In one experiment we have accidentally produced bubbles containing a large number of electrons.  We think this happened in the following way. The apparatus contained a radioactive beta-source. Under normal circumstances, the electron bubble produced by the source will diffuse throughout the liquid and ultimately escape into the metal walls of the cell or into the metal coating on the sound transducer. We suspect that in this particular experimental run the surface of the sound transducer was covered with a layer of non-conducting material, perhaps grease from a fingerprint. This prevented the bubbles reaching the metal coating, and resulted in the transducer being covered with a large number of bubbles each containing a single electron.
When the transducer is driven most of these electron bubbles will explode and grow rapidly.
When they reach sufficient size they will coalesce and bubbles each containing many electrons will move off into the liquid.
Here is a frame from a movie which shows a large multi-electron bubble moving through the liquid helium. The bubble appears as two bright spots; these two spots come from light reflected from the front and from the back of the bubble. The green circle indicates the approximate position of the bubble surface. We think this bubble contains approximately one million electrons.
- ^D. Jin and H.J. Maris, "A Study of the Motion of Particles in Superfluid Helium-4 and Interactions with Vortices", J. Low Temp. Phys. 162, 329 (2011).