Rheological Definition of Friction

I was listening to a record at our university about friction and its rheological definition. For the first moment I thought that its the normal definition

Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other.

However the lecture pointed out that the rheological definition is different from the normal one. The professor who recorded the lecture is out of town now so I can't ask him.

So I just need to know any idea what is meant by the Rheological definition of Friction and what makes it different from the normal one??

Any help is appreciated

• I have never met the phrase logical definition of friction. Can you clarify what is meant by logical definition. Are you asking about the underying physical processes? If so, it's an adhesive process. Apr 13 '16 at 19:32
• There are no "logical" definitions in science, there are only empirical ones. Friction is defined empirically in a number of situations and in other situations it doesn't make sense to talk about friction, at all. The more you know about microscopic physics, the less you will use the term, unless you are in mechanical engineering, where it is unavoidable to deal with. I think you should contact the professor and put him on the spot. As you are portraying it, it seems to make no sense, at all. Apr 13 '16 at 19:34
• It was mentioned in the lecture that logical is the real definition of friction not just that its the force opposite direction to motion
– zozi
Apr 13 '16 at 19:38
• @CuriousOne if we want to get the best definition of friction, what can it be ???
– zozi
Apr 13 '16 at 19:39
• Perhaps you meant to write rheological. That makes a world of more sense. Apr 13 '16 at 19:55

The viscosity is $$\mathrm{stress/rate\ of\ strain}$$ and viscous drag force experienced by a sphere of radius $$r$$ moving in the fluid of viscosity $$\eta$$ with velocity $$v$$ is $$F_d = 6\pi r\eta v$$.