[tt] NS: How a quantum effect is gumming up nanomachines

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How a quantum effect is gumming up nanomachines
http://technology.newscientist.com/article.ns?id=mg19826626.600&print=true

28 June 2008
Saswato Das, New York city

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."

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.

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, DOI:
10.1103/PhysRevLett.100.183602).

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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continuously updated special report.

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, DOI: 10.1103/PhysRevLett.78.5).

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.