Coalescence of bubbles and drops in an outer fluid

TL;DR

This paper investigates the coalescence process of bubbles and drops in an external fluid, revealing that early-stage dynamics are dominated by drop viscosity and identifying conditions where the surrounding fluid's viscosity influences the merging behavior.

Contribution

It provides new insights into the coalescence dynamics in dense fluids, highlighting the dominance of drop viscosity in early stages and introducing a phase diagram for late-time behavior.

Findings

Early coalescence dominated by drop viscosity

External fluid influences late-time coalescence under certain conditions

Identified a dimensionless number governing the transition in dynamics

Abstract

When two liquid drops touch, a microscopic connecting liquid bridge forms and rapidly grows as the two drops merge into one. Whereas coalescence has been thoroughly studied when drops coalesce in vacuum or air, many important situations involve coalescence in a dense surrounding fluid, such as oil coalescence in brine. Here we study the merging of gas bubbles and liquid drops in an external fluid. Our data indicate that the flows occur over much larger length scales in the outer fluid than inside the drops themselves. Thus we find that the asymptotic early regime is always dominated by the viscosity of the drops, independent of the external fluid. A phase diagram showing the crossovers into the different possible late-time dynamics identifies a dimensionless number that signifies when the external viscosity can be important.