  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 proceed. 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 causes initially a Rayleigh-Taylor
instability of the liquid rim. At later times, the rim takes heals into a
circular shape. However for sufficient high concentrations of the surfactant
the unstable rim pinches off microbubbles resulting in a fractal dendritic
structure after coalescence.
