Coalescence of Multielectron Bubbles in Liquid Helium

In 1977, Volodin et al. observed that the electrohydrodynamic instability of charged helium surfaces can lead to the loss of the electrons from the surface in the form of bubbles. These are Multielectron Bubbles (MEBs) which contain electrons pinned on their inner surfaces. The MEBs form a model system for studying electrons on curved surfaces, which are predicted to have many interesting properties. Recent experiments showed that above the Lambda point, MEBs could be trapped using a Paul-trap and their sizes are mainly determined by the amount of vapour present inside. Here, we report the experimental observation of the coalescence of two MEBs which were moving upward in bulk liquid helium-4. The charge and radii of the MEBs were determined before and after the coalescence. The merging of two similar charged bubbles was possible because of the presence of vapour inside the merging MEBs.