Forces out of nothing
Posted on Sunday, January 13, 2008 @ 21:18:09 UTC by vlad
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 When a machine jams, it’s the fault of the engineer - or of physics.
The latter is true at least for the first simple nanomachines which are
slowed down by the Casimir effect. This force only works on the scale
of a few millionths of a centimeter and makes tiny machine parts cling
together.
Scientists from the Max Planck Institute of
Metals Research and the University of Stuttgart have now observed a
similar force in a mixture of two liquids. They have also found a way
to reverse the effect of the force so that blockages might be avoided
in future nanomachines. This will make it possible to miniaturize
machines even further and produce nano-scale mechanical switches or
sensors.
Nothing comes from nothing. Only in physics is
this not always true. For example, two metal plates placed about half a
micrometer apart in a vacuum and at a temperature of absolute zero
exert a mysterious attraction on each other. The force pushing the
plates together comes from the quantum mechanical fluctuations of the
vacuum - so from nothing. This fluctuation represents variations in
electromagnetic waves. These need to have a node on the surfaces of the
two electrically conductive plates, which considerably limits the
number of waves permitted between the plates. Outside of the plates
they can spread without restriction. This results in the attraction
between the plates.
Physicist Hendrik Casimir predicted this effect
in theory as early as 1948; today it is the reason why the components
in nanomachines adhere to each other. Clemens Bechinger, Professor at
the University of Stuttgart and a Max Planck Fellow since the beginning
of the year, Christopher Hertlein and other staff members have now
observed a very similar force in experiments with a mixture of water
and the oily liquid lutidine: the critical Casimir force. "This force
is so weak that it is very difficult to detect it," says Clemens
Bechinger. The results nevertheless agree very well with the values
that Siegfried Dietrich, Director at the Max Planck Institute of Metal
Research in Stuttgart and his team had predicted in theory. The
scientists have now published the results jointly.
The critical Casimir force gets its name from the fact that it
occurs close to a critical point, such as that in a mixture of water
and lutidine. At low temperatures it forms a clear solution. However,
if the solution is heated to around 34 degrees Celsius, it becomes two
separate mixtures; physicists refer to these as two phases: one with a
high water content and the other with a high lutidine content.
The temperature at which this happens is called the critical
temperature. The two phases do not come into being abruptly at this
critical point, like water solidifying into ice. It is more the case
that below the critical temperature areas form in the mixture that
contain more water or more lutidine. The closer the temperature gets to
the critical point, the larger these fluctuating areas grow and the
longer they remain intact. "The way the concentration of water and
lutidine fluctuates in different parts of the mixture is similar to the
quantum mechanical fluctuations in the vacuum," says Siegfried
Dietrich. The fluctuations in concentration should create an attraction
between surfaces in a similar way. The researchers have now proven that
this is exactly what they do.
"We observed a plastic sphere with a diameter of
a micrometer floating in a glass with lutidine and water," says
Christopher Hertlein. The temperature of the solution was initially
much lower than the critical point. The researchers then heated it up
gradually. When the temperature was only 0.2 degrees away from the
critical point, the plastic sphere moved towards the glass surface.
The physicists used evanescent optical fields to determine the
distance of the sphere to the glass surface by scattering them at the
plastic sphere. They shined light towards the glass in a sharp angle so
that it was reflected almost completely. Only a tiny part of the light
leaked into the liquid. How much reaches the plastic sphere and how
much this part is scattered depends very much on the distance of the
sphere to the glass surface.
The researchers succeeded in using the distance of the sphere to
calculate the force working on it. It was tricky: the tiny sphere moved
very rapidly because it was constantly colliding with the heated
molecules of the liquid. The critical Casimir force therefore only
manifests itself in the form of statistical blips towards the glass
surface.
"We can only detect these statistical blips because our measuring
method is several thousand times more sensitive than atomic force
microscopy," says Clemens Bechinger: "That means we can measure in the
range of one femtonewton". Atomic Force Microscopy measures the
attraction which a surface exerts on a fine measuring arm. Using the
optical measuring method, the physicists in Stuttgart have now
established that the critical Casimir force only amounts to 600
femtonewtons, which is less than a millionth of the weight of a flea.
However, this force pushes the plastic sphere to the glass surface
only when the glass and the sphere both prefer water or both prefer
oil. If the two surfaces are coated so that only one of the two
surfaces favors oil, the critical Casimir force pushes the sphere away
from the glass surface. Then areas with a lot of water form on one
surface and some with a lot of oil on the other. However, since it
takes energy to make contact between the water and the oil phases, the
sphere is repelled.
"This is the effect that our theoretical calculations led us to
expect," says Dietrich. The researchers expect that this experimental
proof may offer the possibility of stopping blockages in nanomachines.
These machines, on a scale of a few millionths of a centimeter, could
one day be used as actuators in medicine, for example. They could allow
less invasive operations or medication to be transported directly to a
focus of disease.
However, one of the reasons machines like this have failed up to
now is partly due to the Casimir force of the quantum mechanical vacuum
fluctuation, which brings them to a standstill. "If these machines
would work not in a vacuum, but in a liquid mixture close to the
critical point, that could be changed," says Siegfried Dietrich. Then
the machine parts could be coated so that the Casimir force has a
repelling effect, meaning that the machine runs smoothly. This is one
of the objectives that Dietrich’s theoretical group and Bechinger’s
experimental group want to pursue in the future.
Citation: Christopher Hertlein, Laurent Helden, Andrea Gambassi,
Siegfried Dietrich, Clemens Bechinger, Direct measurement of critical
Casimir forces, Nature, January 10, 2008 (DOI: 10.1038/nature06443)
Source: Max Planck Institute for Metals Research
Via: http://www.physorg.com/news119200202.html
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