Repulsive Casimir Measured

For the first time, physicists have measured a repulsive Casimir force that could be tailored for a wide range of new nanotechnology applications.

The study, which builds on previous work on the quantum mechanical force, was led by Federico Capasso, Robert L. Wallace Professor of Applied Physics at Harvard's School of Engineering and Applied Science (SEAS), and involved researchers from Harvard University and the National Institutes of Health (NIH) in Bethesday, Md.

Shown are researchers Jeremy Munday, postdoctoral scholar, California Institute of Technology, and Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at the Harvard School of Engineering and Applied Sciences. (Image: Eliza Grinnell, Harvard SEAS)

Long considered to be of theoretical interest only, the Casimir force, which is caused by quantum fluctuations of the energy associated with Heisenberg's uncertainty principle, becomes significant when the space between two metallic surfaces, such as two mirrors facing one another, measures less than about 100 nm.

"When two surfaces of the same material, such as gold, are separated by vacuum, air or a fluid, the resulting force is always attractive," said Capasso. Remarkably, but in keeping with quantum theory, when the scientists replaced one of the two metallic surfaces immersed in a fluid with one made of silica, the force between them switched from attractive to repulsive. As a result, for the first time, Capasso and his colleagues measured what they have deemed a repulsive Casimir.

To measure the repulsive force, the team immersed a gold-coated microsphere attached to a mechanical cantilever in a liquid (bromobenzene) and measured its deflection as the distance from a nearby silica plate was varied.

"Repulsive Casimir forces are of great interest since they can be used in new ultrasensitive force and torque sensors to levitate an object immersed in a fluid at nanometric distances above a surface. Further, these objects are free to rotate or translate relative to each other with minimal static friction because their surfaces never come into direct contact," said Capasso.

An artist's rendition of how the repulsive Casimir-Lifshitz force between suitable materials in a fluid can be used to quantum mechanically levitate a small object of density greater than the liquid (Figures are not drawn to scale). Foreground: a gold sphere, immersed in bromobenzene, levitates above a silica plate. Background: when the plate is replaced by one of gold, levitation is impossible because the Casimir-Lifshitz force is always attractive between identical materials. (Image courtesy of the lab of Federico Capasso, Harvard School of Engineering and Applied Sciences)

By contrast, attractive Casimir forces can limit the ultimate miniaturization of small-scale devices known as microelectromechanical systems (MEMS), a technology widely used to trigger the release of airbags in cars, because the attractive forces may push together moving parts and render them inoperable, an effect known as stiction.

Potential applications of the team's finding include the development of nanoscale bearings based on quantum levitation suitable for situations when ultralow static friction among micro- or nanofabricated mechanical parts is necessary. Specifically, the researchers envision new types of nanoscale compasses, accelerometers and gyroscopes.

Capasso's co-authors are Jeremy Munday, formerly a graduate student in Harvard's department of physics and presently a postdoctoral researcher at the California Institute of Technology, and Dr. V. Adrian Parsegian, senior investigator at the NIH. The Harvard researchers have filed for a US patent covering nanodevices based on quantum levitation.

The research is supported by the Center for Nanoscale Systems at Harvard, the National Science Foundation, the Intramural Research Program of the NIH and the Eunice Kennedy Shriver National Institute of Child Health and Human Development.

The work will be published as the Jan. 8 cover story of Nature.

For more information, visit: http://www.seas.harvard.edu/