Merging of soap bubbles and why surfactant matters

Abstract

The merging of two soap bubbles is a fundamental fluid mechanical process in foam formation. In the present experimental study, the liquid films from two soap bubbles are brought together. Once the liquid layers initially separated by a gas sheet are bridged on a single spot, the rapid merging of the two liquid films proceeds. Thereby, the connecting rim is rapidly accelerated into the separating gas layer. We show that, due to the dimple formation, the velocity is not uniform and the high acceleration initially causes a Rayleigh–Taylor instability of the liquid rim. At later times, the rim heals and takes on a circular shape. However, for sufficiently high concentrations of the surfactant, the unstable rim pinches off microbubbles, resulting in a fractal dendritic structure after coalescence.