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Micro-droplet formation from an aperture with a diameter of micrometers is numerically investigated under the cross-flow conditions of an experimental microchannel emulsification process. The process involves dispersing an oil phase into continuous phase fluid through a microchannel wall made of apertured substrate. Crossflow in the microchannel is of non-Newtonian nature, which is included in the simulations. Micro-droplets of diameter 0.76-30 lm are obtained from the simulations for the apertures of diameter 0.1-10.0 lm. The simulation results show that rheology of the bulk liquid flow greatly affects the formation and size of droplets and that dispersed micro-droplets are formed by two different breakup mechanisms: in dripping regime and in jetting regime characterized by capillary number Ca. Relations between droplet size, aperture opening size, interfacial tension, bulk flow rheology, and disperse phase flow rate are discussed based on the simulation and the experimental results. Data and models from literature on membrane emulsification and T-junction droplet formation processes are discussed and compared with the present results. Detailed force balance models are discussed. Scaling factor for predicting droplet size is suggested.

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Slug or plug flow is generally considered as major flow pattern in microchannels in gas-liquid two-phase flow. A new microchannel design has enabled experimental interfacial surface area density exceeding 10,000 m2/m3 based on the two-phase volume in bubbly flow, and mass transfer coefficients exceeding 10sec−1. Numerical simulations as well as experiments are presented with the new microchannel design. The velocity components of secondary flow induced by specially designed angled microgrooves break the gas phase into small bubbles, where otherwise much larger gas pockets/slugs would dominate in flat or smooth wall microchannels. As such, mixing of the two phases and mass transfer are greatly enhanced as a results of increased interfacial surface area density and reduced average mass transfer distance. The Volume-Of-Fluid (VOF) method is used in the numerical computations for different surface feature patterns, gas and liquid flow rates, liquid viscosity and surface tension. In the experiments, nitrogen, carbon dioxide and water are used as the two phase media. The two-phase superficial velocity in the channel is in the range 0.45–2.75 m/s. The results show that the two-phase flow in the microchannel with the angled microgrooves leads to enhanced mass transfer relative to the flat microchannel. Higher flow rates and higher liquid viscosity lead to smaller gas bubbles and in turn enhanced mixing. Opportunities for additional improvement exist with increasing flow rates and optimized processing conditions.