Microfluidic step emulsification techniques based on spontaneous transformation mechanism: A review

[Display omitted] Rapid and robust generation of uniformly sized droplets is a research hotspot for emulsification technology. Compared with conventional microfluidic devices such as cross-junction and flow-focusing, the size of the droplets generated in the step emulsifiers based on the spontaneous transformation mechanism is independent of the flow fluctuation, making this type of devices have many advantages and important application value. To introduce the research progress of step emulsification technology in recent years, this review discusses the device configurations, interfacial evolution for droplet formation mechanism, flow pattern classification, the influence of control variables on the emulsification process and the application advantages of such devices, and illustrates the design and selection principles of such devices, the control laws of manipulation variables, and the future application fields and directions.