流动聚焦及射流不稳定性

流动聚焦是一种有效的微细射流产生方法,其原理可以描述为从毛细管流出的流体由另一种高速运动的流体驱动,经小孔聚焦后形成稳定的锥–射流结构,射流因不稳定性破碎成单分散的液滴.自从1998年流动聚焦被提出以来,陆续发展了单轴流动聚焦、电流动聚焦、复合流动聚焦和微流控流动聚焦等毛细流动技术.这些技术稳定、易操作、没有苛刻的环境条件的要求,能够制备单分散性较好的微纳米量级的液滴、颗粒和胶囊,在科学研究和实际应用中具有重要价值.流动聚焦涉及了多尺度、多界面和多场耦合的复杂流体力学问题,其中稳定的锥形是形成稳定射流的先决条件,过程参数是影响射流界面扰动发展的关键因素,而射流不稳定性分析是揭示射流破碎的最主要理论工具.该文回顾了近二十年来不同结构流动聚焦的研究进展,概述这些技术涉及的过程控制、流动模式、尺度律和不稳定性分析等关键力学问题,总结射流不稳定性的研究方法和已取得的成果,最后展望流动聚焦的研究方向和应用前景.

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.