Dissimilar electro-osmotic flow and ionic current recirculation patterns in porous media detected by NMR mapping experiments.

Random-site percolation clusters were milled into ceramic (polar) and polystyrene (nonpolar) plates as a paradigm for porous media or complex microsystem channel networks. The pore space was filled with electrolyte solutions. Using NMR microscopy techniques, maps of the following quantities were recorded: (i) flow velocity driven by external pressure gradient, (ii) electro-osmotic flow (EOF) velocity, (iii) ionic current density in the presence of EOF, (iv) ionic current density in the absence of EOF. As far as possible, the experiments were supplemented by computational fluid dynamics simulations. It is shown that electro-osmotic flow as well as the electric current density include vortices and recirculation patterns. Remarkably, all transport patterns turned out to be dissimilar, and the occurrence and positions of vortices do not coincide in the different maps.

NMR acceleration mapping in percolation model objects.

An NMR microscopy technique is described that permits direct mapping of local accelerations. The method is tested with water flow through a random site percolation model object and compared with computational fluid dynamics simulations. A general formalism, the "polygon rule," is reported for the design of gradient pulse sequences for phase encoding of higher order motions, or, in other words, for compensation of phase shifts by lower motional orders.