Tan Huanshu, Diddens Christian, Lv Pengyu, Kuerten J G M, Zhang Xuehua, Lohse Detlef
Physics of Fluids Group, Department of Science and Technology, Mesa+ Institute, and J. M. Burgers Centre for Fluid Dynamics, University of Twente, 7500 AE Enschede, The Netherlands;
Department of Mechanical Engineering, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands;
Proc Natl Acad Sci U S A. 2016 Aug 2;113(31):8642-7. doi: 10.1073/pnas.1602260113. Epub 2016 Jul 14.
Evaporating liquid droplets are omnipresent in nature and technology, such as in inkjet printing, coating, deposition of materials, medical diagnostics, agriculture, the food industry, cosmetics, or spills of liquids. Whereas the evaporation of pure liquids, liquids with dispersed particles, or even liquid mixtures has intensively been studied over the past two decades, the evaporation of ternary mixtures of liquids with different volatilities and mutual solubilities has not yet been explored. Here we show that the evaporation of such ternary mixtures can trigger a phase transition and the nucleation of microdroplets of one of the components of the mixture. As a model system, we pick a sessile Ouzo droplet (as known from daily life-a transparent mixture of water, ethanol, and anise oil) and reveal and theoretically explain its four life phases: In phase I, the spherical cap-shaped droplet remains transparent while the more volatile ethanol is evaporating, preferentially at the rim of the drop because of the singularity there. This leads to a local ethanol concentration reduction and correspondingly to oil droplet nucleation there. This is the beginning of phase II, in which oil microdroplets quickly nucleate in the whole drop, leading to its milky color that typifies the so-called "Ouzo effect." Once all ethanol has evaporated, the drop, which now has a characteristic nonspherical cap shape, has become clear again, with a water drop sitting on an oil ring (phase III), finalizing the phase inversion. Finally, in phase IV, all water has evaporated, leaving behind a tiny spherical cap-shaped oil drop.
蒸发的液滴在自然界和技术领域中无处不在，例如在喷墨打印、涂层、材料沉积、医学诊断、农业、食品工业、化妆品或液体泄漏等方面。在过去二十年中，人们对纯液体、含有分散颗粒的液体甚至液体混合物的蒸发进行了深入研究，但对于具有不同挥发性和互溶性的三元液体混合物的蒸发尚未进行探索。在此，我们表明这种三元混合物的蒸发会引发相变以及混合物中一种组分的微滴成核。作为一个模型系统，我们选取了一个静止的茴香烈酒液滴（在日常生活中很常见——水、乙醇和茴芹油的透明混合物），并揭示并从理论上解释了它的四个生命阶段：在第一阶段，球形帽状液滴保持透明，而挥发性更强的乙醇在蒸发，由于液滴边缘处的奇异性，乙醇优先在液滴边缘蒸发。这导致局部乙醇浓度降低，相应地在那里形成油滴核。这是第二阶段的开始，在这个阶段油微滴在整个液滴中迅速成核，使液滴呈现出乳白色，这就是所谓的“茴香烈酒效应”的典型特征。一旦所有乙醇都蒸发完毕，此时具有特征性非球形帽状的液滴再次变清，有一个水滴位于油环上（第三阶段），完成了相转变。最后，在第四阶段，所有水都蒸发了，留下一个微小的球形帽状油滴。
