Partial coalescence of drops on a liquid–liquid interface is a complicated process that is encountered in numerous fields of chemical engineering. It begins when the liquid neck starts to expand and finishes when it contracts to pinch-off. However, mechanisms for liquid neck expansion and contraction are still far from clear. The present work adopts an experimentally verified level-set interface capturing method to investigate the expansion and contraction dynamics of liquid necks during the coalescence process. A simulation case is performed prior to analysis of the normal behavior of the partial coalescence for an aqueous drop on a glycerol aqueous solution–silicon oil interface. When the normal simulation is reinitialized at different instants with the velocity field set to null, the results indicate that the whole coalescence process can be divided into three substages. In the first substage, the liquid column still partially coalesces. In the second stage, the liquid column completely merges. In the third substage, the liquid column partially coalesces again. It suggests that the expansion and contraction of liquid necks are not solely determined by the net capillary pressure arising from the local azimuthal and axial curvature, as traditionally supposed. The downward fluid flow at the liquid neck also produces a negative pressure due to the Bernoulli effect. Quantitative results demonstrate that the liquid neck contracts if the joint pressure arising from the azimuthal curvature and the downward velocity field is larger than that from the axial curvature. Otherwise, the liquid neck expands.

中文翻译：

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液-液界面上液滴合并过程中液颈的膨胀和收缩动力学

液-液界面上的液滴部分聚结是一个复杂的过程，在化学工程的许多领域中都会遇到。它从液颈开始膨胀时开始，到收缩捏断时结束。然而，液颈膨胀和收缩的机制仍远未清楚。本工作采用一种经过实验验证的水平集界面捕获方法，研究了液颈在聚结过程中的膨胀和收缩动力学。在分析甘油水溶液-硅油界面上水滴的部分聚结的正常行为之前，先执行模拟案例。当法线模拟在不同时刻重新初始化时，速度场设置为 null，结果表明整个合并过程可以分为三个子阶段。在第一个子阶段，液柱仍部分聚结。在第二阶段，液柱完全合并。在第三个子阶段，液柱再次部分聚结。它表明，液颈的膨胀和收缩并不像传统上假设的那样完全由局部方位和轴曲率产生的净毛细管压力决定。由于伯努利效应，液颈处的向体流动也会产生负压。定量结果表明，如果方位曲率和向下速度场产生的联合压力大于轴向曲率产生的接头压力，则液颈会收缩。否则，液颈会膨胀。