Since the 1950s physicists have been able to show that quantum fluctuations in a vacuum will cause two surfaces to attract one another, a phenomenon known as the Casimir effect. Now researchers in the US have demonstrated the opposite — that under certain conditions surfaces can also repel each other. Given the importance of the Casimir effect over separations of tens to hundreds of nanometres, the result could lead to new types of nanotechnology device with extremely low levels of friction.
The vacuum force was first predicted by Dutch physicist Hendrik Casimir in 1948. Casimir considered what would happen when two uncharged, perfectly conducting metal plates were placed opposite one another in a vacuum. According to quantum mechanics, the energy of an electromagnetic field in a vacuum is not zero but continuously fluctuates around a certain mean value (equal to half a photon at a temperature of absolute zero). Resonance means that only certain wavelengths will exist between two plates separated by a particular distance. What Casimir worked out was that the radiation pressure of the field outside the plates will tend to be slightly greater than that between the plates and therefore the plates will be attracted to one another.
Casimir’s prediction was generalized for real materials by Evgeny Lifshitz in 1956, whose work was further generalized to show that the vacuum can in fact be replaced by a material. Moreover, it was shown that if the plates and the material between them, generally a liquid, have particular dielectric permitivities the force between the plates will be negative.
It’s all in the permittivity
Dielectric permittivity reflects how easily the atoms and molecules in a material can be polarized. A fluctuating electromagnetic field will induce fluctuating electric dipoles whose strength is proportional to this polarizability, so the force between two materials will be proportional to the product of their permitivities. By making the permittivity of the liquid lower than that of one of the plates (the first plate) but higher than that of the other, it will be attracted to the first plate more than the two plates will be attracted to each other. This will allow it to come between the two plates and therefore in effect make them repel one another.
Now, Harvard University’s Federico Capasso and Jeremy Munday (now at Caltech), and Adrian Parsegian of the National Institutes of Health in Bethesda, Maryland, have demonstrated this effect using gold and silica separated by the liquid bromobenzene. The liquid was placed in a cell between a plate of silica and a 40 µm diameter polystyrene sphere that was coated with a 200 nm thick gold film and suspended from an atomic force microscope cantilever (in principle a sphere produces less accurate results than a second plate, but in practice it is more useful because two plates are so hard to align accurately). By bouncing a laser beam off the cantilever, any bending of the cantilever caused by interactions between the sphere and the plate led to a change in the reflected laser signal (Nature 457 170).
In building their experiment, Capasso and colleagues had to minimize potentially harmful electrostatic effects, such as charge build up on the silica plate. They also had to find a way of calibrating their experiment; in other words find a known force that they could use to convert their reflected laser signals into force measurements. For this they used a hydrodynamic force generated in the liquid, which is proportional to the speed with which sphere and plate are moved apart. “The calibration can be performed with large speeds and at large distances where the Casimir force is relatively small,” says Capasso. “Then the speed can be reduced and the sphere brought closer to the plate to measure only the Casimir force.”
Gold and silica repel
The researchers carried out measurements of the Casimir force using separations from 20 nm up to several hundred nanometres. They found that, as the gold sphere and silica were brought together, they clearly repelled one another. By contrast, they found a clear attraction between the gold sphere and a gold plate that they put in the place of the silica.
Steve Lamoreaux of Yale University, writing in a News and Views article to accompany the research paper, says that by mixing together two or more liquids it might be possible to tune the Casimir force so that it is attractive over large separations but repulsive over shorter distances. “This would provide the means for quantum levitation of an object in a fluid at a fixed distance above another object, and so could lead to the design of ultra-low friction devices,” he says. Lamoreaux also believes that the work could have implications for fundamental physics, pointing out that a Casimir-like force is predicted to be caused by density fluctuations in binary-liquid phase transitions.
