Effect of velocity, fluid properties and drop shape on coalescence and neck oscillation

TL;DR

This study uses numerical simulations to explore how velocity, fluid properties, and drop shape influence the coalescence and neck oscillation of liquid drops, revealing the roles of various forces and identifying transition regimes.

Contribution

It generalizes partial coalescence mechanisms across different drop shapes and elucidates the influence of dimensionless parameters on coalescence behavior and secondary droplet formation.

Findings

Vertical collapse rate affects coalescence outcomes.

Rayleigh-Plateau instability is insignificant in the studied regime.

Multiple neck oscillations delay secondary droplet pinch-off.

Abstract

We perform axisymmetric numerical simulations to investigate the coalescence dynamics of a liquid drop in a deep liquid pool. This study aims to generalize the mechanisms of partial coalescence across a range of drop shapes, elucidate the underlying mechanism of neck oscillations, and examine the roles of inertial, viscous and gravitational forces, quantified by the Weber, Ohnesorge, and Bond numbers, in governing the coalescence behavior. A phase diagram is constructed to delineate the boundaries between partial and complete coalescence regimes based on these dimensionless parameters. Our analysis of the height-to-neck ratio shows that, upon contact with the pool, the primary drop forms an upward liquid column that ultimately pinches off due to inwardly directed horizontal momentum. Additionally, the study suggests that as the dimensionless numbers increase, the effect of the vertical collapse rate plays a significant role in the outcome of the coalescence process. Notably, the Rayleigh-Plateau instability is found to be insignificant in driving partial coalescence within the explored parameter space. We identified a transition regime between partial and complete coalescence, characterized by multiple neck oscillations that delay the pinch-off of secondary droplets. The formation of secondary droplets is most prominent for prolate drops, followed by spherical and oblate drops of comparable volume. Furthermore, we observe that the tendency to form multiple droplets from elongated liquid columns diminishes with an increase in the impact velocity of the primary drop.