Experimental method and process control of capillary flow focusing
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摘要: 作为一种基于毛细流动制备微纳尺度液滴的技术，毛细流动聚焦（Capillary flow focusing）在工程领域具有重要应用。回顾了基于吹气式和吸气式核心装置的毛细流动聚焦实验方法，展示了完整的测试平台，介绍了施加外部激励控制流动聚焦射流破碎的实验方法。同时，给出了同轴界面的拍摄方法，可获得清晰的内外层流体界面图像。在对流体锥形收缩阶段的研究中，探讨了几何参数与流动控制参数对流体锥形的形态与稳定性的影响。在稳定锥形下，研究了流动控制参数对液体射流直径、扰动波长及复合射流界面耦合的影响，并基于光的折射定律，对复合射流外层界面透镜效应所导致的内界面失真进行了修正。在对激励作用下射流破碎的研究中，考察了射流长度随振幅的变化，建立了尺度率关系，探讨了频率对生成液滴的单分散性及粒径的影响规律，为在实际应用中可控制备单分散性微液滴提供了理论与技术支持。Abstract: Capillary flow focusing as one typical kind of capillary flows is able to produce droplets and capsules at micro-scales, offering promising advantages in applications. This work reviews the experimental methods of capillary flow focusing based on the 'air-injecting' and 'air-sucking' devices, and presents the complete platform for studying the fluid cone-jet structures. Moreover, the method of adding external actuations on the flow focusing device to manipulate the breakup of liquid jets is illustrated. The flow visualization methods to capture the interface evolutions of the compound fluid cones and jets are introduced. It is found that the geometric parameters and the external flow parameters significantly affect the morphologies and instabilities of the fluid cone, and would further affect the diameters, perturbation wavelengths and interface coupling of the coaxial liquid jets. The lens effect of the outer interface on the coaxial jets could cause distortion of the inner jet and droplets, and a modified method was developed to remove the distortion based on the law of refraction. For the jet breakup upon external actuation, the relationship between the jet breakup length and the excitation amplitude was studied. The effects of the actuation frequency on the diameters and monodispersity of resultant droplets were studied.
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Key words:
- flow focusing /
- air-injecting device /
- air-sucking device /
- fluid cone /
- liquid jet /
- active actuation
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图 1 流动聚焦核心装置示意图及实物图
Figure 1. Sketches and real pictures of the devices in flow focusing experiments
图 4 不同照明方式下拍摄的同轴流体锥形
Figure 4. The images of coaxial liquid cone under different illumination conditions
图 5 结构参数对同轴流体锥形形态及稳定性的影响(流动控制参数:Qin=1mL/h, Qout=60 mL/h, Δpg=60 kPa, U=1 kV)
Figure 5. Effect of geometry parameters on the profiles and instabilities of the coaxial liquid cone (the flow parameters are fixed at Qin=1 mL/h, Qout=60 mL/h, Δpg=60 kPa and U=1 kV)
图 6 流动控制参数对同轴流体锥形形态及稳定性的影响
Figure 6. Effect of flow parameters on the profiles and instabilities of the coaxial liquid cone
图 9 Δpg对界面扰动波长λ的影响
Figure 9. The effect of Δpg on the perturbation wavelength λ of the jet interface
图 10 同轴射流在不同rQ下破碎时的界面耦合规律
Figure 10. The interface coupling of the coaxial liquid jets under different rQ
图 11 平行光通过射流界面后的折射规律
Figure 11. Refraction of the parallel light after crossing the jet interfaces
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