Researchers heal nanomachines

Physicists reverse the effect of the critical Casimir force

Measurement in the balance: A light beam is totally reflected on a Gef wand, only a little light leaking into the vessel. How much the sphere reflects depends heavily on its distance from the wall and thus on the force pulling it towards the wall. © Ingrid Schofron / Max Planck Institute for Metals Research
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If a machine is stuck, the engineer is to blame - or the physics. The latter applies at least to the first simple nanomachines, which are braked by the so-called Casimir force. This force works only on the scale of a few millionths of a centimeter and allows tiny machine parts to stick together. Scientists have now observed a similar force in a mixture of two liquids. They also discovered a way to reverse the effect of force, so that possible blockages of nanomachines may be avoided in the future.

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This would make it possible to further miniaturize machines and to produce mechanical switches or sensors on a nanometer scale, according to researchers from the Max Planck Institute for Metals Research and the University of Stuttgart in the scientific journal Nature.

Nanomachines stick together

From nothing, comes nothing. It's sometimes different in physics. Thus, two metal plates mysteriously attract each other when they are about half a micron in vacuum and at absolute zero temperature. The force that pushes the plates together is due to quantum mechanical fluctuations of the vacuum - actually out of nothing. Such fluctuations are fluctuations of electromagnetic waves. These must have a node on the surfaces of the two electrically conductive plates. Therefore, the number of allowed waves between the plates is severely limited. Outside the plates, on the other hand, they can spread unhindered. This ultimately results in an attractive force between the plates.

The physicist Hendrik Casimir theoretically predicted this effect as early as 1948, but today he ensures that the components of nanomachines stick together. Clemens Bechinger, Professor at the University of Stuttgart and since the beginning of the year also Max Planck Fellow, Christopher Hertlein and other coworkers have experimentally observed a very similar force in a mixture of water and the oily liquid lutidine: the critical Casimir force. display

"This force is so weak that it is very difficult to detect, " says Bechinger. The results are in very good agreement with values ​​predicted theoretically by Siegfried Dietrich of the Stuttgart Max Planck Institute for Metals Research and his colleagues. The results have now been published jointly by the scientists.

Critical point - critical force

The critical Casimir force owes its name to the fact that it occurs near a critical point. Such a critical point also exists in a mixture of water and lutidine. At low temperatures they form a clear solution. However, heating this solution to about 34 degrees Celsius separates it into two different mixtures - physicists talk of two phases, one containing much lutidine and the other much water.

The corresponding temperature is called the critical temperature. However, at this so-called critical point, the two phases do not arise abruptly, as, for example, water freezes to ice at the freezing point. On the contrary, even below the critical temperature, there are already areas in the mixture which contain more lutidine or more water. However, the further the temperature approaches the critical temperature, the larger these areas become and the longer they persist. Like the concentration of water and lutidine fluctuates in different areas of the mixture, similar to the quantum mechanical fluctuations in the vacuum, "says Dietrich. And like these, the concentration variations should also create an attractive force between surfaces. And they do, as the researchers have now demonstrated.

Optical fields as a tool

"We observed a plastic sphere one micron in diameter that floated in a glass vessel with lutidine and water, " says Hertlein. The temperature of the solution was initially well below the critical point. The researchers gradually heated them up. When the temperature was only two tenths of a degree away from the critical point, the plastic sphere approached the glass wall of the vessel.

Physicists determined the distance between the sphere and the glass wall by means of evanescent optical fields scattered on the plastic sphere. They radiated light at an acute angle on the Gef, so that it is almost completely reflected. Only a tiny part of the light leaks into the liquid. How much of this reaches the plastic sphere and how much this proportion is then scattered depends very much on their distance from the Gef wand.

Statistical Ausrei er aufgesp rt

The researchers have been able to determine the force that acts on them from the distance of the small ball. A tricky affair: The tiny plastic little bucket moves on its own because of its constant agitation with the heated liquid molecules. The critical Casimir force is therefore noticeable only in the form of statistical outliers towards the glass wall.

"We can only determine these statistical outliers because our measurement method is several thousand times more sensitive than atomic force microscopy, " says Bechinger. Atomic force microscopy measures the attractive force that a surface exerts on a fine measuring arm. With the help of the optical measuring method, the Stuttgarters have now found that the critical Casimir force is only 600 Femto-Newton, less than a millionth of the weight of a flea.

This force pushes the plastic ball but only to the glass wall, if glass and plastic ball either water or both bevorl prefer. By contrast, if the two surfaces are coated in such a way that only one of the two surfaces l favors, the critical Casimir force drives the ball away from the glass wall. Then, on one surface, areas with a lot of water tend to form, and on the other, those with a lot of oil. However, since it costs energy to bring the water-rich into direct contact with the oil-rich phase, the bullet is repelled.

Prevent blockage of nanomachines

This effect we have also expected according to our theoretical investigations, says Dietrich. With its experimental proof, the researchers now expect the prospect of preventing the blockage of nanomachines. Such machines on the scale of a few millionths of a centimeter could once serve as actuators in medicine, for example. They could allow surgeries without major intervention or transport drugs to a specific site of illness. So far, however, such machines fail, among other things, the Casimir force of quantum mechanical vacuum fluctuations, which paralyzes their movement.

"If these machines would not work in a vacuum, but in a liquid mixture close to the critical point, that could be changed, " says Dietrich. Then the machine parts could be coated so that the Casimir force repulsive effect and the machine runs around. Achieving this is one of the goals that Dietrich's theoretical group and Bechinger's experimental group will jointly pursue in the future.

(idw - University of Stuttgart / MPG, 11.01.2008 - DLO)