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Nanoscale ‘Levitation’ Discovery Could Lead to Better Nanomachines

“We could use this as a quantum mechanical lubricant,” says Harvard physicist Federico Capasso, one of the authors of a paper published this week in the journal Nature.

The new research uses a force first described by the Dutch physicist Hendrik Casimir in 1948. Casimir predicted that due to quantum energy fluctuations, two metal plates placed very close together — within nanometers of each other — would attract one another.

Casimir’s prediction was borne out in lab experiments, but until recently was interesting only to theoretical physicists. In recent years, however, as scientists have begun to work toward building nanoscale machines, they’ve realized that the Casimir force could cause practical problems — making the tiny parts of those machines stick together and gumming up the works.

Capasso, in fact, first became interested in the problem when working at Bell Labs in the 1990s, building micro electromechanical systems (MEMS), the technology used in airbag sensors, among other things.

“We knew that if we kept scaling down the dimensions, from micrometer to nanometer, at some point these surfaces could stick to each other because of Casimir attraction,” Capasso says.

Since the 1960s, though, researchers also realized that theoretically, if the air between the plates were replaced by the right material, the Casimir force would cause the materials to repel one another rather than attract. However, because the forces at work were so tiny, they were never able demonstrate the repulsive Casimir force in the lab.

“I never really took seriously that it could happen,” says Yale physicist Steve Lamoreaux, who was not involved in the research. “Quite frankly I’m flabbergasted that you could maintain enough sensitivity to measure a force like this.”

Capasso and his colleagues demonstrated the repulsive Casimir force by immersing a tiny gold-coated sphere attached to a cantilever arm in a liquid called bromobenzene, then measuring the force as it was repelled from a nearby silica plate. The repulsion worked because the Casimir attractive force between the bromobenzene and the sphere was stronger than that between the sphere and the plate, so the liquid worked its way between the two and essentially pushed the sphere away from the plate.

The researchers demonstrated only the repulsive force, Capasso says, they did not actually make the sphere levitate.

However, such levitation is “conceptually obvious,” Capasso says, and their next goal is to demonstrate it.

“The levitation is going to work, there’s absolutely no doubt,” he says.

Capasso and his colleagues have already applied for a patent for their work. They call the system “nanobearings.”

“If you ask me ‘when is this going to happen’ — who knows?” Capasso says. “But I think any application could be in the next five to ten years.”

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