SURFACE RELAXATION AND THE LONG-TIME DIFFUSION-COEFFICIENT IN POROUS-MEDIA - PERIODIC GEOMETRIES

The macroscopic diffusion coefficient, obtained in an ideal pulsed-fie ld-gradient spin-echo (PFGSE) experiment in the long-time limit, shoul d exactly equal that derived from the electrical conductivity only whe n the surface relaxivity rho and surface electrical conductivity vanis h. In general, the coefficient derived by PFGSE techniques can be eith er greater or less than its electrical counterpart, depending on the p ore geometry and other factors. Formally, the effect of rho can be see n from the structure of a perturbation expansion based on the rho = 0 time-dependent solutions of the pore space diffusion problem. In addit ion, analytic results for periodic structures with partially absorbing boundary conditions and numerical simulations are used to illustrate the differences between the diffusion coefficients for rho = 0 and rho not-equal 0. In treating disordered media, our simulations are limite d to systems that are not heterogeneous beyond the PFGSE diffusion len gth scale.