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Repulsive Force Could Eliminate Nanofriction

A force that causes nanoscale objects to repel each other has engineers dreaming of quantum levitation.

When two objects are so close together that the distance between them is about the same size as quantum fluctuations called virtual particles, they’re pulled together. This effect, caused by the Casimir force, is not something that humankind has had to worry about until recently. But as researchers develop nanomechanical devices for communications and computation, so-called “stiction” has emerged as a potential stumbling block that might, for example, limit the density of memory chips. But there’s a flip side to the Casimir force that might enable, rather than hinder, nano devices. Hendrik Casimir, who described his eponymous force in 1948, and Evgeny Lifshitz, who expanded his work, predicted that at slightly larger distances, this force should turn repulsive. Now researchers at Harvard University and the National Institutes of Health have seen this repulsive force in the lab for the first time.

In this illustration of the Casimir force, a tiny gold sphere and plate experience “stiction” (right). But with the right combination of materials, as at left, where a gold sphere is paired with a silica plate, the Casimir force reverses, becoming repulsive. Future nanoscale devices might take advantage of this effect. Credit: U. Christensen

The researchers reversed the Casimir force through their choice of materials. Whether the force is attractive or repulsive, it turns out, depends on the relative dielectric permittivities of the two surfaces and of the medium that lies between them. (Dielectric permittivity is a material property that describes how a material interacts with electrical fields.) When the researchers brought together a gold-coated sphere about 40 micrometers in diameter and a silica plate, both submerged in the liquid bromobenzene, they measured a repulsive Casimir force. The gold sphere was attached to an atomic force microscope, which was used to detect this repulsion. These results are described in the journal Nature.

These results suggest that it should be possible to create stictionless, friction-free nanomechanical devices based on what the researchers call quantum levitation. It’s not yet clear what applications will be found for quantum levitation, but according to a press release from Harvard, the researchers have filed a U.S. patent covering nano devices based on the phenomenon. Think friction-free ball bearings and ultrasensitive chemical detectors.

The Harvard researchers were led by Federico Capasso, a physicist who developed the first quantum-cascade laser at Bell Labs in the mid-1990s. He has also been featured in our 10 Emerging Technologies section in 2007 for his work on optical antennas.

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