Step emulsification in microfluidic droplet generation: mechanisms and structures.

The droplet-based microfluidic techniques have been applied widely in functional material synthesis and biomedical information measurements, wherein step emulsification as an integrated system combines the advantages of homogeneity and throughput in monodisperse droplet formation. This paper reviews the mechanisms and classical structures of step emulsification. In terms of droplet formation mechanisms, we describe the droplet size and detachment regimes related to the microchannel geometry. Distinguished by droplet formation, microfluidic step emulsification driven by interfacial tension and centrifugal step emulsification related to buoyancy are introduced respectively, including their improved structures for enhancing the droplet homogeneity and throughput. Finally, the perspectives about the developments of step emulsification in mechanisms, fabrications, and applications are discussed.