A fluorescent-dye based flow visualization technique for liquid microchannel flows is presented in this thesis. Of interest are pressure-driven and particularly electroosmotic flows of Reynolds number on the order of unity. Flows are studied in fused silica capillaries and microfluidic chips, with channels of various cross-sectional geometries, and hydraulic diameters from 20 pm to 200 pm. These parameters are characteristic of the targeted applications of interest, analytical microfluidic chips. Both the determination of fluid crossstream velocity profiles (velocimetry), and the direct visualization of microfluidic processes are pursued in this work.

The primary velocimetry technique developed here utilizes a caged-fluorescent dye which remains non-fluorescent, until exposed to ultraviolet light. A cross-sectional band of fluorescent dye is selectively uncaged in the microchannel by a pulse of ultraviolet laser light intensely focused into a sheet. The transport of this band of dye is visualized with a laser-powered fluorescent microscope and imaged. Recorded image sequences are digitally processed and analyzed to determine the cross-stream velocity profile in the microchannel.

The experimental apparatus, digital imaging system, and accompanying image analysis software are wholly developed as part of this work. The resulting electroosmotic velocity profiles measured here support classical electrokinetic theory, and represent some of the first direct measurements of velocity profiles in these flows. The ability of the technique to resolve electroosmotic fluid velocities up to 2.5 pm from the channel wall is demonstrated. This represents the highest degree of near-wall resolution reported to date for scalar-based measurements of electroosmotic flow.

The method is validated directly through concurrent and independent testing. The method is then applied to study the coupled effects of channel geometry, pressure disturbances, and Joule heating on electroosmotic flow. Additional capabilities are also developed, including a temperature-compensating image analysis method and a new microflow tagging technique.

The microflow visualization setup and the developed analysis software are applied to the direct visualization of injection processes on a microfluidic chip. Three new techniques for the on-chip injection of moderate-to-large samples are developed and presented in turn. These direct visualization studies, which build on each other, significantly extend the capabilities of microfluidic chips.