Liquid-liquid Two-phase Microfluid Flow And Mixing At Low Reynolds Number

Microfluidics is an interdisciplinary subject developed on the bases of the chemical engineering, bioengineering, micro-machinary and nano-materials. It can be widely used in many fields, such as microfabrication by using laminar flow,synthesis of nano-materials, extraction, and separation. To realize these functions,one must be able to control, the flow and mixing of multi-phase microfluids. In this paper, the flow state of two-phase miscible and immiscible fluids at small Reynolds numbers(<0.1) is systematically studied by numerical simulations and experiments.Firstly, the influence of structures of the microchannels on two-phase miscible fluid flows is studied by numerical simulations. A micro-mixer of high efficiency is designed according to the change of mixing mechanism along with the flow state. Localized vortices and strong convection are obtained by changing the size, shape and symmetry of the mixing unit. Mixing between two-phase microfluids flowing at low Reynolds numbers can be largely enhanced in just short periods of time and limited mixing spaces. The mixing index achieved in the downstream flow after two serial mixing unit is as high as 87% after.Secondly, an interface evolving model is established for the oil-water two-phase flows driven by pressure. Numerical simulations and theoretical analyses both indicate that the formation, evolution, and movement of the interface should all be the result of a joint effect of the entrance pressures, the pressure drop due to friction in the channel, and the liquid-liquid and liquid-solid interfacial tensions..The change of flow patterns of the liquid-liquid immiscible two-phase fluid flow along with the flow conditions is investigated. A flow pattern phase diagram is plotted according to both simulated and experimental results. In the experiments,negative pressure is introduced at the channel outlets and heating elements are integrated in the microfluidic device. By controlling the pressure and the interfacial tensions quantitatively, a straight and stable interface between two immiscible fluids has been established. And the position of the interface can be controlled rather freely in the micro-channel.