| The manipulation of liquid flow on a micro-scale is essential to lab-on-a-chip applications in biotechnology. Such a liquid flow is in the regime of very low Reynolds number (Re<<1). The viscous effect is dominant in comparison to inertia effect due to the large surface-to-volume ratio on a micro-scale. As a result, pressure gradient or traditional electro-osmosis can yield only uniform and laminar flow in a microchannel, with no separation or turbulence. However, the need for local control of micro-scale liquid motion arises in numerous applications such as biotechnology. In this study, therefore, a generic surface-charge patterning technology combined with electrokinetics is proposed to overcome this challenge.; A positively-charged poly(allylamine hydrochloride) (PAH) polyelectrolyte is coated onto a negatively-charged silicon oxide surface by the electrostatic self-assembly (ESA) process. Combined with the lift-off photolithography technique, an unlimited number of surface-charge pattern geometries can be produced. Some physical properties of this PAH coating were characterized. This surface-charge patterning technology was integrated into the fabrication process of some microchannel devices. Three types of microchannel devices were fabricated, with different designs of surface-charge patterns on the channel walls, to electrokinetically generate three basic flow patterns, namely bi-directional shear flow, out-of-plane vortex and in-plane vortex. Numerical simulations by CFD-ACE+ were performed to qualitatively and quantitatively compare the experimental results. It is found that both the trajectory and the velocity of the flow tracers were in good consistency with the simulation results. To demonstrate the potential application of this technology, some micromixers were designed and fabricated to study the mixing enhancement in the presence of surface-charge patterns. In comparison to a control experiment without surface-charge patterns, the mixing performance was greatly improved. Both the concentration distribution and the streamlines of the dye/tracer are in agreement with the numerical simulations.; In conclusion, this generic surface-charge patterning technology can be applied in electrokinetically-driven liquid flows to achieve local control of the liquid motions. This study lays out a concrete technological platform to explore the opportunities of electrokinetic flows in lab-on-a-chip applications. |
