HENRI LEZEC has a problem. He has been trying to use the tiny pressure exerted by light to move miniscule mechanical components. A light-powered micromachine could have all sorts of uses but Lezec, a photonics researcher at the US National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland, hasn’t had much luck getting them to work. Frustratingly, the components keep sticking to the optical fibre that is beaming light at them.
Lezec is by no means alone in falling foul of what nanotechnologists call “stiction” – the collective term (derived from “static friction”) for a variety of physical forces that operate at the sub-micrometre scale. Stiction is a growing problem for engineers working with ever tinier devices because it gums up the works of microelectromechanical systems (MEMS) – which are increasingly used to make things like airbag sensors – and also affects computer hard drives and other devices with small moving parts.
“The unthinkable is beginning to happen,” says Federico Capasso, professor of applied physics at Harvard University. “Progress in micromachines and nanotechnology is slowing down because of quantum forces like the Casimir effect.”
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This effect is a ghostly phenomenon whose properties remain poorly understood (see “Under pressure from quantum foam”). However, researchers reckon that the attractive force it creates is a major component of stiction at scales from 300 nanometres down to 10 nanometres. Capasso believes its influence is the reason that MEMS have not been miniaturised as quickly as computer chips – which have no moving parts – and it may block the development of even smaller nanoelectromechanical systems.
Small wonder, then, that dealing with the Casimir effect has become a matter of urgency for nanotechnologists. “Micromachines could run more smoothly, and with less or no stiction at all, if one could manipulate the Casimir force,” says Ulf Leonhardt of the University of St Andrews, UK.
Until recently, that ability might have seemed like a pipe dream. The Casimir effect seems to be such a fundamental consequence of quantum mechanics that it’s hard to imagine how it could be avoided altogether. “Like death and taxes, you can’t escape the Casimir force,” says physicist Robert Jaffe at the Massachusetts Institute of Technology. But while escaping it may not be an option, researchers may be on the verge of taming it: they have recently managed to reduce the strength of Casimir forces markedly, and there are tantalising hints that reversing it may also be possible. That would not only alleviate stiction, but also open up a range of possibilities for nanotechnologists.
“Like death and taxes, the Casimir force is something you cannot escape”
One line of attack is to change the shape of microscopic components. In a paper to be published shortly in Physical Review Letters, a team led by Ho Bun Chan of the University of Florida, Gainesville, describes how the Casimir effect is reduced when the surfaces are not smooth, but patterned.
Chan modified an earlier experiment, in which a gold-plated ball was brought close to a gold-plated see-saw. Thanks to the attractive Casimir forces, the see-saw tilted towards the ball, with a force that peaked at a distance of about 150 nanometres – an effect which could be utilised to detect very small movements (www.tinyurl.com/6c7ljb). The modified version used a silicon plate with regular trenches etched into it instead of the gold see-saw (see Diagram). Chan’s team found that this reduces the strength of the Casimir effect, and that the size of the reduction can be controlled by varying the trench spacing.
Another approach is to suspend the components in a liquid, rather than a vacuum. Theoreticians have calculated that this should make the situation much more complex. Some believe the Casimir force should be greatly reduced, eliminated or even changed from an attractive force to a repulsive one under these circumstances. This is the approach being pursued by Capasso and graduate student Jeremy Munday, who last year tested it using a gold-plated ball and a gold plate, all immersed in a mixture of ethanol and sodium iodide. The Casimir force they observed was just one-fifth of the strength it would have been in a vacuum.
This is quite an achievement, but the researchers are finding it difficult to take the next step – experimental confirmation of a repulsive Casimir force. Capasso’s group has been trying to achieve it for some time but has yet to publish any results, and not everyone is convinced it can be done at all. Jaffe at MIT has carried out repeated calculations, but says “we have found that the Casimir force is always attractive”.
Nevertheless, others remain hopeful. Diego Dalvit and colleagues at the Los Alamos National Laboratory in New Mexico suggest that Casimir repulsion could be achieved using “metamaterials” – materials engineered to have properties not found in nature. “You need a very strongly magnetic material,” says Dalvit. Previous work, such as that of Leonhardt and his colleagues, had suggested that this could only be done by constructing a “sandwich” of materials, but the Los Alamos group’s detailed calculations suggest that this is not necessary (Physical Review Letters, ).
Controlling the Casimir effect would not only help nanotechnologists overcome stiction, but could open up a range of new possibilities. It could be used wherever nanoscale springiness is needed, says Vladimir Aksyuk, professor of electrical and computer engineering at the University of Maryland at College Park. Capasso envisages frictionless ball bearings for micromachines-on-a-chip, or the creation of a nanocompass to give mobile devices a sense of direction. Other possibilities are bound to emerge as the research continues. “The Casimir effect is so universal and subtle,” says Jaffe. “There’s really lots of science still to understand.”
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Under pressure from quantum foam
How can the Casimir effect just appear out of nowhere? The answer is that it doesn’t – at least, not quite. According to quantum mechanics, the vacuum is actually foaming with particles that pop into existence for the briefest of moments. Every particle is paired with an antiparticle, ensuring that they very quickly annihilate each other.
However, during their short lives, the particles bounce off nearby surfaces, exerting pressure on them. The Dutch theorist Hendrik Casimir predicted the existence of this effect in 1948. He deduced that two uncharged metal plates (or mirrors) suspended in a vacuum and separated by less than about 2 micrometres should experience an attractive force. This is because the separation of the plates limits the wavelength of the particles that can appear between them, but there’s no such limit on particles that appear behind each plate. There are therefore more particles pressing in on the plates than pushing them outwards, the net effect of which is an attractive force between the two plates.
For decades, it was widely assumed that the Casimir effect was just a quirk of mathematics, although there were hints of its physical reality as early as 1958. Its existence was accepted only after a convincing demonstration in 1997 by Steve Lamoreaux, now at Yale University (Physical Review Letters, ).
More than a decade later, Casimir forces remain a challenging problem for theoreticians, experimenters and engineers alike, not least because they are mixed in with other small-scale effects, such as surface tension and van der Waals forces. It is proving difficult to pin down exactly how the forces created by the Casimir effect contribute to the static friction – or “stiction” – which sticks together the parts of micromachines, although it is thought to be the dominant influence on scales of between 10 and 300 nanometres.
Attempts to pin down the effect empirically are tough because it is very difficult to make and manipulate the perfectly smooth microscopic plates needed for experiments. In practice, researchers use a ball suspended over a flat surface, but only a handful of groups worldwide have been able to produce reliable results.
