Microfluidic Hydrodynamic Focusing For Flow Cytometry And Diffusion-Mixing

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Microfluidics provides a unique and useful platform for performing analytical measurements. Micro-optics can be imbedded into a microfluidic device, and the analyte can be maneuvered hydrodynamically into the optical interrogation zone. In addition, since flow is laminar, the concentration distributions of the solutes can be explicitly calculated over the volume of the device. In this work, a microfluidic hydrodynamic focusing manifold was developed which focused a solution into the center of a square microfluidic channel. By adjusting the flow-rates of the focusing fluids, the solution-under-focusing was moved up and down relative to the top and bottom walls of the microchannel. In one embodiment, the hydrodynamic focusing manifold was integrated with imbedded optical fibers for use as a micro- flow cytometer. By hydrodynamically aligning the particle stream with the laser beam, absolute counting efficiency of 58+/-8% was demonstrated for fluorescent microparticles at a throughput of 5.5 mu-L/min of bead solution. This device can be used to analyze a multiplexed bead-based assay for point-of-detection biosensing. In a second embodiment, the focusing manifold fed into a long microchannel where solute-mixing took place under pressure-driven laminar flow conditions. A numerical analysis program was developed which modeled the three-dimensional microfluidic channel with a two-dimensional mesh by using the moving mesh method. Using this program, concentration distributions were calculated at distances along the microchannel, and these were compared to an imaging experiment. The numerical analysis program overestimated the diffusion coefficients of fluorescein and Enhanced Green Fluorescent Protein by factors of 1.9 ± 0.4 and 1.4 ± 0.2, respectively. The source of this overestimation was hydrodynamic dispersion, the effect of which could not be properly treated using the moving mesh method due to the assumption transverse-uniform fluid-velocity over the cross-section of the microchannel. Advances in numerical analysis are needed for improved modeling and characterization of microfluidic devices, particularly those for which the two effects of hydrodynamic dispersion and molecular diffusion simultaneously influence the movements of the solutes. This dissertation informs the development of microfluidic analytical devices and the analysis of solute-transport within microfluidic channels.

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