Published online 7 January 2009 | Nature | doi:10.1038/news.2009.4


Quantum force gets repulsive

Casimir–Lifshitz effect could help nanoengineers out of a sticky situation.

gearTiny machines could benefit from a little repulsion.D. PARKER/SPL

A strange quantum force could be used to make tiny machine parts levitate in frictionless nanomachines of the future.

The Casimir effect describes the attraction that occurs between two parallel, uncharged metal plates held very close together in a vacuum. It has been a curiosity of quantum physics since it was postulated 60 years ago by Hendrik Casimir, a Dutch physicist.

Now, Federico Capasso of Harvard University and his co-workers have measured a repulsive version of the force, which Capasso says could allow tiny machines to be made so that their moving parts do not touch.

The force that Casimir predicted is a quantum effect caused by the constant fluctuations in the electromagnetic field between and around the two plates.

The wavelengths of the photons that make up the electromagnetic field are affected by the distance between the two plates, particularly when these are only a few nanometres apart. This makes the electromagnetic field between the plates different from that surrounding them. As the system tries to overcome this imbalance, the plates are squeezed together.

In the 1950s and 1960s, the Russian physicist Evgeny Lifshitz extended Casimir's theory to include real metals, rather than theoretical 'ideal' metals, and dielectric materials — those that are electrically insulating but that can still support an electromagnetic field. Lifshitz predicted that these forces could be repulsive as well as attractive. His name was added to what is now known as the Casimir–Lifshitz force.

So far, only attractive forces have been probed in detail, mostly out of curiosity. But as nanoengineered machines become more popular, the Casimir–Lifshitz effect has begun to cause problems because it causes tiny pieces to stick together. Capasso says that as engineered devices continue to get smaller and smaller, these quantum forces need to be taken seriously.

As tiny components get closer to each other, nanoengineers either have to avoid these interactions to prevent parts getting stuck together — or they could use the Casimir effect to their advantage, he says.

Useful repulsion

Casimir's original theoretical design and Capasso's group's experiment are different. Capasso's team replaced the vacuum with a liquid, bromobenzene, and, instead of metal plates, used a gold-coated polystyrene sphere attached to a cantilever, and a silica plate.

The key to the experiment is the dielectric permittivity of each of these materials. This property represents a material's ability to carry an electric field. To get a repulsive force out of the system, the dielectric permittivity of one plate must be higher than that of the surrounding liquid, and the dielectric permittivity of the second plate must be lower than that of the surrounding liquid. "We're talking about a repulsion that is controlled by the ordering of the dielectric properties of the materials, not the shape," says Capasso.

In the set-up used by Capasso's group, gold has the highest dielectric permittivity, followed by bromobenzene, followed by silica. The Casimir-Lifshitz force works so that the liquid is attracted into the gap between the two, forcing them apart.

Capasso used the cantilever attached to the gold-coated sphere to measure the size of the repulsive force. A change in a beam of light reflected off the top of the cantilever signalled movement in the system, and revealed that as the gold sphere was brought close to the silica plate it got pushed back. The results are published in Nature1.

The effect occurs only at separations of less than about 120 nanometres. The researchers found the force to be strongest — around 150 trillionths of a newton — when the gold and silica were about 20 nanometres apart.

"This paper is a milestone in experiments on Casimir forces and quantum fluctuations," says physicist Ho Bun Chan at the University of Florida in Gainsville, who works on microelectromechanical systems.

Steve Lamoreaux of Yale University was one of the first to accurately measure the attractive Casimir–Lifshitz force, and is impressed by Capasso and colleagues' experiment. The Casimir–Lifshitz force has become a "bit of a nuisance" for nanomachines, Lamoreaux says, and harnessing the repulsive force may provide a solution, stopping tiny clean surfaces from sticking together.

Chan adds that the approach would be particularly useful "if future research can demonstrate repulsive forces with a liquid that is more user-friendly than bromobenzene."

The next step is to use the phenomenon to levitate a tiny piece of gold, or other material, in the liquid. "The levitation experiment should be straightforward," says Capasso, and, if so, could be exploited in tiny machines within 10 years. "I have a hunch that something useful will come out of this," he adds. 

  • References

    1. Munday, J. N., Capasso, F. & Parsegian, V. A. Nature 457, 170–173 (2009).
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