It was recalled that, while the same atomic mechanism was usually assumed to apply to both viscous flow and diffusion in a liquid, viscous flow was found to be many orders of magnitude faster in all cases where mass transfer was caused by a pressure gradient. A more detailed study showed that Fick’s law had to be modified for the case of self-diffusion in liquids. The use of a corrected form of Fick’s law was found to be equivalent to using the Navier-Stokes equation while assuming the validity of the Einstein equation. Hence, the rate of transport as calculated on the basis of self-diffusion was then the same as that based upon viscous flow. The analysis seemed to provide both the Navier-Stokes equation and the Einstein equation with a stronger physical basis. However, it was not exactly a continuum theory because the position of the first peak of the radial distribution function was involved.

F.Yang, J.C.M.Li: Journal of Applied Physics, 1996, 80[11], 6188-91