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

The present study has experimentally investigated the two-dimensional distribution of ζ-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 ζ-potential measurement. The fluorescent dye in the vicinity of the wall is excited by the evanescent wave, which decays exponentially from the wall. The ζ-potential is obtained from the fluorescent intensity because the distribution of fluorescent dye near the wall is related to the ζ-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 ζ-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 ζ-potential distribution at the silica glass and PDMS wall, which was dependent on the Na⁺ transport in the flow field.