Strange kinetics, porous media, and NMR

Nuclear magnetic resonance (NMR) techniques cover a broad range of length and time scales on which dynamic properties of fluids confined in porous media can be investigated. This report refers to field-cycling NMR relaxometry, field gradient NMR diffusometry and NMR microscopy. The objective was to examine diffusion, hydrodynamic dispersion, flow, and thermal convection under the influence of geometrical confinements and surface interactions in porous media. The anomalous character of these phenomena will be demonstrated and discussed in comparison with computer simulations and theoretical concepts. The first part of this presentation is devoted to nanoporous samples. It is shown that molecular Levy walks along inner surfaces occur under certain conditions. Mutual "obstruction" of molecules in molecular sieves and zeolites is another source of diffusion anomaly known as single-file diffusion which can be described by Gaussian propagators with a diffusion coefficient depending on time in a certain limit. In the case of polymers confined in narrow artificial tubes of a porous solid matrix, the characteristics of reptation were experimentally verified. The second part mainly refers to "trapping" effects as a source of anomalous transport characterised by non-Gaussian propagators. Model objects fabricated on the basis of percolation cluster models were examined with respect to flow, diffusion, thermal convection and hydrodynamic dispersion. The elucidation of transport laws in model systems of well defined and mathematically describable geometries is considered to be a promising way for the exploration of the structure/dynamics relationship in porous media as a long-term objective. (C) 2002 Elsevier Science B.V. All rights reserved.