Abstract A sessile binary mixture droplet (BMD) has a relatively simple geometry; it is in the shape of a spherical cap. However, the contact line motion of a BMD during evaporation shows complex transient behavior that cannot be modeled using the current droplet evaporation model. The present study aims to quantitatively investigate the influence of initial ethanol concentrations on the contact line dynamics of an ethanol-water BMD. We use surface plasmon resonance imaging (SPRi) to simultaneously measure contact line motion and ethanol concentration during BMD evaporation. The visualization results show that non-monotonic contact line motions during evaporation can be subdivided into three stages: I) an initial spreading stage, II) a rapid sliding stage, and III) a moderate sliding stage. Results show behavior that is quite different from the typical motion of an evaporating DI water droplet, i.e., a simple pinning-depinning contact line motion. A weighted ethanol flux is newly introduced to consider the effect of spatial and temporal evaporative flux on the complex sliding motion of a BMD. Moreover, the weighted ethanol flux is quantitatively correlated with the receding velocity of the contact line in stage II.

中文翻译：

---

使用表面等离子体共振成像定量分析蒸发二元混合物液滴的接触线行为

摘要 固着二元混合液滴（BMD）具有相对简单的几何形状；它呈球形帽状。然而，蒸发过程中 BMD 的接触线运动显示出复杂的瞬态行为，无法使用当前的液滴蒸发模型进行建模。本研究旨在定量研究初始乙醇浓度对乙醇-水 BMD 接触线动力学的影响。我们使用表面等离子体共振成像 (SPRi) 在 BMD 蒸发过程中同时测量接触线运动和乙醇浓度。可视化结果表明，蒸发过程中的非单调接触线运动可以细分为三个阶段：I) 初始扩散阶段，II) 快速滑动阶段，和 III) 中度滑动阶段。结果显示的行为与蒸发去离子水滴的典型运动完全不同，即简单的钉扎-脱钉接触线运动。新引入了加权乙醇通量以考虑空间和时间蒸发通量对 BMD 复杂滑动运动的影响。此外，加权乙醇流量与阶段 II 中接触线的后退速度定量相关。