Microfluidic emulsification through a monolithic integrated glass micronozzle suspended inside a flow-focusing geometry

Microfluidic devices have shown remarkable success in generating emulsions with precise control over their size. Yet, highly sensitive nature of generation mechanism to surface wettability requires such devices to be built out of specific materials showing homogenous wettability that favors the continuous phase rather than the dispersed phase. Moreover, the need to switch the continuous phase and the dispersed phase requires switching the device wettability by applying a suitable surface treatment. Here, we demonstrate a microfluidic device that can generate water-in-oil and oil-in-water emulsions without the necessity of surface treatment. The device features a suspended glass micronozzle integrated inside a flow-focusing geometry formed by silicon and poly(dimethylsiloxane) channels where drops of the dispersed phase can be sheared off at the micronozzle tip without touching channel walls in a coflow of the continuous phase. The micronozzle structure is a partially released segment of a self-enclosed capillary entirely built in phosphosilicate glass and with a cylindrical lumen ∼1.5 μm in diameter. Owing to high fluidic resistance of such fine capillary, emulsion generation in the device takes place in a dripping process and no noticeable jet formation of the dispersed phase has been observed throughout the tested flow rates. The effect of the flow rates on the diameter of the emulsions and their rate of generation has been experimentally investigated and found to show a similar trend to that of a simple physical model based on the critical Capillary number.