Helium Drops Contain Tiny Tornadoes

SLAC National Accelerator Laboratory Artist's impression of the quantum vortexes packing tiny liquid helium droplets.

The world's smallest whirlpools have been found, hidden inside drops of liquid helium cooled to temperatures barely above absolute zero and spun to unimaginable speeds.

When helium is cooled below 2.2K (-271 C) it starts to behave in very strange ways, becoming what is known as a superfluid, where there is no viscosity at all. As the easiest superfluid to create and study liquid helium can provide us with insights into other superfluids, which may include the interiors of neutron stars and, according to an astonishing  theory, spacetime.

One of the theoretical aspects of superfluids experimentalists have been chasing in liquid helium is vortexes. These may serve as models for the disturbances in the material of the very early universe that eventually became galaxies, but there creation has taken decades.

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. 

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