Viscous fingering in complex fluids

Viscous fingers form when a less viscous fluid pushes a more viscous fluid in a linear channel. The instability of the interface results from a competition between viscous and capillary forces. We show here that by using complex fluids such as polymer or surfactant solutions one can act on the viscosity or the surface tension and modify the instability drastically. Two different polymer solutions, that exhibit either shear thinning or normal stress effects, are used. For the first fluid the viscous forces are altered leading to finger narrowing, whereas for the second fluid the viscous forces are supplemented by normal stresses, which leads to finger widening. For the surfactant solutions the modification of the capillary forces leads to finger widening.