van Gaalen R T, Diddens C, Wijshoff H M A, Kuerten J G M
Department of Mechanical Engineering, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.
Department of Mechanical Engineering, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands; Faculty of Science and Technology (TNW), University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands.
J Colloid Interface Sci. 2020 Nov 1;579:888-897. doi: 10.1016/j.jcis.2020.06.099. Epub 2020 Jul 3.
There are two different sharp-interface models for moving contact lines: slip models and precursor film models. While both models predict a mostly constant contact angle during the evaporation of pure droplets, it is expected that they behave differently when surfactants are present, because of the inherent dissimilarities in their respective interface definitions.
Both contact line models are numerically implemented using lubrication theory to analyze evaporating droplets. A convection-diffusion equation is implemented for insoluble surfactants. For pure droplets the models are compared with experiments performed by Nguyen et al. (2012).
The two contact line models show results comparable to the experiments with pure droplets. If insoluble surfactants are present, the slip model increasingly shows pinning-like behavior as the initial surfactant concentration is increased. This 'quasi-pinning' is found to be consistent with experimental results in literature. The precursor film model, in contrast, shows no significant change when surfactants are added. This lack of change is a result of surfactant flowing from the droplet into the precursor film and vice versa. While suggesting potential solutions to this unphysical behavior, it is concluded that in the context of surfactants, slip models are preferable over precursor film models given the current state of the art.
对于移动接触线存在两种不同的尖锐界面模型：滑移模型和前驱膜模型。虽然这两种模型都预测在纯液滴蒸发过程中接触角大多保持恒定，但预计当存在表面活性剂时它们的行为会有所不同，这是由于它们各自界面定义存在内在差异。
两种接触线模型都采用润滑理论进行数值实现，以分析蒸发液滴。针对不溶性表面活性剂实施对流扩散方程。对于纯液滴，将模型与Nguyen等人（2012年）进行的实验进行比较。
两种接触线模型显示的结果与纯液滴实验结果相当。如果存在不溶性表面活性剂，随着初始表面活性剂浓度增加，滑移模型越来越多地表现出类似钉扎的行为。发现这种“准钉扎”与文献中的实验结果一致。相比之下，前驱膜模型在添加表面活性剂时没有显著变化。这种缺乏变化是表面活性剂从液滴流入前驱膜以及反之亦然的结果。在提出针对这种非物理行为的潜在解决方案的同时，得出结论：就表面活性剂而言，鉴于当前技术水平，滑移模型比前驱膜模型更可取。
