Time-series velocity measurements of electroosmotic flows with nonuniform zeta-potential using evanescent wave and volume illumination

Velocity measurements of transient electroosmotic mecrochannel flows with nonuniform electrostatic charge at the wall, i.e., the zeta-potential, have been conducted using fluorescent submicron particles. Near-wall and bulk velocity measurements were performed by the evanescent wave and the volume illumination, respectively. Particle tracking velocimetry with phase sampling was used to increase the velocity information at low number density of tracer particles, and achieved the spatial resolution of 19.6 μm ×19.6 ×m. The temporal resolution of 0.4 ms was obtained using a high speed CMOS camera. The fluid velocity with the uncertainty of approximately 0.1 mm/s was measured subtracting the velocity of particle electrophoresis from the observed particle velocity. Two straight microchannels with and without surface modification to generate step-change zetapotential at the wall were prepared. The results show detailed time evolution of flows with an order of 10^-3 s, which agrees with the relaxation time based on the momentum diffusion. The flow without surface modification shows a profile in agreement with the flow by slip velocities. The flow of the modified channel shows temporally constant velocity gradients generated from the patterned interfacial flows. It is found that depth-wise velocities and inner pressure gradients were generated by the mass conservation.