3D Velocity Measurement by Orthogonal-Plane Micro-PIV for Electrokinetic Enhancement of Surface Reaction

In microfluidic systems, surface reaction is diffusion-limited because the effect of convection on mass transport decreases due to low Peclet number. It is indicated that an externally induced flow toward the reactive interface is effective to enhance the efficiency of the surface reaction. However, it is difficult to evaluate the flow velocity normal to the substrate, which directly contributes to the enhancement of the surface reaction, due to the monolithic dimension of microfluidic device. This paper reports the development of a 3D flow velocity measurement method by orthogonal-plane micro-PIV to evaluate the contribution of flow distortion by alternating-current electrokinetic phenomena on the reaction enhancement. 3D velocity field is reconstructed from two orthogonal velocity fields measured by 2D micro-PIV with different measurement planes; one is based on normal observation with the measurement plane parallel to the bottom wall and the other is based on a lateral observation with the plane perpendicular to the bottom wall through a sidewall of a fluidic channel made of PDMS (100 × 50 μm). Complete 3D velocity field is determined by scanning the measurement plane in each observation scheme. Validity of orthogonal-plane micro-PIV for the 3D velocity measurement was confirmed by the measurement of three component velocities in a tilt rectangular microchannel. Then, we investigated AC-driven electrothermal (ACET) effect induced by the property gradient of fluid due to temperature nonuniformity under an electric field application. Effective flow structure of ACET for the enhancement of surface reaction, a circular stirring fluid motion which conveys bulk fluid to the surface region, was observed. This stirring motion of fluid could improve the binding opportunities between suspended and immobilized species and result in the promotion of reaction efficiency. It is clarified that 3D flow of ACET contributes to the localized enhancement of the surface reaction efficiency.