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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.
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Bonn et al. (1997) studied this question.
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