Viscoelastic free-boundary problems: Non-Newtonian viscosity vs normal stress effects

We study the viscous fingering instability and the motion of long bubbles in tubes for two different polymer solutions. For similar non-Newtonian viscosities, the results are strikingly different: Flexible polymers widen the viscous fingers, whereas stiff polymers narrow the fingers. A study of the motion of air bubbles in a tube filled with the polymer solution shows that the difference is due to normal stress effects; these are large for the flexible polymer and undetectable for the stiff polymer.