Baldwin Kyle Anthony, Fairhurst David John
School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, UK.
Soft Matter. 2015 Feb 28;11(8):1628-33. doi: 10.1039/c4sm02642j.
Although the evaporation mode of sessile droplets is almost universally characterized as either constant contact radius (CCR) or constant contact angle (CCA), here we investigate two alternatives where the contact line speed is either constant or inversely proportional to the droplet radius. We present supporting evidence from our experiments on poly(ethylene oxide) (PEO) polymer solutions and blood, and from literature on pure and binary liquids, colloidal suspensions, soft substrates, reactive dewetting and hole nucleation. We introduce the use of novel "clock-drop" images to visualize droplet evolution and dimensionless height-radius plots to characterize the evaporative pathways. Combining these with a simple scaling argument, we show that receding speed is inversely proportional to the three-phase contact radius R, with a constant of proportionality A, which is dependent on the drying conditions and drop shape, but independent of drop volume. We have shown that this is equivalent to a linear decrease in contact area with time. By varying only A, which we achieved experimentally by choosing solutions whose precipitate constricts after deposition, the evaporation mode can be altered continuously to include the two established modes CCR and CCA, and two new modes which we term "slowly receding" and "rapidly receding", which are characterised by fully dried "doughnut" and "pillar" deposits respectively.
尽管固着液滴的蒸发模式几乎普遍被描述为恒定接触半径（CCR）或恒定接触角（CCA），但在此我们研究另外两种情况，即接触线速度要么恒定，要么与液滴半径成反比。我们展示了来自聚环氧乙烷（PEO）聚合物溶液和血液实验的支持证据，以及来自纯液体和二元液体、胶体悬浮液、软基底、反应性去湿和空穴成核等方面文献的支持证据。我们介绍了使用新颖的“时钟液滴”图像来可视化液滴演变，以及使用无量纲高度 - 半径图来表征蒸发路径。将这些与一个简单的标度论证相结合，我们表明后退速度与三相接触半径R成反比，比例常数为A，A取决于干燥条件和液滴形状，但与液滴体积无关。我们已经表明，这等同于接触面积随时间呈线性减小。通过仅改变A（我们通过选择沉积后沉淀物收缩的溶液在实验中实现这一点），蒸发模式可以连续改变，以包括两种已确立的模式CCR和CCA，以及两种新的模式，我们将其分别称为“缓慢后退”和“快速后退”，其特征分别是完全干燥后的“甜甜圈”和“柱状”沉积物。
