The mysterious attraction between two neutral, conducting surfaces in a vacuum was first described in 1948 by Hendrik Casimir and cannot be explained by classical physics. It is instead a purely quantum effect. Tiny though it is, the Casimir effect becomes significant at distances of microns or less and can cause parts in nano- and micro-electromechanical systems (NEMS and MEMS) to stick together. Being able to control the force would thus be a boon for NEMS/MEMS designers.

One way to understand the Casimir force is to consider the zero-point energy associated with the quantum fluctuations of the electromagnetic field between the two interacting objects. These fluctuations exert a "radiation pressure" on the surfaces. An important point is that the electromagnetic waves inside a cavity have different wavelengths than those outside. As a result, the force from the inside becomes smaller than the force from the outside, so drawing the surfaces together.

van der Waals interaction
"Alternatively, for simple, smooth objects (such as a flat plate and sphere), the Casimir force can also be derived by dividing the objects into small constituents and summing up the van der Waals interaction between them," explained team member Ho Bun Chan of the University of Florida. This approach, which takes into account the speed of light, is called "pairwise addition approximation" of retarded van der Waals interactions and has worked well for previous experiments. The new experiments are the first in which this approximation breaks down. "We found that the attraction between a gold sphere and a nanostructured silicon surface is larger than the pairwise addition of van der Waals forces," declared Chan.

During the past decade, researchers have been measuring the Casimir force in high-precision experiments. In their case, Chan and co-workers used a micromechanical torsional oscillator to measure the force between a glass sphere coated with gold and a silicon surface with nanoscale trenches. The sphere was glued onto a micromechanical plate that tilts in response to the force on the sphere. Additional electrodes are placed underneath the plate for detecting the tilt and a piezo-electric stage controls the distance between the plate and the silicon surface.

Strong shape dependence
According to the researchers, the shape of the interacting bodies is crucial in determining the force because it depends on the zero point energy of the electromagnetic modes confined by the interacting objects. "The width of the nanoscale trenches are similar to the wavelength of the relevant electromagnetic modes, making it difficult for the electromagnetic force to penetrate, which gives rise to the strong shape dependence," explained Chan.

Although the experiment will not lead to any immediate applications, it is a first step in controlling the Casimir force through geometry tailoring, he adds. "Our experiments could one day help to reduce the tendency of moveable, nanoscale components from coming into contact with each other due to quantum effects."

The measurements deviate from theoretical predictions that are based on perfectly metallic surfaces because the silicon and gold surfaces do not reflect light perfectly at short wavelengths. "We are now working with theorists to achieve better agreement," revealed Chan. "We will also look into other ways of 'engineering' the Casimir force based on the shape of the interacting bodies."

The researchers reported their work in Physical Review Letters.