I Fluid Viscosity & Viscous Friction Between Plates

[Dario56]

When fluid is placed between two parallel plates such that one plate is moving and other is stationary, fluid will start flowing. Between plate and the fluid there is viscous friction given by equation: $$ F = -\eta A \frac {dv} {dy} $$

where $ \eta $ is fluid viscosity, $A$ is area of a plate and $\frac {dv} {dy} $ is a velocity gradient

Since fluid viscosity is a measure of intermolecular or cohesive forces in a fluid how can its value determine viscous friction between fluid and the plate since interactions between fluid and the plate aren't the same like between fluid molecules?

 

[Chestermiller]

At the plates, the boundary condition is no-slip (perfect adhesion). That is all that is required.

 

[Baluncore]

Yes, there is no relative motion between fluid and the plate at that point. However, this doesn't really answer my question.

All the shear is between fluid and fluid. There is no shear movement between metal and fluid.
For that reason the viscous friction is a function of fluid viscosity only.

If we misunderstand your question, then you may need to ask your question again in a different way.

 

[Dario56]

All the shear is between fluid and fluid. There is no shear movement between metal and fluid.
For that reason the viscous friction is a function of fluid viscosity only.

If we misunderstand your question, then you may need to ask your question again in a different way.

Everything is understood. No slip condition explains my question.

 

[boneh3ad]

This is why I don't like when textbooks call this phenomenon "friction." It is, I suppose, related to static friction, but that makes it confusing to some because everything is moving so students tend to get a bit wishy washy. I prefer describing it through shear stresses.