Experimental investigation of electrokinetic phenomena in planar and porous substrates
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Nowadays, there are various electrokinetic phenomena which are utilized in a wide range of applications, ranging from microfluidics and colloid and interface science to electrochemistry. However, even after 200 years of research on electrokinetic phenomena, there are still open questions with respect to fundamental understanding. The focus of this thesis is on three different phenomena, i.e., streaming potential, streaming current and electroosmosis. Hence, the thesis is divided in two parts: The first part focuses on the applicability of the classical Helmholtz-Smoluchowski theory on streaming potential and streaming current measurements of poly(methyl methacrylate) (PMMA) wafers, to infer the zeta potential of this substrate in contact with liquids of defined pH and ionic strength. In detail, we perform electrical impedance spectroscopy measurements to infer the electrical resistance in a PMMA microchannel and derive novel correlations for the electrokinetic characterization of the substrate. We conclude that convection can have a significant impact on the electrical double layer configuration which is reflected by changes in the surfaces conductivity. The second part of the thesis is concerned with electroosmotic flows in porous substrates where we develop a phenomenological correlation which is based on dimensional reasoning. A large set of experiments is carried out using a relatively simple and cost-effective setup including different sintered packed beds of borosilicate microspheres. A centre-of-mass model of the experimental setup allows for the interpretation of various effects. Streaming current measurements result in a correlation for the zeta potential of borosilicate depending on ionic strength and pH of the liquid. Finally, a quantitative expression for electroosmotic flow in packed beds of granular material is derived from the experiments. This correlation can be employed with other materials as well.