The development of MEMS requires deliberate designs for controlling fluids in the low Reynolds number regime. Arranging surface charges in rectangular channels to obtain in-plane or out-of-plane vortices have been studied by previous researchers. However, previous surface modification techniques require different signs of zeta potentials from the other wall surfaces which made it difficult in selecting and coating microchannels. Previously, the opposite polarities are usually adjusted by changing the pH value of the solution with acid chemicals in other researches which made the solution complicated and difficult to simulate a real application. Meanwhile the acid chemicals may also destroy the coating. It is convenient to use same polarity patches if a vortex flow can also be generated. However, it is not clear if the patterned surface charges have the same polarity of zeta potentials as the other walls, what kind flow pattern will be generated and what mechanism behind the flow pattern. Furthermore, the cross-section of previously studied microchannels is usually limited to a rectangular shape. Therefore, the surface charge patterns are usually in 2D since the sidewalls of the rectangular microchannels are difficult to be patterned. However, a channel with round cross-section has better leak-proof performance of the membrane valve. Furthermore, a round channel is also advantageous in mimicking the human vein when a vascular structure is needed in tissue scaffolding, the round microfluidic channel is considered as a good candidate for an artificial capillary vessel. It is anticipated that there will be no stagnation occurs at the corner edges, which occurs at the corners of a rectangular channel, for a round microchannel owing to the perfectly symmetrical velocity profile. This is important when the microfluidic chip is subjected to a separation process such as liquid chromatography. In this paper, effects of patterned surface modification on 3D vortex flows generation in a micro capillary tube under very low Reynolds number have been investigated. Microfabrication technology was successfully employed to pattern surface charges on inner surfaces of round capillary tubes, which form non-uniform zeta-potentials. This technique extends the heterogeneous surfaces from flat surface to curved surface. 3D vortices are visualized and measured at the vicinity of tube walls when an electric field is applied across the surfaces utilizing micro resolution PIV. It demonstrated that 3D vortices can also be generated by the patterned surface charges with a same polarity. Experimental results have been compared with the numerical simulations using CFD-ACE+.
