Viscous force depends on relative motion of the two surfaces and also the surface area. Does friction also depend on these two factors? (I'm not sure.)
What's the main difference between both forces according to a perspective from molecular level and what's the reason behind that?
Best Answer
In the extreme case of a gas, viscosity comes mostly from the diffusion of molecules between layers of the flow (this diffusion transports momentum between the layers) — this is why a hard sphere gas is viscous for example. In the opposite extreme of the friction between two solids, this clearly does not happen: in that case, friction comes mostly from short distance repulsion between molecules or atoms.
The problem to go further is that there is a continuum of material between solids and gases, not to mention that a thin layer of the surface of a solid can melt under friction.