are strikingly different; flexible polymers widen the viscous fingers, whereas stiff polymers narrow the fingers. We show that the difference is due to normal stress effects; these are large for the flexible polymer and undetectable for the stiff polymer. For the surfactant systems, two different regimes are found. For low surfactant concentrations, the dominant effect occurs at the surface; the advection of surfactant molecules from the interface results in dynamic surface tension. For high concentrations, volume effects become important, notably the non-Newtonian flow properties of the surfactant phases, which leads to the formation of extremely thin fracture-like fingers.