I was reading an example problem in my fluid mechanics text book that talks about the difference between liquids and gases. I am not sure why this is the case, if someone can explain that'd be great. So the problem is about two coaxial cylinders that rotate. There is a fluid in the gap between these 2 cylinders. The cylinders rotate and a constant shear stress is maintained, which warms up the fluid. One of the walls (it does not mention which) of the fluid is insulated thermally. The other is at room temp. In the problem, it states that for a LIQUID, the temperature increases, viscosity decreases and velocity increases as time increases for the insulated wall. So here I don't understand why the viscosity decreases. Can someone explain? Since it's constant shear, velocity has to increase because velocity and viscosity are inversely proportional. The example then goes on to say that for a GAS, everything is backwards (i.e., visocisty increases, velocity decreases), except the temperature of the insulation wall still increases. Why the difference? Does it have something to do with the way molecules are arranged in the 2 fluids?