1 Spatial Distribution of Electrokinetically Driven Flow Measured by Micro-PIV (An Evaluation of Electric Double Layer in Microchannel)

The effect of varying pH on electrokinetically driven flow has been investigated by an optical measurement system using fluorescent submicron particles. The objectives of the present study are to establish quantitative measurements of electroosmotic velocity and zeta-potential, and to apply a spatially averaged time-resolved particle tracking velocimetry (SAT-PTV) technique to particle stacking affected by pH gradient. SAT-PTV can detect an unsteady microchannel flow, eliminating the errors associated with Brownian motion without losing temporal resolution. A flow is driven by the electric field in a T-shaped microchannel that is comprised of a poly(dimethylsiloxane) (PDMS) chip and a fused silica cover glass. 500 nm diameter particles were included in buffer solutions. Electroosmotic velocity was obtained by subtracting particle electrophoretic velocity from the observed particle velocity, and the zeta-potential of particles and each wall was calculated by using electrophoretic and electroosmotic velocities, respectively. With increasing pH, electroosmotic velocity and the zeta-potential of glass and PDMS surface was increased due to deprotonation at each surface. Figure 1 depicts time evolution of an accumulation of submicron particles affected by pH gradient. The uniform concentration of submicron particles was observed at t = 0. On application of electric field, particles were moved toward the left hand side region, which is confirmed by velocity profiles of in the transverse direction detected by SAT-PTV, as shown in figure 2. At t = 7.8 s particles were accumulated along the interface between different pH buffer solutions. It is pointed out from these results that the electric potential may influence depletion or accumulation of submicron particles. (a) t = 0 sec (b) t = 7.8 sec position, x [µm]-150-100-50 0 50 100 150T ra ns ve rs e ve lo ci t