Discovery in Helium Droplets

Liquid helium, when cooled down nearly to absolute zero, exhibits unusual properties that scientists have struggled to understand: it creeps up walls and flows freely through impossibly small channels, completely lacking viscosity. It becomes a new state of matter — a “superfluid.”

Now a large, international team of researchers led by scientists at USC Dornsife, Stanford and Berkeley has used X-rays from a free-electron laser to peer inside individual droplets of liquid helium, exploring whether this liquid helium retains its superfluid characteristics even at microscopic scales — such as in tiny droplets of helium mist.

“One of the ways that superfluidity manifests is through the formation of quantum vortices, but they have never been experimentally observed in droplets,” said Andrey Vilesov, professor of chemistry and physics at USC Dornsife and co-corresponding author of the study, which appeared in Science on Aug. 22.

Vilesov’s co-corresponding authors include Christoph Bostedt, senior staff scientist at the SLAC National Accelerator Laboratory, and Oliver Gessner of the Lawrence Berkeley National Laboratory.

The team found that quantum vortices, or whirlpools, form in spinning helium nanodroplets in unprecedented quantities, leading to extreme deformation of the droplets into flat discs resembling a thick pancake. In addition, the group also noticed anomalously large numbers of vortices in the droplets, indicating they even behave differently than other superfluids.

“Now we are able to study this form of matter in a new way that allows us to see how quantum mechanics manifests itself in the motion of an isolated superfluid,” Gessner said.

If a drop of normal liquid spins, centrifugal force pushes the liquid away from the center, leading to equatorial and polar deformation — much like the way the spinning of the Earth makes it deviate slightly from a perfect sphere. If a planet spins fast enough, it distorts into a peanut shape before flying apart, unable to sustain the strain of its own motion.

Spinning superfluid helium similarly distorts its shape, but also forms a honeycomb consisting of countless quantum vortices, in contrast to the single vortex formed in a normal liquid. The lack of viscosity in the superfluid allows these vortices to persist indefinitely.

As such, in a superfluid droplet, the rotation is evenly distributed in vortices, which allows the droplet to withstand stunningly large rotation speeds without forming lobes or disintegrating.

“Even though it is a liquid, there’s a higher degree of order — and we were able to image that. It’s far beyond what people originally thought free electron lasers were able to do,” Bostedt said.

Some speculate that studying superfluids could help understand the origins of the universe; in the moments following the Big Bang, the universe was uniform in density and similar to a superfluid. The creation of density fluctuations in the form of quantum vortices may be what led to the early formation of galaxies.

“A cosmos hides within each helium droplet, and we looked inside over 10,000 of them,” said Curtis Jones, USC Dornsife chemistry graduate student and research assistant.

Vilesov and his colleagues also plan to study how the presence of quantum vortices affects chemical reactions and cluster formation inside helium nanodroplets.

This research was funded by the National Science Foundation (grant CHE-1112391), the U.S. Department of Energy (Contract No. DE-AC02-05CH11231) and the Max Planck Society.