US Nuclear Weapons Laboratory Discovers How to Suppress the Casimir Force

The Casimir effect causes microscopic machines to stick fast. Now physicists have succesfully tested a way to suppress this force

The Casimir effect is a strange and mysterious force that operates on the tiniest scales. It pushes together small metal objects when they are separated by a tiny distance.

That’s a problem because engineers are increasingly interested in building tiny machines with parts that move against each other on precisely the scale. For some years now, they’ve been thwarted by a problem called stiction in which the tiny cogs, gears and other parts in these machines stick together so tightly that the device stops working.

The culprit in these strange stiction events is often the Casimir effect. But since it is poorly understood, physicists and engineers have never known how to prevent it.

That looks set to change thanks to the work of Francesco Intravaia at Los Alamos National Laboratory in New Mexico and a few pals who have discovered a way to reduce this force and showed that it works for the first time.

Los Alamos is best known as a nuclear weapons laboratory but physicists there are also intensely interested in micromachines because they can be used as switches inside weapons that, unlike transistors, cannot be destroyed by intense electromagnetic fields

This ability to reduce the Casimir force could have a profound effect on the way microscopic and nanoscopic machines are designed and built in the near future and on their reliability.

The Casimir effect comes about because the universe at the smallest scales is filled with virtual particles leaping in and out of existence. When two metal plates are close together, the gap between them is so small that some of these particles cannot form. That creates an excess of virtual particles on the other sides of the plates which pushes them together.

This force is impressive. At distances of around 10 nm, the force is equivalent to about 1 atmosphere of pressure. But it drops off dramatically as the distance increases and so becomes more or less negligible on the scale of the few hundred micrometres.

Indeed, the problems associated with measuring forces over these distances mean that the effect was only observed for the first time in 1997.

One curious feature of the Casimir effect is that it is hugely sensitive to the shape of the parts involved. Physicists have developed a numerical model called the proximity force approximation to calculate the Casimir force between objects of different shapes.

It’s straightforward to work out what this force should be between two infinite parallel metal plates. But start changing the shape of these objects and the calculations become mind-bogglingly complex and unreliable.

Various physicists speculate that by choosing the right combination of geometries, it may be possible to make the force repulsive. That would be handy in preventing problems such as stiction. But nobody has been able to say for sure how this might be done.

Intravaia and co have a slightly less ambitious goal. Instead of making the Casimir force repulsive, they’ve looked for ways to reduce its strength.

Their idea is simple in theory. Instead of using a flat metal sheet, these guys use a metal grating instead. This is a metal sheet in which many parallel grooves have been etched, or on which many ridges have been grown.

Any nearby object only comes into close contact with the top of the ridges. And the Casimir force generated by these peaks is much less than the force that would be generated by an equivalent smooth plate.

That’s the theory but testing this idea is difficult in the extreme. The first problem is to create metal ridges of the required dimensions. These ridges have to be tall so that the Casimir forces associated with the valleys below are negligible.

In practice, that means the ridges have to be anywhere from 200 to 500 nanometres high but less than 200 nanometres wide. Constructing these is at the very edge of materials science technology today.

Intravaia and co did it by coating a sheet of gold with a template layer in which they carved into the required ridge shapes. They then filled these voids with gold and removed the template layer to leave a grid.

The next step was to measure the force itself, another challenging task. They did this by moving a gold ball towards the grid and comparing the force it experienced with the known forces generated by electrostatic effects.

The results are impressive. Intravaia and co say their grid structure dramatically reduces the size of the Casimir force at these scales. What’s more, they say this reduction is more than twice the amount predicted by the best theoretical model, the proximity force approximation.

That’s handy because it paves the way for further experiments that could help prevent the stiction problems Casimir forces produce. But it also raises an interesting question— what’s wrong with the proximity force approximation?

Without a good way to model the physics involved, the design of new nano and micromachines is going to be more of a black art than a science.

So there is a pressing need to find out where the model is going wrong and to fix it. Over to the theorists.

Ref: arxiv.org/abs/1202.6356 : Strong Casimir Force Reduction Through Metallic Surface Nanostructuring