Surface bubble coalescence

We present an experimental study of bubble coalescence at an air-water interface and characterize the evolution of both the underwater neck and the surface bridge. We explore a wide range of Bond number, Bo, which compares gravity and capillary forces and is a dimensionless measure of the free surface's effect on bubble geometry. The nearly spherical Bo << 1 bubbles exhibit the same inertial-capillary growth of the classic underwater dynamics, with limited upper surface displacement. For Bo > 1, the bubbles are non-spherical - residing predominantly above the free surface - and, while an inertial-capillary scaling for the underwater neck growth is still observed, the controlling length scale is defined by the curvature of the bubbles near their contact region. With it, an inertial-capillary scaling collapses the neck contours across all Bond numbers to a universal shape. Finally, we characterize the upper surface with a simple oscillatory model which balances capillary forces and the inertia of liquid trapped at the centre of the liquid-film surface.

Automated summary

An experimental study was conducted on bubble coalescence at an air-water interface, examining the evolution of underwater neck and surface bridge. The research explored a wide range of Bond numbers to compare gravity and capillary forces, finding different behaviors for nearly spherical bubbles with Bo << 1 and non-spherical bubbles with Bo > 1. By observing inertial-capillary growth and using a simple oscillatory model, the study characterized the dynamics of both the bubble neck and upper surface in various Bond number scenarios.