It was recalled that, in liquid diffusion experiments, the diffusion path could be partially obstructed by bubbles, or by oxide layers between the components of a diffusion couple. There was also the possibility of a so-called wall effect, in which the diffusivity was thought to depend upon the distance, from the container wall, out to macroscopic ranges. Numerical simulations were made of the evolution of the concentration field, in a 2-dimensional diffusion sample, in response to deviations from 1-dimensional transport. It was found that, due to smoothing of the concentration distribution by radial diffusion, significant deviations from the ideal 1-dimensional concentration field occurred only in the axial direction. Upon evaluating the concentration field for the semi-infinite capillary method, and for other methods, it was concluded that local obstructions of the transport path had to be greater than half of the transport path's cross-section in order to produce apparent diffusivities which were less than 0.95% of the true diffusivity. It was further concluded that the radial concentration gradients which were found in solidified diffusion samples did not constitute evidence for the existence of a wall effect, and were more likely to reflect convection effects.

On the Insensitivity of Liquid Diffusivity Measurements to Deviations from 1D Transport L.B.Jalbert, F.Rosenberger, R.M.Banish: Journal of Physics - Condensed Matter, 1998, 10[32], 7113-20