Advances on Microfluidic Methods for Producing Compound Droplets

Microfluidics is advantageous for one-step fabricating compound droplets at micro scale with various structures and functions. This review surveys several microfluidic methods commonly used for producing compound droplets, including coaxial electrospray, co-flow focusing, lab on a chip and glass microcapillary. The principles and relevant progresses of these methods are briefly summarized and analyzed. In these flows, different types of fluids are stretched into microjets which eventually break up into compound droplets in channels with certain geometric structures or under external force fields. In coaxial electrospray and compound flow focusing techniques, the fluids flowing from the mouth of capillary are able to form steady cone-jet configurations, and different flow modes can be obtained as the external forces change. In lab on a chip and glass microcapillary devices, the fluids are often restricted in different channels, and the flow morphologies can be changed for different channel geometries. In these methods, the purely physical principles are employed, the processes are robust and easy to operate, and the compound droplets with controllable size, nice uniformity and designable microscopic structures can be fabricated. Therefore, they have significant applications in various science and technology areas.