Influence of a ‘Step’ on the liquid–liquid flow patterns and flow phenomena in a microfluidic Y-Y channel

Microfluidic multiphase flows are being increasingly used in many mass transfer applications because of the numerous advantages of operating in the microscale such as greater flow stability and low cost. Among the various flow regimes, parallel flow in the microscale is considered to be advantageous for extraction applications, especially radioisotope transfer, because efficient transfer and purification are possible as long as the interface position remains stable throughout without leakage at the outlets. Therefore, some papers have worked with asymmetric microchannels with different depths for the two fluids in mass transfer applications. The flow phenomena in such channels, however, have not been studied in detail. This paper focuses on these asymmetric microchannels, which we have termed ‘step’ channels, and the influence such an asymmetry has on the flow phenomena. We perform experiments on this channel and compare the flow maps with experiments in a channel of uniform depth (standard channel). The step channel was found to favour parallel flow and interestingly, to produce stable parallel flow without leakage at low Capillary numbers. This is contrary to the results observed in a standard channel, where slug flow is observed. Volume-of-fluid simulations showed the role of interfacial tension in obtaining stable parallel flow in a step channel. Additionally, flow maps were also plotted for step channels of different widths and degrees of asymmetry, where it was found that smaller widths and higher degrees of asymmetry favour stable parallel flow.