Evanescent-wave/volume illuminated velocity measurements of transient eletrokinetically driven flow with nonuniform wall electrostatic potential

Time-resolved velocity measurements using fluorescent submicron particles have been conducted to examine transient electrokinetically driven flow with nonuniform wall electrostatic potential, i.e. the zeta-potential. Evanescent wave- and volume illumination-based optical systems were used, respectively, to measure the near-wall and bulk velocities. Particle tracking velocimetry with phase sampling at low concentration of tracers achieved a spatial resolution of 9.8 µm × 36.1 µm with images of high signal-to-noise ratio. A microchannel having a step-change wall zeta-potential with intervals of 50 µm, which was generated by the surface modification on the wall, was prepared. Since particles do not follow the fluid flow due to electrophoresis, the time evolution of the particle and fluid flows were investigated with a temporal resolution of 0.4 ms. Measurement uncertainty was 0.05 mm s−1 for the particle velocity, and 0.11 mm s−1 for the fluid velocity. Near-wall fluid velocities with the step-change zeta-potential of 20% difference generated 50% difference of the particle velocities due to electrophoresis. The depth-wise velocities were induced by the mass conservation. It is found that the transient flow field is characterized by the ratio of the stream-wise to depth-wise velocities. The fluid flow depending on the momentum diffusion yielded the temporally constant velocity gradients derived from the step-change zeta-potential