Near-wall motion of caged fluorescent dye in microchannel flows obtained from evanescent wave molecular tagging

A molecular tagging technique utilizing evanescent wave illumination was developed to investigate the motion of a caged fluorescent dye in the vicinity of the microchannel wall surface in electroosmotic and pressure-driven flows. A line pattern in a buffer solution was written by a pulsed UV laser and the uncaged dye was excited by the evanescent wave with total internal reflection inside the glass wall using an objective lens. The velocities calculated by the measured displacement of the near-wall tagged region were compared with the results of molecular tagging using volume illumination, which represents the bulk flow information. Concerning electroosmotic flow, the micro-PIV technique using a confocal microscope system was applied to the microchannel rinsed by the caged fluorescein beforehand in comparison with a pure glass-PDMS microchannel to examine the effect of dye adsorption to the wall on the electroosmotic mobility. The electroosmotic mobility obtained by evanescent wave molecular tagging (EWMT) showed close to the micro-PIV measurement result near the glass wall for the rinsed case and the uncaged dye at the almost constant velocity remained in the depthwise illumination region. On the other hand, the dye velocity in pressure-driven flow by EWMT increased rapidly with respect to time. The uncaged dye convected to the streamwise direction dispersed toward the wall due to the concentration gradient of the dye, which was confirmed by the numerical simulations.