Droplet microfluidics is a branch of microfluidics that uses a two-phase liquid system (i.e. oil and water, generally speaking) to distribute or sequester materials of interest into a multitude of immiscible droplets. This is typically achieved by flowing one solution containing the material into a microfluidic device then injecting a second solution in spurts at relatively high flow rate through both sides of a cross-junction. The effect is to pinch off droplets of the first solution and suspend them in the second, creating an emulsion. Drops can be maintained separate from each other as long as they continue to flow through a narrow channel, thereby preventing them from touching and mixing. Addition of small amounts of drop-stabilizing surfactant to the second phase can reinforce this immiscibility effect even after the channel widens or that phase is removed.
This technique has been put to use in a wide variety of applications from microscale chemical reactions to diagnostics to biomolecule synthesis. It has a multitude of advantages, such as the high monodispersity of droplet size achievable through controlling the oil phase flow rate, the ability to perform high-throughput assays since each droplet can serve as its own independent testing ground, and the high heat transfer/diffusion/mixing rates inherent to the system due to the high surface area to volume ratio of the droplets. Scientists have only just begun to explore the possibilities for making additional modifications to the droplets once they're created, for example, by taking advantage of their electrical instability of their surfactant membranes to inject additional reactants (Keyword Ref. 3).
In Image A to the right (Keyword Ref. 1), low-MW polymer solution is injected in bulk into a microchannel, broken into droplets by the periodic injection of an oil phase, and later crosslinked to form polymer gel droplets. Images B and C show droplets of two different sizes produced by controlling the oil phase.