Comment on Slip Velocity at a Fluid-Solid Boundary

The assumption that fluid velocity near a solid boundary is equal to boundary velocity, i.e., the no-slip assumption, is argued to be physically untenable. The assumption is critically examined firstly for dilute gases and we show that the no-slip assumption is incompatible with even the most elementary kinetic theoretical considerations. The effective viscosity coefficient relating shear stress exerted by the solid on the gas to the velocity gradient in the gas differs from the viscosity coefficient applicable to the bulk of the gas. Qualitative arguments are used for liquids and similar conclusions are reached. A macroscopic model, applicable to both gases and liquids, consistent with the physical requirement of nonzero slip, is then proposed. The shear stress at the boundary is then interpreted as a frictional force proportional to the slip velocity and viscosity is assumed to have the same constant value everywhere.