VISCOELASTIC FREE-BOUNDARY PROBLEMS - NON-NEWTONIAN VISCOSITY VS NORMAL STRESS EFFECTS

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