SEAS Researchers Measure Quantum Repulsive Force

Large-scale levitation may only be possible using smoke and mirrors, but researchers at the School of Engineering and Applied Sciences have measured a repulsive force resulting from quantum fluctuations that could be used to levitate objects on the nanometer scale.

It is common knowledge that like charges repel and unlike charges attract, but it is less obvious that even charge-neutral objects have fluctuating charges.

The charges result from shifting electromagnetic fields that are always present, even in a vacuum, said Jeremy N. Munday, one of the study’s authors.

It was already known that when the two neutral materials are brought into contact, the fluctuating charges can interact and cause an attractive force—the Casimir-Lifshitz force.

However, this was the first study to measure a repulsive force between two charge-neutral objects.

When Munday was a graduate student at SEAS, he set out to measure this repulsive force with V. Adrian Parsegian, a senior investigator at the National Institutes of Health, and Federico Capasso, a professor of applied physics at SEAS.

“What we did was measure the Casimir-Lifshitz force between a metal and an insulator submerged in a fluid and found that a repulsive force can be obtained between particular materials,” Munday said.

When he measured the force between a gold sphere and a gold plate in bromobenzene, he found that there was an attractive force, as expected.

“If you have two similar objects, like two dancers following steps, they tend to attract each other,” said Parsegian. “But with two unlike objects there is the possibility that they repel.”

Indeed, when Munday replaced the gold plate with a silicon dioxide plate, the force became repulsive.

“This is very exciting,” Munday said. “Now we have shown that we can change this force in a dramatic way—from attractive to repulsive.”

Capasso said he hopes that these repulsive forces can be used with microelectromechanical systems (MEMS).

“MEMS are gizmos—for example, they can be used in cars to release the airbag,” Capasso said.

When MEMS are built on the nanometer scale, the Casimir-Lifshitz force becomes dominant and can cause the metallic parts to stick together.

This is known as “stiction,” and prevents the device from working properly.

“By learning how to modify this force, it can be incorporated into the design of the devices rather than just being a hindrance,” Munday said.

If certain properties of the two materials and liquid are chosen appropriately, the liquid will insert itself between the two solids, Capasso said, and this could help reduce stiction.

Munday added that in future experiments they hope to levitate small spherical particles in fluid above a plate, but that the force “cannot be used to levitate trains or frogs or anything like that.”

“This could lead to the development of ultra-low static friction devices and sensors based on engineering these quantum fluctuations,” Munday said.

—Staff writer Alissa M. D’Gama can be reached at adgama@fas.harvard.edu.