The coalescence process of two liquid droplets where one is placed initially over the other is investigated. The lower drop is placed over a horizontal surface in a sessile configuration. The liquids of interest selected are water, glycerin, and Cs-alloy. The two liquid drops merge under atmospheric conditions. The substrate is superhydrophobic with respect to the three liquids, the equilibrium contact angle being 150°. For the combined drop, the Bond number is ∼0.2. Numerical simulations have been performed in an axisymmetric coordinate system along with supporting experiments. A variety of contact line models reported in the literature have been adopted and compared. Experiments are carried out for validation against simulation with water as the liquid medium. The coalescence phenomenon is recorded by a high-speed camera. The two drops coalesce spontaneously and generate interfacial shapes, velocity fields, footprint, and wall shear stress in time. In water, the combined drop recoils from the surface before spreading over the surface and approaching equilibrium. This trend, including the instant and height of recoil, is correctly realized in the contact line models. Additionally, two distinct timescales originate during the coalescence process. These are associated with inertia and surface tension at small times and inertia–viscosity for longer durations. The instantaneous footprint radius and the average wall shear stress fall to zero during recoil, increase then to a maximum, and diminish to zero with damped oscillations over the longer timescale. Recoil is seen in water as well as Cs-alloy, but not in glycerin. Despite differences in the instantaneous data, these predictions are broadly reproduced by each of the contact line models.

中文翻译：

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垂直排列的液滴在超疏水表面上的聚结

研究了两个液滴的聚结过程，其中一个液滴最初放置在另一个液滴上。较低的液滴以固定配置放置在水平表面上。所选择的感兴趣的液体是水、甘油和铯合金。两滴液滴在大气条件下合并。基材对三种液体具有超疏水性，平衡接触角为 150°。对于组合液滴，邦德数为 ~0.2。在轴对称坐标系中进行了数值模拟以及支持实验。文献中报道的各种接触线模型已被采用和比较。进行实验以验证以水作为液体介质的模拟。聚结现象由高速摄像机记录。两滴自发聚结并及时产生界面形状、速度场、足迹和壁面剪切应力。在水中，组合的液滴在扩散到表面并接近平衡之前从表面反冲。这种趋势，包括后坐力的瞬时和高度，在接触线模型中得到了正确的体现。此外，在合并过程中产生了两个不同的时间尺度。这些与小时间的惯性和表面张力以及较长时间的惯性粘度有关。瞬时足迹半径和平均壁面剪应力在反冲期间降至零，然后增加到最大值，并随着阻尼振荡在更长的时间范围内减小到零。在水中和 Cs 合金中都可以看到反冲，但在甘油中则没有。尽管瞬时数据存在差异，