Quasistatic Microdroplet Production in a Capillary Trap

We develop a method to produce aqueous microdroplets in an oil phase, based on the periodic extraction of a pending droplet across the oil-air interface. This interface forms a capillary trap inside which a droplet can be captured and detached. This process is found to be capillary based and quasistatic. The droplet size and emission rate are independently governed by the injected volume per cycle and the extraction frequency. We find that the minimum droplet diameter is close to the injection glass-capillary diameter and that variations in surface tension moderately perturb the droplet size. A theoretical model based on surface energy minimization in the oil-water-air phases is derived and captures the experimental results. This method enables robust, versatile, and tunable production of microdroplets at low production rates.