Mysterious quantum forces unraveled -- How to keep micromachines’ parts from sticking together.

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Post by Cr6 on Sun Aug 06, 2017 1:22 am

Mysterious quantum forces unraveled (a bit dated)

MIT researchers find a way to calculate the effects of Casimir forces, offering a way to keep micromachines’ parts from sticking together.

Larry Hardesty, MIT News Office
May 11, 2010

Discovered in 1948, Casimir forces are complicated quantum forces that affect only objects that are very, very close together. They’re so subtle that for most of the 60-odd years since their discovery, engineers have safely ignored them. But in the age of tiny electromechanical devices like the accelerometers in the iPhone or the micromirrors in digital projectors, Casimir forces have emerged as troublemakers, since they can cause micromachines’ tiny moving parts to stick together.

MIT researchers have developed a powerful new tool for calculating the effects of Casimir forces, with ramifications for both basic physics and the design of microelectromechanical systems (MEMS). One of the researchers’ most recent discoveries using the new tool was a way to arrange tiny objects so that the ordinarily attractive Casimir forces become repulsive. If engineers can design MEMS so that the Casimir forces actually prevent their moving parts from sticking together — rather than causing them to stick — it could cut down substantially on the failure rate of existing MEMS. It could also help enable new, affordable MEMS devices, like tiny medical or scientific sensors, or microfluidics devices that enable hundreds of chemical or biological experiments to be performed in parallel.

Ghostly presence

Quantum mechanics has bequeathed a very weird picture of the universe to modern physicists. One of its features is a cadre of new subatomic particles that are constantly flashing in and out of existence in an almost undetectably short span of time. (The Higgs boson, a theoretically predicted particle that the Large Hadron Collider in Switzerland is trying to detect for the first time, is expected to appear for only a few sextillionths of a second.) There are so many of these transient particles in space — even in a vacuum — moving in so many different directions that the forces they exert generally balance each other out. For most purposes, the particles can be ignored. But when objects get very close together, there’s little room for particles to flash into existence between them. Consequently, there are fewer transient particles in between the objects to offset the forces exerted by the transient particles around them, and the difference in pressure ends up pushing the objects toward each other.

In the 1960s, physicists developed a mathematical formula that, in principle, describes the effects of Casimir forces on any number of tiny objects, with any shape. But in the vast majority of cases, that formula remained impossibly hard to solve. “People think that if you have a formula, then you can evaluate it. That’s not true at all,” says Steven Johnson, an associate professor of applied mathematics, who helped develop the new tools. “There was a formula that was written down by Einstein that describes gravity. They still don’t know what all the consequences of this formula are.” For decades, the formula for Casimir forces was in the same boat. Physicists could solve it for only a small number of cases, such as that of two parallel plates. In recent years, researchers around the world attacked the problem of finding Casimir forces between more general shapes and materials. For instance, in 2006, MIT physics professors Robert Jaffe and Mehran Kardar — with whom Johnson continues to collaborate — and Thorsten Emig of the University of Köln in Germany showed how to calculate the forces acting between a plate and a cylinder; the next year, they demonstrated solutions for multiple spheres. Meanwhile, Johnson and his collaborators explored various numerical methods that can be applied to a wide variety of geometries. However, the full power of existing tools for classical electromagnetic calculations had not yet been brought to bear on the Casimir problem.

The power of analogy

In a paper appearing this week in Proceedings of the National Academy of Sciences, Johnson, physics PhD students Alexander McCauley and Alejandro Rodriguez (the paper’s lead author), and John Joannopoulos, the Francis Wright Davis Professor of Physics, describe a way to solve Casimir-force equations for any number of objects, with any conceivable shape.

The researchers’ insight is that the effects of Casimir forces on objects 100 nanometers apart can be precisely modeled using objects 100,000 times as big, 100,000 times as far apart, immersed in a fluid that conducts electricity. Instead of calculating the forces exerted by tiny particles flashing into existence around the tiny objects, the researchers calculate the strength of an electromagnetic field at various points around the much larger ones. In their paper, they prove that these computations are mathematically equivalent.

For objects with odd shapes, calculating electromagnetic-field strength in a conducting fluid is still fairly complicated. But it’s eminently feasible using off-the-shelf engineering software.

“Analytically,” says Diego Dalvit, a specialist in Casimir forces at the Los Alamos National Laboratory, “it’s almost impossible to do exact calculations of the Casimir force, unless you have some very special geometries.” With the MIT researchers’ technique, however, “in principle, you can tackle any geometry. And this is useful. Very useful.”

Since Casimir forces can cause the moving parts of MEMS to stick together, Dalvit says, “One of the holy grails in Casimir physics is to find geometries where you can get repulsion” rather than attraction. And that’s exactly what the new techniques allowed the MIT researchers to do. In a separate paper published in March, physicist Michael Levin of Harvard University’s Society of Fellows, together with the MIT researchers, described the first arrangement of materials that enable Casimir forces to cause repulsion in a vacuum.

Dalvit points out, however, that physicists using the new technique must still rely on intuition when devising systems of tiny objects with useful properties. “Once you have an intuition of what geometries will cause repulsion, then the [technique] can tell you whether there is repulsion or not,” Dalvit says. But by themselves, the tools cannot identify geometries that cause repulsion.


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Post by Cr6 on Sun Aug 06, 2017 1:27 am

US Nuclear Weapons Laboratory Discovers How to Suppress the Casimir Force
Oct 10, 2013

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: : Strong Casimir Force Reduction Through Metallic Surface Nanostructuring


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