Diffusion processes in unsaturated porous media studied with Nuclear Magnetic Resonance techniques

Abstract

Unsaturated porous media form two-phase systems consisting of the liquid and its vapor. Molecular exchange between the two phases defines an effective diffusion coefficient which substantially deviates from the bulk value of the liquid. The objective of the present thesis is to study self-diffusion under such conditions by varying both the filling degree of the porous medium and the diffusion time. The experimental tool was a combination of two different NMR field gradient diffusometry techniques. For comparison, diffusion in a porous medium was modeled with the aid of Monte Carlo simulations. The NMR diffusometry techniques under consideration were the pulsed gradient stimulated echo (PGStE) method, the fringe field stimulated echo (FFStE) method, and the magnetization grid rotating frame imaging (MAGROFI) method. It is shown that water or cyclohexane in Vycor, show opposite tendencies as a function of the filling degree. Upon reduction of the filling degree of cyclohexane in VitraPor#5, the effective diffusion coefficient increases by a factor of up to ten times the value in the bulk liquid due to the vapor phase contribution. On the other hand, the effective diffusion coefficient of water first decreases and then increases when the filling degree is reduced. The different dependences on the filling factor for polar and non-polar adsorbate species are attributed to different effective tortuosities represented by different exponents in Archie’s law anticipated in the two-phase exchange theory presented. NMR microscopy of VitraPor#5 partially filled with water or cyclohexane reveals heterogeneous distributions of the liquid on a length scale much longer than the pore dimension. This is attributed to the spatial variation of the granular microstructure visible in electron micrographs. As a consequence of the inhomogeneous filling degree, the effective transverse relaxation time varies, which in turn leads to NMR imaging contrasts.