Friday, March 03, 2017

The Quantum Storm Inside of a Superfluid

The mini tornadoes that form in superfluids won’t send any cows flying through the air, but the scientists from Newcastle University behind a new study were surprised to see that these mini twisters can create quite a tangled storm. Their results suggest that superfluids have a deeper connection to everyday fluids than previously thought, and will soon be published in the American Physical Society’s journal Physical Review Letters.

Two different views of the surface of a wire (yellow) and resulting vortices (red). In the top figure, note the turbulent boundary layer up to approximately the height of the tallest mountains and the region of small vortex rings above it. If a vortex happens to form a coil (which they often do), this can detach and escape as a vortex ring.
Image Credit: G. W. Stagg, N. G. Parker, and C. F. Barenghi, Physical Review Letters, Superfluid boundary layer.

By splashing, bathing, painting, and helping to make cookies as kids, most of us develop an intuitive understanding of liquids and how they respond to a mixing spoon or the bathtub wall. Scientists study how liquids flow in a more formal way, using mathematical equations to describe and predict how fluids behave. Although liquids vary in many ways—think oil versus water—most of them flow according to the same basic rules.

However, if you cool liquid helium down to near absolute zero, those rules no longer apply. The liquid can flow with no resistance, earning it the name superfluid. If you’ve ever tried to cook with honey you know how difficult it is to get every drop out of a measuring cup—that’s because it is so viscous. A superfluid has zero viscosity: Stir a cup of a superfluid and it will continue rotating forever. This strange behavior is the result of quantum mechanical effects that arise because helium stays a liquid at such low temperatures.

Understandably, scientists have been intrigued by superfluids since their discovery about eighty years ago. They are currently used in some high-tech applications, but the better we understand how superfluids move and behave, the better we can exploit their unique properties.

The flow of a fluid is usually classified as turbulent or laminar (smooth). This is true for regular fluids and superfluids, although turbulence looks different in the two cases. In regular fluids, swirling eddies can take on any shape, size, or strength. In superfluids, quantum mechanics limits the eddies to a fixed strength and size (on the scale of nanometers) and they take the form of mini-tornadoes called vortices.

To better understand superfluids, scientists often generate turbulence by moving a wire or grid through the fluid. You can’t see the details of turbulence formation directly, so scientists have to infer what is happening by studying changes in the superfluid’s properties. The problem is that, without being able to see exactly what’s going on, it can be difficult to interpret the results of turbulence experiments.

This lead researchers Nick Parker, Carlo Barenghi and doctoral student George Stagg to create a computer model of the three-dimensional surface of a wire used in an actual turbulence experiment. While the wire looks smooth to the naked eye, zoom in to the nanoscale—where vortices form—and you’ll see that the surface is covered in sharp grooves and steep ridges, kind of like a mountain range. Once they modeled this surface, they ran a massive computer simulation of a superfluid flowing past the wire.

“This was the most intensive simulation we ever performed in our group, taking around one year to get the final results. Even processing the data was a slow task due to the sheer size of the data set,” says Parker. “But it was worth the effort as we saw something unexpected - the vortices were created at the mountains, got tangled up with each other, and formed a striking dense layer of knitted vortices, stuck to the boundary.”

When normal fluid encounters an object, it slows down—think of water flowing through a river on which there is a barge. The water near the barge slows down due to viscous forces, but the water far from the barge maintains its original speed. The result is a layer of water with different flow properties than the rest of the river, called a boundary layer.

With this work, the Newcastle University scientists demonstrate that boundary layer can also form in a superfluid. Boundary layers look different in superfluids, like a storm of tangled vortices, but two follow-up simulations confirm that their behavior is surprisingly similar. Scientists have long assumed that viscosity plays an essential role in forming boundary layers—but superfluids have no viscosity. How can this be? Could superfluids and regular fluids be two different manifestations of some universal principle? We don’t know yet.

In the meantime, there are some concrete predictions for experimentalists to dig into. The simulation shows that if a vortex forms a coil, which happens often, it can escape and travel into the surrounding fluid as a vortex ring. These should be detectable by experiments. In addition, the results suggest that wires and other objects used to create turbulence, and even the walls of the containers that hold superfluids, may be lined with a thin, dense layer of tangled vortices. Is this the case? And if so, what are the experimental implications?

They may be ridiculously small compared to the tornadoes that make the news, but these tiny twisters sure have a lot to teach us: on top of their practical applications, superfluids provide the strongest known analogy for the quantum vacuum—the "substance" of spacetime that matter and energy represent excitations of. The vortices that form in superfluid Helium-3 behave a lot like quarks and electrons, the fundamental particles of our universe, and the collective behavior of the fluid acts a lot like the electromagnetic and gravitational fields. Perhaps by studying and simulating these systems, we can learn not just about the behavior of this strange state of matter, but about the universe overall.

Kendra Redmond

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