A 3D electrokinetic flow structure of solution displacement in microchannels for on-chip sample preparation applications

Abstract

Electroosmotic flow with solution displacement in microchannels is often encountered in many lab-on-a-chip devices where washing procedures are designed using one solution to displace another different solution. In order to investigate the detailed flow structures for a displacement process between two different electrolyte solutions, a three-dimensional numerical model is developed in this paper. KCl solution and LaCl3 solution are used as sample solutions. A 2 mm long straight microchannel with a rectangular cross-sectional area (height 100 µm and width 200 µm) was employed in this study. The governing equations of the applied electrical field, flow field and concentration field are numerically solved based on a finite-control-volume scheme. The fluid flow coupled equations are solved using the semi-implicit method for pressure-linked equation (SIMPLE) algorithm. The observed electrokinetic flow structures include back flow in the center of the channel and distortion of the plug-like electroosmotic velocity profile, which are investigated in detail. It is found that distortion in the flow field is due to the induced pressure gradient, which results from the nonuniformity of electroosmotic mobilities and electrical conductivities of the two solutions. Finally, the displacement between a pair of different solutions, distilled ultra-filtered (DIUF) water and LaCl3 solution, is briefly studied.