Expansion and Contraction Dynamics of Liquid Necks during the Merging of Drops on a Liquid–Liquid Interface

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.