Department of Chemical Engineering, Loughborough University, Loughborough LE11 3TU, UK.
Langmuir. 2013 Aug 13;29(32):10028-36. doi: 10.1021/la401578v. Epub 2013 Aug 1.
The simultaneous spreading and evaporation of droplets of aqueous trisiloxane (superspreader) solutions onto a hydrophobic substrate has been studied both experimentally, using a video-microscopy technique, and theoretically. The experiments have been carried out over a wide range of surfactant concentration, temperature, and relative humidity. Similar to pure liquids, four different stages have been observed: the initial one corresponds to spreading until the contact angle, θ, reaches the value of the static advancing contact angle, θad. Duration of this stage is rather short, and the evaporation during this stage can be neglected. The evaporation is essential during the next three stages. The next stage after the spreading, which is referred to herein as the first stage, takes place at constant perimeter and ends when θ reaches the static receding contact angle, θr. During the next, second stage, the perimeter decreases at constant contact angle θ = θr for surfactant concentration above the critical wetting concentration (CWC). The static receding contact angle decreases during the second stage for concentrations below CWC because the concentration increases due to the evaporation. During the final stage both the perimeter and the contact angle decrease. In what follows, we consider only the longest stages I and II. The developed theory predicts universal curves for the contact angle dependency on time during the first stage, and for the droplet perimeter on time during the second stage. A very good agreement between theory and experimental data has been found for the first stage of evaporation, and for the second stage for concentrations above CWC; however, some deviations were found for concentrations below CWC.
已通过视频显微镜技术对含三硅氧烷（超润剂）水溶液液滴在疏水性基底上的同时铺展和蒸发进行了实验和理论研究。实验在宽范围的表面活性剂浓度、温度和相对湿度下进行。与纯液体类似，观察到了四个不同的阶段：初始阶段对应于铺展，直到接触角θ达到静态前进接触角θad的值。这个阶段持续时间很短，在此阶段的蒸发可以忽略不计。在接下来的三个阶段中，蒸发是必不可少的。在铺展之后的下一个阶段，在此文中被称为第一阶段，以恒定周长发生，并且在θ达到静态后退接触角θr时结束。在下一个第二阶段中，对于超过临界润湿浓度（CWC）的表面活性剂浓度，周长以恒定接触角θ=θr减小。对于 CWC 以下的浓度，由于蒸发导致浓度增加，静态后退接触角在第二阶段减小。在最后阶段，周长和接触角都减小。在下面，我们只考虑最长的第一阶段和第二阶段。所开发的理论预测了第一阶段接触角随时间的变化以及第二阶段液滴周长随时间的变化的普适曲线。理论与实验数据在蒸发的第一阶段以及 CWC 以上浓度的第二阶段之间具有非常好的一致性；然而，在 CWC 以下的浓度下发现了一些偏差。
