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High-Tech Helium Tricks May Benefit Earth And Space

Superfluid helium is a liquid with unique and fascinating properties. Below a temperature of about 2 Kelvin 4He, the most common isotope of helium, it completely looses its viscosity. Once set in circular motion, for example, it will keep on flowing forever - without external forces necessary. Unlike all other chemical elements helium does not solidify when cooled down near absolute zero. Physicists explain this phenomenon by extremely weak attractive forces between the almost "perfectly round" atoms and by their rapid motion which is due to Heisenberg's uncertainty principle.
Pasadena - May 18, 2001
Imagine turning on your faucet and watching water flow out and then flow right back up into the faucet. NASA scientists have observed a similar phenomenon by using superfluid helium-4 in laboratory research that could improve earthquake prediction and spacecraft navigation.

The team of scientists, led by Dr. Dave Pearson of NASA's Jet Propulsion Laboratory, Pasadena, Calif., is the first ever to observe this phenomenon, called the Josephson effect, in superfluid helium-4, the most common type of helium. Superfluids allow matter to flow without friction in the same way that electricity flows without resistance in a superconductor.

The Josephson effect was first predicted in 1962 by Nobel Prize Laureate Brian Josephson. For ordinary fluids, a pressure difference along a pipe causes fluid to flow from the high pressure to low pressure area; thus, water comes down out of a faucet and stays there. But with the Josephson effect, when pressure is applied, fluids begin to oscillate back and forth, or up and down, at a rate in direct proportion to the pressure difference. In essence, this effect enables a fluid to defy gravity.

"My colleagues and I used very high-resolution thermometers to precisely control the superfluid temperature to approximately two degrees above absolute zero," Pearson said. Absolute zero is the temperature at which scientists think that no further cooling can occur.

At this extreme cold, helium-4 enters a quantum state, in which its behavior becomes very odd. By using electrostatic forces to create a pressure difference across a pipe, Pearson and his colleagues saw that the fluid began to oscillate from one end to the other.

This strange effect is created because the fluid begins obeying laws of quantum mechanics, which govern how atoms behave at super-low temperatures. "What we see is quantum mechanics on the macroscopic scale," Pearson said. "This was very exciting for us, because we thought various technical factors would prevent the Josephson effect from occurring."

The successful observation of the Josephson effect in superfluid helium-4 allows measurements of very small rotation, enabling scientists to measure very precisely how fast Earth rotates. Monitoring Earth's rotation speed could yield information on minute movement of tectonic plates, which may eventually help predict earthquakes.

In addition, this research could lead to extremely precise, yet simple, gyroscopes to navigate spacecraft. Among the NASA missions that may benefit is the Terrestrial Planet Finder, which may use multiple spacecraft flying in very precise formation to image planets around other stars, looking for Earthlike planets that may harbor life.

The Josephson effect had been observed in superconductors in 1963, then in isotope helium-3 in 1987, but it has eluded researchers for 35 years in helium-4.

Certain properties of helium-4 make it easier to work with in laboratories and in space. The research by Pearson and his team was conducted under a grant from NASA's Biological and Physical Research Program.

Pearson co-authored the quantum experiment paper, which appears in the May 17 issue of the journal Nature, with Drs. Talso Chui and Kalyani Sukhatme of JPL and collaborator Dr. Yury Mukharsky of CEA-DRECAM (Commissariat a l' Energie Atomique, Département de Recherche sur l 'Etat Condensé, les Atomes et les Molécules) from Saclay, France.

Related Links
Office of Biological and Physical Research
Fun Physics
Atoms and Ions in Superfluid Helium
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Lubricating Nanoscale Machines Challenges Conventional Expectations
Atlanta - Feb. 21, 2001
The unique and often unexpected properties of fluids confined to very small spaces will force designers of future nanometer scale devices to reexamine conventional expectations regarding lubrication and fluid flow.

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