Photo-injection based sample design and electroosmotic transport in microchannels

Techniques for facilitating chemical reactions and transporting reactants in microfluidic applications are becoming increasingly important. Combined experimental and numerical studies of a photochemical injection process and the subsequent electroosmotic transport of products are presented here. In the experiments, intense focusing of ultraviolet light performs local photolysis of a caged fluorescent dye in a 25 μm i.d. capillary. The advection and diffusion of this sample in electroosmotic flow are imaged using a micro-flow visualization system. Independent numerical simulations of the sample transport are conducted with a custom-designed code. Strong agreement between the numerical predictions and the experimental results is established. Further comparisons demonstrate that near-ideal, diffusion-limited sample transport has been achieved. Focusing on diffusion, numerical simulations show that alternative sample concentration profiles may be obtained through the diffusive interactions of multiple, photo-injected Gaussian sample concentration profiles. In particular, a compact, flat-topped sample profile, exhibiting a constant concentration plateau is predicted. The ability to produce this sample profile using multiple photo injections was demonstrated experimentally in agreement with numerical simulation results.