Relaxing the vacuum

There are few topics in physics more prone to misrepresentation than the Casimir force. In popular discourse, the term is commonly preceded by 'ghostly', as though there is something barely credible about the manifestation of an attractive interaction between two surfaces separated by a vacuum. Interpretations in terms of virtual particles or suppressed quantum fluctuations of the electromagnetic field only encourage that view. But regarded as the familiar dispersion force resulting from induced dipoles, 'slowed down' by the finite speed of photons, the Casimir force becomes altogether more prosaic.

All the same, proposals to alter its influence — to engineer it — have about them something of the marvellous, as though the inescapable exigencies of nature are somehow being cheated. This possibility, however, was already implicit in Evgeny Lifshitz's recasting of the Casimir force in 1956, when he worked out the theory for real materials with finite dielectric permittivity (that's to say, finite conductivity). It's easy to see from Lifshitz's theory that, for certain choices of plate materials and media separating them, the Casimir force can actually be made repulsive. All the same, it wasn't until earlier this year that the right combination of materials — silica, gold and an organic liquid — was found¹. (It's often overlooked that a classical analogue of this repulsive force, due to density fluctuations of a fluid at its critical point between two surfaces, was seen some time ago in superfluid helium².)

This raises the prospect of 'quantum levitation' and of ultralow friction and contactless bearings for micro- and nanoelectromechanical systems (MEMS and NEMS, respectively). But the reality is trickier. The choice of materials, for example, is commonly dictated by other engineering considerations. Transparent dielectric surfaces such as silica will in themselves reduce the Casimir attraction relative to reflective metals, even if they don't alter its sign. But they also have a tendency to accumulate surface charges in air, which, on non-conductive media, cannot be dissipated and create a strong electrostatic attraction. A thin film of noble metal such as gold will allay that issue, but at the expense of constraining the dielectric function and leaving little scope for tuning the Casimir force.

Davide Iannuzzi and colleagues at the University of Amsterdam have now shown that it is possible to combine the best of both worlds³. Conductive transparent metal oxides such as indium tin oxide (ITO), indispensable for semiconductor display technology, offer amenable dielectric properties while dispersing surface charges in air. The researchers have used a customized atomic-force microscope to measure the force between a gold-coated polystyrene microbead and a flat surface coated with gold or ITO. In both cases, the Casimir force clearly dominates over any residual Coulombic force in ambient conditions for separations down to about 60 nm. But for ITO the attractive force is about a factor of two smaller. This, they say, should leave plenty of scope for tailoring the interaction in MEMS/NEMS applications. It's an intriguing example of how the right choice of materials can alter the basic physics.