ABSTRACT
The slip velocity measured experimentally in flow through capillaries of sufficiently small diameters, with solid surfaces made repellent to the liquid, is examined. An answer to the question that whether the slip occurs directly on the surface of the solid or there exists a gap between the solid and liquid surfaces is sought. The slip velocity, when the molecules move directly on the solid surface, can be obtained as the product of the gradient of the chemical potential and the mobility coefficient. However, comparison with experiment provides values too high for the surface diffusion coefficient. This suggests that slip does not occur directly over the solid surface but over a gap. Such a gap is generated when the liquid and the solid have different natures (one of them hydrophobic and the other hydrophilic), and may be increased in thickness by the release of the gas entrained in the flowing liquid and/or the desorption of the soluble gas.
