Progress and application of flow focusing

Abstract

Flow focusing (FF) is one of capillary flows, which is first proposed in 1998, characterized by the formation of a steady meniscus in the core of an extensional high-speed fluid focused by a small hole when a fluid is injected through a capillary needle. A very thin jet is issued from the vertex of the meniscus, passes through the hole and breaks up into monodisperse droplets at a certain distance outside the hole. The FF technique is steady, controllable and does not require rigorous conditions in producing microparticles, such as droplets, bubbles, particles and capsules down to the micrometer dimension and below. It has important applications in fields of science, technology and engineering. This paper reviews the progress of flow focusing involving experiments, theories and numerical simulations. The applications are also briefly reviewed. Finally, some future issues are presented.