Question
can the coefficient of viscosity be negative?
Answer
Not for the viscosity at a molecular level. The viscosity represents the tendency of deformation in a fluid to convert kinetic energy into heat. If the viscosity was negative, heat would be converted into kinetic energy at a constant temperature. We could then use the kinetic energy say to generate electricity. This is a perperual motion machine that violates the second law of thermodynamics. <BR><BR>Note that fluid dynamicists was the concept of eddy viscosity to represent the net effect of turbulence on macroscopic flow. If there were somehow an excess amount of energy in small eddies, it might feed energy into macroscopic flow. Then one might use a negative eddy viscosity, but I have not seen this done with a real problem. I deal mostly with fluids that flow too slowly to be turbulent.
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