同轴流动聚焦中射流不稳定性的理论研究

Theoretical Investigation on Flow Instability of Liquid Jets in Co-flow Focusing

壳核结构的微胶囊在医学药学材料食品农业等领域具有广泛的应用前景,其制备方法一直是相关领域关注的焦点.同轴流动聚焦(co-flow focusing)是一种新型制备技术,利用复合射流的破碎制备微胶囊具有包裹率高过程量化可控参数域广产率高等诸多优势.在实验中,复合射流的破碎受到多个过程参数的影响,并涉及了多层界面的耦合效应.利用简化的物理模型,在时间和时空域中分析了三相水-油-水复合射流不稳定性的发展和演化.在黏性流体线性稳定性理论中,同轴射流和驱动液体的基本速度型分别基于管流和误差函数构造,并通过数值方法求解满足相应边界条件下的线化小扰动控制方程.结果表明:增加内外层界面的界面张力均有利于射流的破碎;流体的黏性对同轴射流的稳定性均有着促进作用;越大的黏性越小的内界面张力对应着越大的射流破碎波长;内外界面的耦合作用以及复合液滴的包裹情况均与内外射流的半径比息息相关;绝对-对流不稳定性转换的临界Weber数随Reynolds数内层界面张力的增大而增大,随内层和驱动流体的黏性增大而减小.这些结果将有助于提高液体驱动下同轴流动聚焦技术的过程控制,为实际应用提供理论指导.

Microcapsules with core-shell structures have greatly potential applications in various areas including medicine, pharmaceutical industry, material science, food and agriculture. Liquid driven co-flow focusing is an emerging technique for producing monodisperse microcapsules due to the break up of compound l iquid jets. Experimental studies have been per-formed to capture the morphology of compound jets. A linear instabi lity analysis of three-phase water-oil-water jets has been carried out based on a simplif ied physical model. The results indicate that the inter facial tension of both inner and outer l iq-uids suppresses the stability of the jets, while the l iquid viscosities promote it. A lower inner inteifacial tension and higher l iquid viscosities result in a longer wavelength of perturbations for the coaxial jet breakup. The coupling effects of the inner and outer interfaces and the encapsulation process of the compound droplets are both closely related to the radius rat io of in ner and outer liquid jets. The crit ical Weber number for the absolute/convective transition increases as the Reynolds number and the inner inter facial tension increase. The results will provide theoretical guidance to practical applications by improving the process control of liquid driven co-flow focusing.