Maps of electric current density and hydrodynamic flow in porous media: NMR experiments and numerical simulations

Current density imaging sequences with separation of mobile-ion current from immobile-ion current

Three sequences for electric current density imaging (CDI) are studied by testing their ability to selectively detect the electric current of different types of charge carriers. Two of the methods are standard DC-CDI and AC-CDI sequences, and a new AC-DC-CDI sequence is also introduced. The methods rely on the application of square voltage pulses in combination with refocusing RF pulses that can, depending on their position with respect to the voltage pulses, eliminate resistive, capacitive or neither of the current components from the detection pathway. It is demonstrated by experiments on a model system that the DC-CDI sequence detects only the resistive current of mobile ions, the AC-DC-CDI sequence detects only the capacitive current of fixed ions, while the AC-CDI sequence detects both current components. The three sequences introduce a new MRI contrast mechanism that depends on the ion mobility of the sample. As such, the methods may play an important role in the study of electric conductivity phenomena in biological systems.

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.

Visualization of transport: NMR microscopy experiments with model objects for porous media with pore sizes down to 50 microm

Hydrodynamic flow and electric currents through model porous media were investigated. The transport rates through the individual pathways of the pore network are determined by the local width of the pore channels and by the driving mechanism. The model objects represent quasi two-dimensional random site percolation clusters. The calculated design was realized by milling the structure in polystyrene sheets. Velocity maps of stationary flow and current density maps of stationary currents through the cluster were acquired by magnetic resonance imaging methods. The findings were compared to the results of numerical simulations based on the same structure. Since the difference in the transport patterns of the different driving mechanisms are expected to be more pronounced in smaller pore spaces, ultra deep X-ray lithography has been used for the fabrication of downsized model objects with a spatial resolution of better than 50 microm and an aspect ratio as large as 20. First results obtained with these objects are reported.