State Key Laboratory of Engines, Tianjin University , Tianjin, 300072, China.
Department of Chemical Engineering, Imperial College London , London, SW7 2AZ, United Kingdom.
Langmuir. 2017 Oct 31;33(43):12140-12148. doi: 10.1021/acs.langmuir.7b01901. Epub 2017 Aug 17.
The impact of droplets on liquid films is ubiquitous in natural and industrial processes, and surfactants can significantly alter the impact process by changing the local surface tension. Here we study the impact of droplets on liquid films in the presence of surfactant using high-speed photography, and reveal the flow pattern by dye-tracing. The effects of the droplet size and speed, and the initial film thickness on the impact process are elucidated. The results show that the flow is significantly affected by adding surfactant to the droplet, the liquid film, or to both phases. In particular, the film dye patterns form concentric circles and flower-shaped structures at low and high droplet Weber numbers, respectively. We also show how surfactant-induced Marangoni stresses modify these flow patterns, and alter the characteristics of the phenomena associated with the impact process, such as the propagation of capillary waves, the evolution of the crown, and the formation of secondary droplets. During the impact of surfactant droplets on thin water films, the Marangoni stresses can be sufficiently strong so as to drive film dewetting.
液滴对液体薄膜的冲击在自然和工业过程中无处不在，而表面活性剂可以通过改变局部表面张力显著改变冲击过程。在这里，我们使用高速摄影研究了存在表面活性剂时液滴对液体薄膜的冲击，并用示踪染料揭示了流动模式。研究了液滴尺寸和速度以及初始薄膜厚度对冲击过程的影响。结果表明，向液滴、液体薄膜或两相中添加表面活性剂会显著影响流动。特别是，在低和高液滴韦伯数下，薄膜染料图案分别形成同心环和花形结构。我们还展示了表面活性剂引起的马兰戈尼应力如何改变这些流动模式，并改变与冲击过程相关的现象的特征，例如毛细波的传播、冠的演化和二次液滴的形成。在表面活性剂液滴撞击薄水膜时，马兰戈尼应力可能足够强，足以导致薄膜去湿。
