电场作用下同轴流动聚焦的复合射流不稳定性特性研究

The project proposes the method for combination of three coaxial techniques involving co-flow focusing, coaxial electrospray and co-electrospinning, where an electric field is applied in the region of compound cone and jet in co-flow focusing. By means of the advanced microscopic system combined with high-speed video camera and laser sheet "section-cutting" visualization techniques, we aim to obtain the sharp images and quantitative data of gas-liquid compound jets moving with high velocity. The instability characteristics of compound jets under multiphase flow and multi-field coupling conditions are also investigated theoretically. As a whole, the physical rules and dynamic mechanisms of co-flow focusing under an electric field are expected to be obtained. According to the research in this project, the experimental platform for studying the coaxial capillary flows under complex conditions will be established; the data of electrified compound jet for both Newtonian and non-Newtonian fluids including the diameter, the velocity, the growth rate of disturbances and the unstable modes will be explored in a range of different control parameters; the flow modes and corresponding parametric regimes will be given; the effects and physical rules of main influencing factors on the transitions between different dominant unstable modes will be obtained. The results will provide theoretical guidance to practical applications. Compared to each of these coaxial techniques, the method reported here is more advantageous for producing microcapsules and compound fibers with micro-/nanometer scales, and it is hopeful of stabilizing the atomization, widening the range of applied parameters and reducing the diameter of productions. As a result, this method possesses significant and extensive application prospects.

本申请提出将同轴结构的流动聚焦、电雾化和电纺丝三种技术相结合的方法，在同轴流动聚焦的复合锥形和射流区域施加电场作用，采用先进的显微系统结合高速摄影机和激光"切片"流场显示技术，获得高速运动复合气-液射流的清晰图像和定量数据，并利用理论分析方法研究在多相流和多场耦合条件下带电复合射流的不稳定性特性，力求获得电场作用下同轴流动聚焦的物理规律和力学机理。通过本项目研究，建立复杂条件下同轴结构毛细流动的实验研究平台，获取不同控制参数下牛顿流体和非牛顿流体带电复合射流的直径、速度、扰动发展和失稳模态，得到流动模式及其参数域，并给出优势模态之间相互转变的主要影响因素和规律，为实际应用提供理论指导。与单一的同轴技术相比，该方法在制备微纳米尺度的胶囊和复合纤维方面更有优势，有望使雾化更稳定，可应用的参数范围更广，制备的产物直径更小，因此具有重要而广泛的应用前景。

具有壳核结构的微纳米液滴和胶囊在科学研究和工程实践中具有重要的应用前景。本项目组结合流动聚焦、电雾化和电纺丝技术的优势，提出了同轴流动聚焦、同轴电流动聚焦等主动控制的制备方法，可有效控制和调节产物的形貌、尺寸、结构、分散性甚至功能，并系统开展了深入的实验和理论研究工作，在过程控制、流动模态、尺度律和不稳定性机理等方面取得了系列创新研究成果。首先，自行设计加工了新颖的复合针头和核心装置，搭建了可靠的实验测试平台，实现了复合锥形和复合射流的可控生成和流场诊断。获得了锥形、射流以及制备产物的清晰图像和微观结构，得到了不同流动模态及其工作域，发现了单核及多核微胶囊的形成条件，建立了产物粒径和控制参数之间的尺度律关系。其次，提出了包含有粘、有电场、非均匀基本流、同轴结构或非轴对称扰动等复合射流的不同物理模型，建立了气驱和液驱同轴射流和带电射流的不稳定性理论分析方法，得到了射流失稳的主要无量纲参数和对应的临界值，阐明了惯性力、电场力、表面张力、粘性力等不同作用力的竞争机制。最后，给出了电场力和惯性力共同作用下界面发展和失稳的主要影响因素和演变趋势，阐明了电场强度和表面电荷分布对锥形和射流稳定性的作用机制，得到了最不稳定扰动的波长、绝对/对流不稳定性以及轴对称/非轴对称不稳定性的转换边界，获得的理论预测与实验结果一致。受本项目资助，已申请了多项国家专利，正式发表16篇重要国际期刊SCI论文，包括Physics of Fluids、Applied Physics Letters、Lab on a Chip、Microfluidics and Nanofluidics等，受邀在《力学进展》发表长篇综述论文，多次受邀在国际和国内学术会议上做口头报告。项目负责人司廷于2017年获得国家基金委优秀青年科学基金资助，近年来也凝炼了研究方向，拓展了研究内容，并取得了突出研究进展，具有一定的国际影响力。