# Why is the no-slip condition valid for fluids but not for solids?

You can obviously move a solid at a different speed along the surface of another solid, so how come the velocity of the fluid at the fluid-solid interface must be equal to that of the solid? What physical property dictates that the no-slip is valid for fluids but not solids? The Wikipedia page has the following:

Particles close to a surface do not move along with a flow when adhesion is stronger than cohesion. At the fluid-solid interface, the force of attraction between the fluid particles and solid particles (Adhesive forces) is greater than that between the fluid particles (Cohesive forces). This force imbalance brings down the fluid velocity to zero.

which didn't make sense to me since I'm not sure how they prove that this is true for any fluid-solid interface.

• No-slip condition is only an assumption, and it does fail for some special interfaces. – user99917 Jan 4 '16 at 0:01
• No-slip is only reasonable in viscous non-rarefied fluids; once this is no longer the case (i.e. when $\mathrm{Kn}>1$ in e.g. microflows) then there may be slip between the fluid particles and the solid boundary. – nluigi Jan 4 '16 at 10:14