As devices get ever smaller, nanotechnologists' concerns about the forces that come into play at the nanoscale grow ever larger. The Casimir force, one such concern, is a purely quantum mechanical effect. Two closely spaced plates constrain the wavelengths of the virtual particles that pop into and out of the vacuum between them, resulting in a net force outside the plates where the wavelengths are not limited. The plates thus appear to attract one another.
A collaboration between researchers from the University of Florida and Bell Laboratories in the US has now shown that the magnitude of the Casimir force can be manipulated by changing the surface structure of the plates [Chan et. al., Phys. Rev. Lett. (2008) 101, 030401]. The team prepared highly doped Si plates either smooth or with one micron deep, 200 nanometer wide trenches every 400 nm – reducing the plate's effective surface area by half.
To measure the minuscule Casimir force, the team used tiny Au spheres suspended from a micromechanical torsional oscillator. The experiment measures the shift in the resonant frequency of the oscillator as an indirect measure of the influence of the corrugated plate on the spheres. The gradient of the force, as a function of distance between plate and sphere, can be compared between the corrugated and flat plates.
What the team found was that the measured Casimir effect was indeed lower for the corrugated plates—but only by 30 to 40%, not by the 50% that was expected from surface area considerations. The authors suggest that this deviation belies a complex geometry-dependent theory for the effect.
While earlier experiments have shown the effect of surface composition on the lateral Casimir force (that is, along the plates rather than between them), these are the first results showing the effects on the normal Casimir force.
Much theoretical work remains to devise an analytical expression for the effect as a function of surface structure. But being able to manipulate the effect for the benefit of micro- and nanoelectromechanical systems may leave nanotechnologists with one less concern.