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Researchers verify 70-year theory of turbulence in fluids

When you touch milk in a cup of coffee, you will see fluid turbulence in action – quick mixing that has defied deep scientific understanding.

A collaboration between researchers at the University of Otago, New Zealand and the University of Queensland, Australia, set out to learn more about the daily riddle of turbulence using the superfluid's remarkable properties, strange quantum fluids that can flow endlessly without friction.

The team's breakthrough, just published in Science, can have implications for our understanding of quark-gluon plasmas, solids electrons, and the endurance of Jupiter's Great Red Spot or can help create more efficient transportation.

Co-author Dr. Ashton Bradley, lead researcher at the Dodd Template Center for Photonics and Quantum Technologies, says the group never observed any negative temperature conditions for quantum vortices in an experiment.

"Although it is important for the modern understanding of turbulent fluids, these states have never been observed in nature. They contain significant energy, but appear to be high-ranking, defying our common beliefs about statistical mechanics disorder," Bradley says.

He describes understanding of fluid turbulence as a challenging problem.

"Despite a long history of studies, the chaotic nature of the turbulence has challenged a deep understanding. So much so that the need for a full description has been recognized as one of the Clay Mathematics Institute's unresolved" Millennium Problems ".

"Fluid turbulence plays an important role in our everyday lives. About 30 percent of carbon dioxide emissions come from transport, with fluid turbulence playing an important role. A deeper understanding of the turbulence can eventually contribute to creating a more sustainable world by improving transport efficiency."

An interesting aspect of the turbulence is that it has universal properties, which means that turbulent systems on a scale from microscopic to planet lengths seem to share with similar descriptions and properties.

Nobel laureate Lars Onsager came up with a toy theory for two-dimensional turbulence in 1949. Simply put, it says that if you add enough energy to a 2D system, turbulence will give rise to giant warts – just like in the atmosphere of Jupiter.

His theory, however, applies only directly to superfluids, where the warts rotate in discrete (quantum) steps and are almost particle-like.

Seventy years ago, the Queensland-Otago collaboration has observed Onsager's predictions.

Dr. Bradley says they took advantage of the high control available in the Bose-Einstein condensation laboratory in Queensland's Center of Excellence for Engineered Quantum Systems, using optical manipulation techniques that are groundbreaking there.

They created a superfluid by cooling a gas of rubidium atoms down to almost zero temperature and keeping it in focus by laser beams. The developed optical techniques allow them to adjust the vortex in the fluid – much like touching the milk in your coffee.

Lead author Dr. Tyler Neely, from Queensland, says the "amazing" thing is that the group achieved this with light and on such a small scale.

"The nuclei of the vortexes created in our system are only about 1/10 of the diameter of a human blood cell," he says.

One of the more bizarre aspects of Onsager's theory is that the more energy you add to the Vortice system, the more concentrated they become the giant warts. It turns out that if you consider the vortices as a gas of particles moving inside the superfluid, there are vortex gaps in absolutely negative temperature conditions, under absolutely zero.

"This aspect is very strange. Absolute negative temperature systems are sometimes described as" hotter than hot "because they really want to give up their energy to a normal system at positive temperature, which also means that they are extremely fragile.

"Our study anticipates this intuition by showing that, since the warts are sufficiently isolated within the superfluid, the negative temperature heat clusters can persist for nearly ten seconds," says Dr Neely.


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University of Otago

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