Flow focusing and jet instability

Flow focusing is an effective method to form thin jets. It can be characterized by the formation of a steady cone-jet configuration in the core of a focusing high-speed fluid stream, as the focused fluid is continuously supplied through a capillary needle. The jet issued from the vertex of the cone passes through an orifice, and eventually breaks up into monodisperse droplets due to flow instability. First proposed in 1998, the flow focusing principle has been adopted to develop a series of capillary flow techniques such as single flow focusing, electro-flow focusing, co-flow focusing and microfluidic flow focusing. These techniques are steady, controllable and gentle in producing monodisperse droplets, particles and capsules down to micrometer scale and below. Therefore they have great significance in science, technology and engineering applications. In flow focusing, the formation of the stable cone is the prerequisite condition to form the stable jet; the process parameters influence the perturbations deposited on the jet interface; and the growth of perturbations results in the breakup of the jet. This is a complex problem in fluid mechanics due to its multi-scale, multi- interface and multi-coupling characteristics. Jet instability analysis is the most useful tool for exploring the mechanisms of jet breakup. In this paper, we review the progress of flow focusing with different geometrical structures during recent two decades, and summarize the key mechanics problems of flow focusing including process control, flow modes, scaling laws and instability analyses. The methods and achievements in the study of jet instability are also briefly described. Finally, some future research topics and opportunities for applications are provided.