Maris and Wei Guo, a doctoral student, took advantage of the bubbles that form around electrons in supercold liquid helium. Using sound waves to expand the bubbles and a coordinated strobe light to illuminate them, Guo was able to catch their movements on a home video camera.

A free electron repels the atoms that surround it, creating a small space, or bubble, around itself. In conventional liquids, the bubble shrinks to nothing because the surface tension of the liquid works against the repulsive force. Superfluid helium has very little surface tension, so the bubble can become much larger. The two opposing forces balance when the diameter of the bubble is about 40 angstroms – still far to tiny to see.

The researchers used a planar transducer – basically, a loudspeaker that produces flat, not focused, sound waves – to pummel the whole volume of liquid helium with sound. As each wave overtook an electron bubble, it alternately increased and decreased the surrounding pressure. Under negative pressure, the bubbles expanded to about eight microns, the size of a small speck of dust, then shrank again as the next wave of high pressure washed over them. A strobe light, synchronized to the sound pulse, illuminated the bubbles without overheating the chamber.

Running a camcorder in “super night mode,” Guo and Maris were able to record the approximately 2,000 photons they estimate were scattered by the expanded bubbles, producing a series of electron-bubble images on each frame of videotape.