  We have developed 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
quasi-static. 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 was derived and captures the experimental results. This method enables
robust, versatile and tunable production of microdroplets at low production
rates.
