Experimental study on morphology and flow structure of liquid cone in flow focusing
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摘要: 流动聚焦(flow focusing)是一种制备单分散性微纳米尺度液滴、颗粒和胶囊的毛细流动技术，小孔上游稳定的液体锥形的形成是产生射流并高效制备微液滴的前提条件。采用量纲分析方法得到了被聚焦液体流量、驱动气体压差、毛细管与聚焦小孔距离对锥形稳定性的影响，利用吸气式流动聚焦装置观测了锥形界面形态及稳定性，验证了理论分析结果，通过调控主要过程参数获得了锥形稳定的参数区间。在被聚焦液体内部添加示踪粒子，采用高速摄影技术拍摄了流场图像并进行定量分析，探究了锥形内部的回流区结构及其变化规律，发现回流区的产生与锥形界面两侧的切向速度分布密切相关，被聚焦液体流量、驱动气体压差、毛细管与聚焦小孔距离对回流区的大小均具有显著影响。Abstract: Flow focusing is one of capillary flow techniques that can produce monodisperse droplets, particles and capsules at micro-/nano-scales. In flow focusing, the formation of a stable liquid cone upstream of the small orifice is a prerequisite for further jet generation and efficient preparation of microdroplets. In this work, dimensional analysis is first used to analyze the effects of the focused phase flow rate, driving gas pressure difference, and the distance between the capillary and the focusing orifice on the stability of the liquid cone. Based on the aspirating flow focusing experimental platform, the morphology and stability of the liquid cone are observed, verifying the theoretical analysis. In addition, the stable parameter range of the liquid cone is obtained by adjusting the main process parameters. Moreover, the flow field is visualized by adding tracer particles into the fluid of the focused phase, and the high-speed photography is employed to capture the flow field images, which are analyzed quantitatively to explore the structure of the recirculation zone inside the liquid cone. It is found that the generation of the recirculation cell is closely related to the tangential velocity distribution on both sides of the cone interface, and the size of the recirculation cell is affected significantly by the driving gas pressure difference, the flow rate of focused phase, and the geometric parameters.
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Key words:
- flow focusing /
- cone morphology /
- cone stability /
- recirculation cell
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图 3 不同气体压力差下的锥形形态图
Figure 3. Cone morphology diagram at different gas pressure differences
图 4 不同管孔距离下的锥形形态图
Figure 4. Cone morphology diagram at different heights between the tube and the hole
图 6 不同毛细管流量的回流区结构
Figure 6. Recirculation cell structure at different liquid flow rates
图 7 不同气体压差下的回流区结构
Figure 7. Recirculation cell structure at different gas pressure differences
图 8 不同管孔距离下的回流区结构
Figure 8. Recirculation cell structure at different heights between the tube and the hole
表 1 实验材料的物理属性（20 ℃）
Table 1. Physical properties of the experimental materials（20 ℃）
实验材料 动力黏性系数
μ/（Pa·s）密度
ρ/（kg·m–3）表面张力系数
γ/（N·m–1）去离子水 1.01×10–3 998.8 71.4×10–3 空气 1.79×10–5 1.29 — 
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