Electro-hydrodynamic investigations of fluids in complex systems by NMR mapping experiments and computer simulations

Nuclear Magnetic Resonance (NMR) techniques offer valuable insights into the transport phenomena in complex systems like porous media. This cumulative thesis presents a number of experimental NMR mapping techniques for retrieving and recording valuable information about transport quantities in simple and complex geometries. NMR investigation protocols have been developed for velocity mapping, acceleration mapping, ionic current density mapping and electro-hydrodynamic mapping. Complementary information about the same objects can be obtained in most situations with the aid of Computational Fluid Dynamics (CFD) simulations. This procedure allowed us a direct comparison between the experimental and simulated data. The simple objects presented here are to be seen as test devices for microfluidics, and the complex percolation structures as model objects for real porous materials. The acceleration mapping technique developed here allows the direct measurement of spatial acceleration distributions in simple and complex test objects. Experimentally recorded acceleration maps highlighted the tortuous pathway of the fluid in site-correlated percolation objects. The correlated site-percolation models presented here try to mimic the high connectivity of the real porous materials, as they occur in nature. Well established NMR investigation protocols and CFD simulations permitted us to characterize the test objects only from the geometrical (percolation threshold, correlation length, fractal dimension) but also from the dynamic point of view. Transport phenomena under combined action of pressure and electrical field gradients are of interest for many microfluidics applications. Electroosmotic flow patterns are strongly modified by hydrodynamic pressure gradients and ionic current density distributions built up in the pore system. The appearance of closed loops and vortices was detected. Electro-osmotic flow and current density mapping reveal dissimilarity in the Helmholtz/Smoluchovski equation for such complex systems. All NMR experiments mentioned above (velocity, acceleration, electro-osmosis and current density mapping) can be performed independently of each other but in the same model object offering a broad range of information on the existing transport phenomena. (orig.)