Setting Micro Gears In Motion

The University of California, Riverside's Umar Mohideen, associate professor of physics, has performed the first demonstration of the lateral Casimir force (a shape-dependent Casimir force) in his laboratory. His findings will be published in an upcoming issue of Physical Review Letters. The Casimir force has its origins in virtual particles that exist in empty space.

"The virtual particles are also called quantum fluctuations," Mohideen said. "They are predicted for all the fundamental forces in physics." Mohideen's experiment deals with the electromagnetic force and relies on virtual photons or particles of light, which are intermediaries for the electromagnetic force.

The Dutch physicist Hendrik Casimir realized that the properties of the virtual particles could be controlled by introducing boundaries. He predicted that if two parallel metallic plates are brought very close to each other then the Casimir force, named after him, comes into play.

Mohideen explained that the force arises because the number of virtual photons between the plates is smaller than the number of photons outside the plates. "The photons outside bounce off the plates," he said. "In doing so, they push the plates together, effectively leading to an attractive force between the plates. This is the normal Casimir force, where normal means perpendicular. If the shape of the plates is changed, however, you get different forces."

Indeed, the Casimir force can also manifest itself as a repulsive force. While the force is attractive in nature for two parallel plates, for two hemispherical shells, whose circular rims can be placed against each other to form one sphere, the force is repulsive and tends to break the sphere apart. The lateral Casimir force, which Mohideen's laboratory has demonstrated and which MIT physicist Meharan Kardar predicted in 1997, is another such shape-dependent Casimir force. Its result: a horizontal sliding motion between two surfaces.

"The lateral Casimir force has wide applications," Mohideen said. "One can envision a device fabricated with two corrugated surfaces allowing for a sliding motion between the two surfaces. The normal Casimir force would move the membrane up and down in the vertical plane, while the lateral Casimir force would slide it back and forth. Thus, on a silicon chip you can have vertical and sliding motions of a micro device."

The Casimir force would have vast implications for micromachines. "The effect of the force on the individual parts of the machines would need to be considered," said Mohideen. "This would be important in the silicon chip industry."