3D vortex flows in micro capillary tubes with patterned surface charges

Flow in microfluidic devices usually has very low Reynolds number. As a result the flow is inherently limited to laminar flow regime and the mixing is solely diffusion dominated. Surface heterogeneity originated mixing is one of the enhancement methods drawn much attention in recent years since it can be employed at very small scale and it requires no moving parts. In this research, microfabrication technologies were successfully employed to pattern surface charges on inner surfaces of round capillary tubes. When contacting with aqueous solutions, these surfaces have nonuniform zeta-potentials. 3D vortices can be generated at the vicinity of tube walls when an electric field is applied across the surfaces. Motion of fluorescent particles induced by external applied electric field and heterogeneous surface charge distributions was captured by an image recording system. The cross-correlation of consecutive frames was computed by commercial PIV program. The obtained velocity field proved the existence of vortices generated by the patterned surface charges with the same polarity. Application of these vortices on mixing enhancement was then simulated by using commercial software CFD-ACE+ (CFDRC, USA). The mechanisms of its mixing enhancement effects were analyzed. A high performance mixer was proposed and its mixing performance was compared with that in rectangular channels. The comparison showed that round tubes with patterned surface charges had better mixing performance than rectangular channels with similar patterns. To my best knowledge, this is the first research that has successfully fabricated patterns in round capillary tubes using microfabrication technologies. Based on the fabricated samples, the research on heterogeneous surfaces has been extended from flat surface to curved surface and complex 3D vortex flows have been obtained. The experimental work visualized the flow field and provided solid support to the numerical models. 3D Simulation of mixing enhancement by the heterogeneous surface has provided a deep understanding of the mechanisms. The results are useful guidelines for the design of high performance mixers