Diffusion past an entropy barrier

Diffusion in a tube or channel of varying cross section has been treated, by Fick and Jacobs, as one-dimensional diffusion past an entropy barrier determined by the tube cross-sectional area or channel width. The Fick-Jacobs diffusion equation is rederived, along with corrections involving the curvature of the tube or channel. On comparison with two exactly solvable examples, the Fick-Jacobs equation appears to be quite reliable as long as the curvature is not too great. 9 refs., 4 figs