Tandem emulsification for high-throughput production of double emulsions

Core-shell double emulsions produced using microfluidic methods with controlled structural parameters exhibit great potential in a wide range of applications, but the low production rate of microfluidic methods hinders the exploitation of the capabilities of microfluidics to produce double emulsions with well-defined features. A major obstacle towards the scaled-up production of core-shell double emulsions is the difficulty of achieving robust spatially controlled wettability in integrated microfluidic devices. Here, we use tandem emulsification, a two-step process with microfluidic devices, to scale up the production. With this method, single emulsions are generated in a first device and are re-injected directly into a second device to form uniform double emulsions. We demonstrate the application of tandem emulsification for scalable core-shell emulsion production with both integrated flow focusing and millipede devices and obtain emulsions of which over 90% are single-core monodisperse double emulsion drops. With both mechanisms, the shell thickness can be controlled, so that shells as thin as 3 μm are obtained for emulsions 50 μm in radius.