Helium Drops Contain Tiny Tornadoes
Dr Daniel Rolles of the Deutsches Elektronen-Synchrotron sprayed liquid helium into a vacuum chamber through a nozzle so fine the drops were 0.2-2 micrometers across and let evaporation cooling bring the temperature to the point where superfluidity was achieved. They were then hit with an X-ray laser flash that produced images on detectors sensitive enough to measure individual photons at 120 images a second.
“The analysis of the images shows that a surprising number of drops were not spherical as expected, but were pulled length-wise by rapid rotation,” says Rolles. “In fact, some drops possessed more of a shape resembling a thick wheel with two almost parallel sides.”
Rapid is an understatement, with speeds up to 14MHz, caused by the expansion of liquid helium inside the nozzle. Any ordinary fluid would be pulled apart spinning at such speeds, but as a superfluid the helium instead formed quantum vortexes the researchers compare to whirlpools above a drain. The whirlpools were made visible with the addition of xenon to the liquid helium
Something similar had been seen in larger quantities of liquid helium, but co-author of a Science paper announcing the finding, Professor Andrey Vilesov of the University of Southern California, says, “the quantum vortices are surprisingly 100,000 times more densely packed than in the larger samples of superfluid helium that were previously studied.” This makes the cores of the vortexes just an atom across.
This is interesting but does not provide a clear description of quantum vortices(apart from the size and fast rotation). Could someone provide a clearer explanation?