微流控技术中双重乳粒尺寸调控规律的研究

Controlled production of double emulsion by microfluid technique

在采用乳液微封装技术制备惯性聚变用聚-α-甲基苯乙烯(PAMS)靶丸芯轴过程中,以氟苯为油相溶剂,水溶性聚合物水溶液为外水相制备水包油包水(W1/O/W2)双重复合乳粒,对复合乳粒进行固化干燥得到PAMS靶丸芯轴.本文设计搭建了一套双重同轴乳粒发生器,用微流控技术产生PAMS靶丸复合乳粒,该乳粒发生器采用两种不同结构:"两步法"通道与"一步法"通道.研究了利用此乳粒发生器制备复合乳粒过程中,乳粒形成机理及三相流速对乳粒尺寸调控规律.实验结果显示,乳粒发生器结构上的细小差异会极大地影响乳粒形成机理以及尺寸变化规律.在"两步法"通道结构中,内水相流速对复合乳粒的形成及外径无明显影响,而外径随外水相流速的变化规律与单乳粒实心液滴(O/W2)尺寸变化规律相同;在固定体系中,乳粒尺寸取决于内水相与油相流速之和及外水相流速,而与内水相和油相流速之比无关.然而在"一步法"通道中,由于W1-O界面的存在,内水相流速对复合乳粒外径的影响非常大;复合乳粒外径不仅与内层相界面的界面张力大小有关,还与内水相与油相流速之比有关.最后,将实验中的双重复合乳粒置于水溶性聚合物水溶液中进行固化,得到毫米级空心聚合物微球.

All planned inertial confinement fusion （ICF） capsule targets except machined beryllium require plastic mandrels with tight requirements on which the ablator is built. In this paper, the fabrication of poly（α-methylstyrene） （PAMS） mandrel is studied. PAMS mandrels are produced by using microencapsulation technique. This technique involves producing a water droplet （W1） encapsulated by a flourobenzen （FB） solution of PAMS （O） with a droplet generator, and this droplet is then flushed off by external phase （W2）, forming a water-in-oil-in-water （W1/O/W2） compound-emulsion droplet, which is suspended in a stirred flask filled with external phase to cure. The encapsulation process is based on a microfluid technique, which can achieve the controlled production of millimeter-scale PAMS mandrels. In this work, capillaries-based co-flowing microfluidic triple orifice generator is designed and built to fabricate W1/O/W2 droplets. Two configurations of the droplet generator：one-step device and two-step device, are employed in this experiment. In one-step device, the end of oil phase capillary is located at the same position as the end of inner water phase capillary. So the core droplet and the shell droplet break off from their capillaries ends at the same time, forming a W1/O/W2 droplet. While in the two-step device, the W1 phase capillary tip is located upstream to the W2 phase capillary tip. As a result, the core droplet and the shell droplet depart from the ends of their capillaries respectively, forming a W1/O/W2 droplet as well. Differently, the shell droplet contains only one core droplet in one-step generator, while several core droplets are contained in the shell droplet in two-step generator. In this paper, the mechanism of the droplet formation and the effect of the flow rate on the size of the droplet are studied with these two configurations. Results show that tiny difference between the two generators will lead to great differences in droplet formation mechanism and size control. In the two-step generator, the inner phase flow rate has little influence in the outer diameter of the compound-emulsion droplet. The diameters of the compound-emulsion droplets have a similar change to the diameters of the single droplets （O/W2）. In one-step device, the inner phase flow rate has a significant influence on the outer diameter of the double-emulsion droplet because of the existence of W1-O interface. Finally, the compound-emulsion droplets fabricated in this experiment are cured in external phase, after which PAMS mandrels are fabricated. The diameters of the final PAMS mandrels are measured with optical microscope. The distribution of the diameters well concentrates in an area of （2000±10） μupm, which is favorable for producing the PAMS mandrels with a diameter of 2000 μupm.