| Over the past decade, numerous chemical and biochemical assays have been demonstrated on microfluidic devices by integrating chemical reactions, separations, sample processing and detection etc., onto the same microchip. In most applications however, the separation unit is an integral part of the analysis system and often determines the overall footprint size of the micro-device. Miniaturization of separation geometries therefore, remains a key issue in the design of such micro-systems and is often limited by solutal dispersion in the analysis channel. One such dominant source for dispersion arises due to electrokinetic flow of solute samples around curved channel geometries. In this case, a shorter path length and a larger analyte velocity along streamlines at smaller radii of curvature, leads to a stretching of the solute slugs. While such dispersion may be reduced by decreasing the channel width around the turn geometry, here we investigate the effect of a sinusoidal wavy inner wall in the curved section.; Besides electrokinetic dispersion around turn geometries, broadening of analyte bands also occurs during their pressure-driven transport through closed conduits. The no-slip flow condition at the channel walls in such systems leads to the band broadening as convection proceeds in the axial direction. Solutal dispersion in pressure-driven open-channel liquid chromatographic devices also occurs due to similar reasons. In this case however, solute molecules are slowed down near the channel walls both due to the no-slip flow condition as well as their retention in the stationary phase coating the inner walls of the device. While the slug dispersivity arising from such variations in the solute velocity across the channel cross-section scales with the square of the Peclet number based on the narrower dimension of the conduit, its dominant contribution in a large aspect ratio channel geometry results from diffusion limitations in the wider direction. Here, we propose simple modifications in the geometry of the channel side-regions to minimize this contribution in straight channel segments. In addition, we have also examined the use of a pressure-driven back flow to reduce solutal spreading in open-channel liquid electro-chromatographic systems. The imposition of a back flow in such a system tends to speed-up the slow moving molecules near the channel walls by increasing fluid convection around these regions, thereby, minimizing variations in the solute velocity across the channel cross-section.; Apart from designing strategies to reduce dispersion on micro-analytic devices, protein separations at prep-scale have also been explored in this dissertation using a novel electrophoretic technique, Binary Oscillatory Cross-Flow Electrophoresis (BOCE). This technique relies on the interaction of an oscillatory electric field and a transverse oscillatory shear flow in the analysis channel. Appropriate selection of frequency and phase of oscillation of the two interacting driving forces provides an effective binary filter that allow proteins, either higher or lower than chosen electrophoretic mobility to pass through the device. While simple binary separations had been demonstrated previously in our research group, the BOCE technique has been extended to ternary separations in our current work. |
