Relationship between Viscous Flow and Diffusion

Abstract

A MOST peculiar paradox in the general area of viscous flow and diffusion is the fact that while the absence of slip between the surface of very large diffusing particles and the surrounding medium is usually taken for granted, at the same time in the case of diffusing particles that are identical as those of the medium, that is, in self-diffusion, it is the presence of slip between the ‘diffusing particles’ and the ‘medium’ which is a triviality. It amounts to nothing more than the generally accepted fact that in a fluid the molecules are in translatory motion and, hence, move past each other. However, in those cases of diffusion where neither of these two extreme situations is encountered the question as to the presence or absence of ‘slip’ does not always appear to be a trivial one.