1 Supporting Online Material Materials and Methods Fabrication of the Microfluidic Devices and Data Collection. We fabricated the

microfluidic devices using standard photolithographic and soft-lithographic methods.(S1-S5) We created a master with photoresist on a silicon wafer, and then exposed the surface to a fluorinated silane.(S6) Polydimethylsiloxane (PDMS) poured onto the master and allowed to cure in a 60 °C oven formed the stamp that contained the channels. After being exposed to an oxygen plasma for 1 minute, the stamp was sealed irreversibly to a glass microscope slide that also had been exposed to the plasma for 6 minutes. We used a high-speed CCD video camera capable of recording up to 160,000 frames per second (Phantom V7, Vision Research) mounted on a microscope (Leica) to capture movies of the behavior of the system. We analyzed the movies using Matlab and software created in-house. Controlling the Flow of Liquids in the System. We controlled the flows of both the aqueous phase and the hexadecane by using nitrogen gas to pressurize the reservoirs of the two liquids. An electronically controlled valve (Pneutronics OEM-EPC 15 PSI, Parker Life Sciences) also metered the pressure of the gas applied to the aqueous phase. This valve was powered by a power supply set to 24 V. House nitrogen supplied th