In the present study, flow fields in the threedimensional, tangentially crossing micro-channels were studied. The effect of the relevant geometrical parameters such as the aspect ratio, contact surface area, surface to volume factor, flow rate and cross angle on the flow turning was reported. When the geometries and the flow conditions of the two crossing channels were the same, the fraction of turning flow was found to be dependent on the aspect ratio of the channel as reported previously in the literature. However, if the configuration and flow conditions of the two channels were different, the results need to be clarified. A parameter of non-dimensionalized surface to volume ratio was devised to characterize the flow turning. And the parameter was tested against its validity using numerical simulation and the available experimental data. The experiments on the crossing angle were conducted to show that larger angle in general yielded higher turning flow ratio. The results are expected to be useful in the passive control of flow in micro-fluidic devices among others.

In this study, we have demonstrated that effective mixing can be achieved in simple and regular crossing channels. The dynamical processes combining split-and-recombine and chaotic mixing elements of stretching and folding were discussed. In channels with simple turns (up/down/right/left), flow could be stretched; on the other hand, tangential crossings could yield folding. Theoretically, repeating stretching and folding then result in the increase of interfacial area and decrease of striation thickness which facilitate the eventual mixing by diffusion. Practically, the structural sequence of turning and/or crossing affects the performance of the mixing. Three types of micromixers were studied, and the simulated performance was favorably compared with the published data. Experiments were carried out to qualitatively demonstrate the typical performance in the mixers.