Are we on the cusp of frictionless mechanical systems?

Mechanical systems, such as gears and wheels, are core components of machines that we use daily. But these machines often require expensive maintenance or replacement because friction caused by physical contact — gears grinding or wheels rubbing against another material — can wear these systems down, causing them to malfunction or work inefficiently.

But now, Berkeley Engineering researchers have developed a novel approach to eliminate friction in mechanical systems by harnessing a phenomenon of theoretical quantum physics, known as the Casimir effect. Their discovery paves the way for exploring new forms of non-friction mechanics and quantum mechanical applications. A paper describing their work appears today in Science.

“Until recently, people thought of the Casimir effect as an attractive force. But for the first-time, our work proposed theoretically and demonstrated experimentally a repulsive Casimir force that eventually leads to a stable Casimir trap, without input of extra energy,” said Xiang Zhang, professor of mechanical engineering and corresponding author of the study. Zhang is also the vice-chancellor and president of the University of Hong Kong.

“This ultimately means that the attraction between two objects of the same material can be reversed at short and preserved at long distances, so they can remain without physical contact for a specific distance, reaching the stable trapping equilibrium,” said Sui Yang, a research scientist in Zhang’s lab and one of the lead authors of the paper.

The Casimir effect, named for Dutch physicist Hendrik Casimir, is based on the fact that in quantum physics, a vacuum in space isn’t empty. It is filled with fluctuating electromagnetic waves (light) that can’t be completely eliminated. These waves exist in a range of lengths, and their presence implies that empty space contains a certain amount of energy.

“In addition to being a fundamental scientific advancement, this discovery could have a variety of applications, such as contact-free nanomachines and ultrasensitive force sensors.”

In 1948, Casimir found that when mirrors are placed facing each other in a vacuum, some waves will fit between them, bouncing back and forth, and others won’t. As the two mirrors move closer together, the longer waves will no longer fit, so the total amount of energy between the mirrors is less than elsewhere in the vacuum. This shift in energy is what causes the mirrors to attract to each other, a phenomenon known as the Casimir effect. Meanwhile, the energy between the mirrors is known as Casimir force.

“Since the discovery of Casimir force, the stable Casimir equilibria (trap) has been long pursued by the scientists. However, the study only remained theoretical with extremely complicated design. In [our] study, we theoretically propose and experimentally demonstrate that stable Casimir equilibria can be achieved by a simple coating method,” said Rongkuo Zhao, a former postdoctoral researcher in Zhang’s Lab and one of the lead authors of the paper.

In addition to mirrors, the Casimir effect occurs between objects of the same material that are placed in a vacuum. In their paper, Zhang and his colleagues demonstrated these effects with gold. When they coated gold objects with Teflon, a relatively low-reactive index thin film, they discovered that the Casimir force between the two objects repelled at short distances and attracted at longer ones.

Zhao noted that the refractive index is not a constant number; it changes as a function of wavelength. And the index contrast created between gold and Teflon allowed the Berkeley researchers to control the phase of the virtual electromagnetic field through the Casimir interaction. So that at short wavelengths, the phase contribution of Teflon’s low-refracted index became significant and gave a repulsive force. At longer wavelengths, the influence of gold’s higher-refractive index became significant and caused the attraction.

By confining these quantum fluctuations between two gold objects just nanometers apart, the researchers essentially created a Casimir trap without needing any additional energy.

“This trap is totally passive, and the trapping distance can be precisely controlled by the thickness of the coating layer. The same principle can cover a huge family of materials, and there are almost infinite opportunities for creating such non-contact Casimir trapping equilibria at the nanometer scale,” said Yang.

The researchers note that this discovery, in addition to being a fundamental scientific advancement, could have a variety of applications, such as contact-free nanomachines and ultrasensitive force sensors.