IMAGING OF PRESSURE-DRIVEN AND ELECTROKINETICALLY-DRIVEN FLOWS THROUGH OPEN CAPILLARIES

A new tool for imaging both scalar transport and velocity fields in li quid flows through microscale structures is described. The technique e mploys an ultraviolet laser pulse to write a pattern into the flow by uncaging a fluorescent dye. This is followed, at selected time delays, by flood illumination with a pulse of visible light which excites the uncaged dye. The resulting fluorescence image is collected onto a sen sitive CCD camera. The instrument is designed as an oil immersion micr oscope to minimize beam steering effects, The caged fluorescent dye is seeded in trace quantities throughout the active fluid, thus images w ith high contrast and minimal distortion due to any molecular diffusio n history can be obtained at any point within the microchannel by sele ctively activating the dye in the immediate region of interest. We rep ort images of pressure- and electrokinetically driven steady flow with in round cross section capillaries having micrometer scale inner diame ters, We also demonstrate the ability to recover the velocity profile from a time sequence of these scalar images by direct inversion of the conserved scalar advection-convection equation.