Bittermann M R, Deblais A, Lépinay S, Bonn D, Shahidzadeh N
Van der Waals-Zeeman Institute, IoP, Science Park 904, Amsterdam, Netherlands.
Sci Rep. 2020 Sep 10;10(1):14863. doi: 10.1038/s41598-020-71964-1.
The processes in which droplets evaporate from solid surfaces, leaving behind distinct deposition patterns, have been studied extensively for variety of solutions. In this work, by combining different microscopy techniques (confocal fluorescence, video and Raman) we investigate pattern formation and evaporation-induced phase change in drying oil-in-water emulsion drops. This combination of techniques allows us to perform drop shape analysis while visualizing the internal emulsion structure simultaneously. We observe that drying of the continuous water phase of emulsion drops on hydrophilic surfaces favors the formation of ring-like zones depleted of oil droplets at the contact line, which originate from geometrical confinement of oil droplets by the meniscus. From such a depletion zone, a "coffee ring" composed of surfactant molecules forms as the water evaporates. On all surfaces drying induces emulsion destabilization by coalescence of oil droplets, commencing at the drop periphery. For hydrophobic surfaces, the coalescence of the oil droplets leads to a uniform oil film spreading out from the initial contact line. The evaporation dynamics of these composite drops indicate that the water in the continuous phase of the emulsion drops evaporates predominantly by diffusion through the vapor, showing no large differences to the evaporation of simple water drops.
液滴从固体表面蒸发并留下独特沉积图案的过程，已针对多种溶液进行了广泛研究。在这项工作中，我们通过结合不同的显微镜技术（共聚焦荧光、视频和拉曼），研究水包油乳液滴干燥过程中的图案形成和蒸发诱导的相变。这种技术组合使我们能够在同时可视化内部乳液结构的情况下进行液滴形状分析。我们观察到，亲水性表面上乳液滴的连续水相干燥有利于在接触线处形成贫油滴的环状区域，这是由弯月面对油滴的几何限制所致。随着水蒸发，由表面活性剂分子组成的“咖啡环”从这样一个贫化区域形成。在所有表面上，干燥都会通过油滴聚结导致乳液不稳定，这种聚结从液滴周边开始。对于疏水性表面，油滴的聚结会导致从初始接触线向外扩展形成均匀的油膜。这些复合液滴的蒸发动力学表明，乳液滴连续相中的水主要通过蒸汽扩散蒸发，与简单水滴的蒸发没有太大差异。
