Mixing in polymeric microfluidic devices

Microfluidics has tremendous potential in the areas of chemical and biological analysis, environmental monitoring and biotechnology. In each of these areas, mixing plays a vital role in the operation of the microfluidic devices. The goals of this dissertation are to better understand microfluidic mixing, investigate the use of electroosmotic flow as a mixing technique, provide quantitative comparison of common microfluidic mixer techniques in terms of mixing efficiency and required power, and evaluate the use of porous polymer structures as microfluidic mixers.; Theoretical results are presented for electroosmotic flow in long-and-narrow channels with step changes in cross section and zeta potential. By manipulating the conditions of the channel, complex velocity profiles are possible. The addition of an overall pressure drop allows for the velocity profiles within different regions of the channel to be tailored. In some cases, a flat velocity profile is produced, which could be useful for situations when longitudinal dispersion needs to be minimized.; Results for the manipulation of the zeta potential of the substrate used for device fabrication are presented. Materials were selected to produce positive and neutral zeta potentials on a surface that was initially negative. Experiments investigating the role of exposure time on zeta potential were also performed using 2-(dimethylamino)ethyl methacrylate as the graft material. The results show that, with the appropriate choice of graft material and exposure time, the zeta potential can be tailored to a specific value.; Simulations are presented for the evaluation of three types of microfluidic mixers. The mixers were selected based on their compatibility with the microfluidic device fabrication process used in the department. The three mixers investigated are based on electroosmotic flow with nonuniform zeta potential, hydrodynamic focusing and physical constrictions. Relevant dimensionless groups are discussed, and the effects of the dimensionless groups on the extent of mixing and required power are examined. For all three mixers, a straight channel without a mixer was used as a basis of comparison. The hydrodynamic focusing mixer and the physical constriction mixer produce a high extent of mixing and a power requirement lower than for the electroosmotic flow mixer. In cases where the mixing liquids do not contain particulates, a physical constriction mixer is recommended. For cases with particulates, a hydrodynamic focusing mixer is recommended. For situations where the channel dimensions are on the order of microns, electroosmotic flow mixers should be considered since the power requirements for electroosmosis and pressure-driven flow become comparable at that size scale.; Finally, porous polymer monoliths were considered for several applications. Experiments were performed to evaluate porous polymers as reactors, valves and static mixers. Serving as a reactor, the porous polymer underwent surface grafting and staining. Confocal microscope revealed uniform staining, suggesting that the surface grafting reaction took place throughout the entire porous polymer volume. As a valve, the porous hydrogel monoliths were actuated using liquids of different pH. The response time of the valve is quicker than those in literature, but the valve began to degrade after 8 cycles. As a static mixer, the porous monoliths produced extents of mixing significantly higher than for a channel without a mixer. Mixers with large irregular pores were shown to yield the highest extent of mixing. Experiments investigating the monolith lengths found that for monolith with a pore size distribution of 53 - 180 microns, both 1 mm and 2 mm monoliths produced nearly complete mixing, suggesting that the additional length of the 2 mm monolith was unnecessary. Effective dispersivities for the monoliths made with different pore size distributions were shown to increase linearly with flow rate through the monolith...