This is the first movie that we made.[1,2] The camera is looking through a window into the liquid helium cell in the cryostat. The bottom surface of the sound transducer can be seen just below the top of the window. Each electron is seen as a series of white dots starting near the top of the cell and moving down. The heat dissipated in the transducer gives a downward heat flow (flow of the normal fluid component of the helium) which drags the electron bubbles towards the bottom of the cell.
A typical frame from the movie is shown here. Each bright dot corresponds to an explosion of the electron by a sound pulse. The electron is moving with a velocity of a few mm per second.
Most of the electrons that we see move along slightly curved paths following the heat flow. However, a small number move on snake-like paths. We believe these electrons have become trapped on quantized vortices. The motion of these electrons is the sum of the motion of the electron along the vortex together with a motion of the vortex itself.
Most of the electrons that we see are the result of cosmic rays passing through the liquid. The cosmic ray muons produce electrons in an interesting way. The muon knocks electrons (red dots) off helium atoms leaving the atoms as positive ions (green dots).
The electrons travel a small distance away but are then pulled back by the field of the positively charged-helium ions.
When the electrons return to the atoms they came from high energy photons are emitted.
One or more of these photons may hit the bottom of the transducer and cause an electron to be ejected into the liquid. It is these electrons that we see in the experiment.
We also see some number of electron images which start in the interior of the liquid, at a point far from the surface of the transducer. We believe that these come from electrons which are knocked off atoms when a gamma-ray undergoes Compton scattering. Note that in the image shown below we have recorded only the path of the electron; the purple line shows one possible trajectory of the gamma ray.