Summary Microfluidic two-phase flows are increasingly being used in many mass transfer applications because of the numerous advantages of operating in the microscale such as stability of the interface and low cost. Such two-phase flows have multiple applications in various fields such as medicine, metal extraction, chemistry, oil and gas, and handling of industrial effluents. It is particularly important in both the transport and extraction of substances from one fluid to another fluid. The main reasons for this are the short diffusion distances and large surface-volume ratios when using multiple chips in parallel.

Among the various flow regimes, parallel flow in the microscale is considered to be advantageous for extraction applications, especially radioisotope transfer. In this regime, the two fluids move parallel to each other in a microfluidic channel. If the fluid-fluid interface remains stable throughout, efficient transfer is possible without necessitating a step to separate the two fluids. This benefit offered by microfluidics is very important for radioisotopes with short half-lives, as the absence of a separation step ensures that radioisotopes can be transferred efficiently in a quick time, thereby maximizing their utility for different applications such as pharmaceuticals.

However, stable parallel flow is hard to achieve as it is contingent on several factors. In a microfluidic channel with two inlets, a rectangular main channel and outlets, the ideal scenario for efficient mass transfer involves a stable fluid-fluid interface to be located exactly in the middle of the rectangular channel, followed by the two fluids flowing to their respective outlets without any fluid leaking to another outlet. Considering the utility of such a regime, it is important to study the underlying flow phenomena which govern the regime and leakage. This thesis, therefore, focuses on using simulations and experiments to study parallel flow in microfluidic channels, followed by an analysis of the mass transfer when using such a regime for radioisotope extraction...@en

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.@en

The lattice-Boltzmann method (LBM) is becoming increasingly popular for simulating multi-phase flows on the microscale because of its advantages in terms of computational efficiency. Many applications of the method are restricted to relatively simple geometries. When a more complex geometry is considered—circular and inclined microchannels—some important physical phenomena may not be accurately captured, especially at low capillary numbers. A Y-Y micro-fluidic channel, widely used for a range of applications, is an example of a more complex geometry. This work aims to capture the various flow phenomena, with an emphasis on parallel flow and leakage, using the Rothman–Keller (RK) model of the LBM. To this purpose, we modify the forcing term to implement the surface tension for use at low capillary numbers. We compare the simulation results of the RK model with and without the force modification with experiments, Volume of Fluid and the phase field method and observe that the modified forcing term is an improvement over the current RK model at low capillary numbers, and it also captures parallel flow and leakage more accurately than the other simulation techniques.

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