Measurement of zeta-potential at microchannel wall by a nanoscale laser induced fluorescence imaging

A nanoacale laser induced fluorescence technique was proposed by using fluorescent dye and evanescent wave with total internal reflection of laser beam. The present study focused on the two-dimensional measurement of zeta-potential at the microchannel wall, which is an electrostatic charge of the wall surface and a characteristic property of an electroosmotic flow field. The evanescent wave, which decays exponentially from the wall, was used as an excitation light of the fluorescent dye. The fluorescent intensity in the vicinity of wall detected by a CCD camera is closely related to the zeta-potestial. Two kinds of fluorescent dye solution with different ionic concentration were injected into a T-shaped microchannel, and formed a mixing flow field in the junction area. The two-dimensional distribution of zeta-potential was measured at the microchannel wall in a pressure driven flow field. A transverse gradient of zeta-potential was observed in the mixing field, and the two-dimensional distribution of zeta-potential was changed by the difference in the averaged velocity. To understand the ion motion in the mixing flow field, the three-dimensional structure of flow field was analyzed by velocity measurement using micron-resolution particle image velocimetry and numerical simulation. It is concluded that the two-dimensional distribution of zeta-potential at the microchannel wall was dependent on the ion motion in the flow field, which was governed by convection and molecular diffusion.