Hua Zesheng, Man Jia, Liu Guangxu, Li Jianyong, Zhou Chenchen, Xia He, Li Jianfeng
State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, P. R. China.
Langmuir. 2022 Sep 13;38(36):10994-11002. doi: 10.1021/acs.langmuir.2c01460. Epub 2022 Sep 1.
Phase separation technology has attracted extensive scientific interest because of its intriguing structure changes during the phase separation process. Phase separation inside emulsion droplets in continuous surroundings has been well studied in recent years. Many investigations have also been conducted to study the droplet phase separation phenomena in noncontinuous surroundings. However, studies on the phase separation phenomena and the spreading behavior of suspended droplets at the air-liquid interface were rarely reported. In this study, PEGDA-glycerol suspended Janus droplets with a patchy structure were produced by utilizing solvent evaporation-induced droplet phase separation at the air-liquid interface. By altering the glycerol/PEGDA volume ratio, the initial proportion of ethanol, and the concentration of surfactants, suspended droplets with different morphologies can be achieved, which include filbert-shaped droplets (FSDs), half lotus seedpod single-phase Janus droplets (HLSDs), lotus seedpod single-phase Janus droplets (LSDs), lotus seedpod-shaped droplets (LSSDs), multiple-bulge droplets (MBDs), and half gourd-shaped droplets (HGSDs). A patchy structure was generated at the air-droplet interface, which was attributed to the Marangoni stresses induced by nonuniform evaporation. Furthermore, a modified spreading coefficient theory was constructed and verified to illustrate the phase separation at the air-droplet interface, which was the first research to predict the phase separation phenomena at the air-liquid interface via spreading coefficients theory. Moreover, we studied the factors that led to the droplets being able to float by designing the combined parameters, including three interfacial tensions and the equilibrium contact angles. Therefore, a simple and versatile strategy for creating suspended Janus droplets has been developed for the first time, which holds significant potential in a variety of applications for material synthesis, such as the electrospinning solution behavior when sprayed from the nozzle into the air.
相分离技术因其在相分离过程中有趣的结构变化而引起了广泛的科学关注。近年来，连续环境中乳液滴内的相分离已得到充分研究。也有许多研究致力于研究非连续环境中的液滴相分离现象。然而，关于气液界面处悬浮液滴的相分离现象和铺展行为的研究却鲜有报道。在本研究中，通过利用气液界面处溶剂蒸发诱导的液滴相分离，制备了具有斑状结构的聚乙二醇二丙烯酸酯-甘油悬浮双面液滴。通过改变甘油/聚乙二醇二丙烯酸酯的体积比、乙醇的初始比例和表面活性剂的浓度，可以实现具有不同形态的悬浮液滴，包括榛子形液滴（FSDs）、半莲蓬单相双面液滴（HLSDs）、莲蓬单相双面液滴（LSDs）、莲蓬形液滴（LSSDs）、多凸起液滴（MBDs）和半葫芦形液滴（HGSDs）。在气-液滴界面处产生了斑状结构，这归因于不均匀蒸发诱导的马兰戈尼应力。此外，构建并验证了一种改进的铺展系数理论，以阐明气-液滴界面处的相分离，这是首次通过铺展系数理论预测气液界面处的相分离现象。此外，我们通过设计包括三个界面张力和平衡接触角在内的组合参数，研究了导致液滴能够漂浮的因素。因此，首次开发了一种简单通用的策略来制备悬浮双面液滴，这在材料合成的各种应用中具有巨大潜力，例如从喷嘴喷入空气中时的静电纺丝溶液行为。
