Water-in-oil emulsification in a microfluidic impinging flow at high capillary numbers

The formation of water-in-oil disperse flows is investigated in a microfluidic device. The liquid-liquidflow mainly differs from these presented in the existing literature in its high capillary number (between3 and 14). The pressure-driven high flow rate, the viscous oily continuous phase, and the head-on impingingflow contribute to oppose a maximal shear force to the interfacial tension. Interfacial tension effectsare significant at scales smaller than the capillary length within microfluidic devices, but not dominant inthe present work. Moreover, the impinging channel is designed to deliver a low water fraction. As a result,the obtained water-in-oil emulsion is finely dispersed. A channel geometry is selected among threedifferent items, with a physical explanation of its observed finer emulsification. In comparison withexperimental data, two complementary approaches are used to model the Ohnesorge number from fluidsand flow parameters. Finally, an in situ optical diagnostic using polarized laser light provides a preciseknowledge of the emulsifying area and the overall flow pattern within the microchannel.