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-liquid flow mainly differs from these presented in the existing literature in its high capillary number (between 3 and 14). The pressure-driven high flow rate, the viscous oily continuous phase, and the head-on impinging flow contribute to oppose a maximal shear force to the interfacial tension. Interfacial tension effects are significant at scales smaller than the capillary length within microfluidic devices, but not dominant in the 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 three different items, with a physical explanation of its observed finer emulsification. In comparison with experimental data, two complementary approaches are used to model the Ohnesorge number from fluids and flow parameters. Finally, an in situ optical diagnostic using polarized laser light provides a precise knowledge of the emulsifying area and the overall flow pattern within the microchannel.