DIFFUSION IN POROUS SYSTEMS AND THE INFLUENCE OF PORE MORPHOLOGY IN PULSED GRADIENT SPIN-ECHO NUCLEAR-MAGNETIC-RESONANCE STUDIES

We report a combined experimental, theoretical, and simulation study of pulsed gradient spin-echo (PGSE) nuclear magnetic resonance (NMR) for fluid saturated porous media. A simple pore hopping theory is developed on the basis of the assumption that diffusion within pores is very much faster than diffusion between pores. For suitable periodic media, the theoretical results are found to be in good agreement with random-walk simulations. The theory for glasslike media is then used to analyze experimental PGSE NMR data for a water-saturated random loose pack of nearly monodisperse polystyrene spheres. The structural parameters extracted by this method are consistent with the known geometry of such packings. An important observation from the simulations is that the long-time effective diffusion constant is already accessed at times so short that a single spin will only have diffused across one pore width.