Effect of ion motion on zeta-potential distribution at microchannel wall obtained from nanoscale laser-induced...

The present study has experimentally investigated the two-dimensional distribution of zeta-potential at the wall, which dominates electroosmotic microchannel flow. Nanoscale laser-induced fluorescence imaging using fluorescent dye and the evanescent wave with total internal reflection was developed for the zeta-potential measurement. The fluorescent dye in the vicinity of the wall is excited by the evanescent wave, which decays exponentially from the wall. The zeta-potential is obtained from the fluorescent intensity because the distribution of fluorescent dye near the wall is related to the zeta-potential by the Boltzman distribution. Two kinds of solution at different Na+ concentrations were mixed in a T-shaped microchannel composed of PDMS and silica glass. The zeta-potential distribution at the silica glass wall was measured with the uncertainty of 4.7 mV. The motion of Na+ in the microchannel was estimated by the numerical analysis using the velocity information obtained by micrometer-resolution particle image velocimetry. It is concluded that electroosmotic flow was generated by the zeta-potential distribution at the silica glass and PDMS wall, which was dependent on the Na+ transport in the flow field.