Casimir effect creating Quantum Trap for tiny object

Physicists in the US had their way of making the Casimir force a repulsive or attractive one, depending on the size of the gap separating two gadgets. It indeed makes a potential energy minimum with actually no energy input – which the researchers believe it could be beneficial in the operation and creation of a range of nanomechanical systems – .

The Casimir effect is an odd phenomenon in which two electrically neutral surfaces held a tiny distance apart experience a force from quantum fluctuations. Named after the Dutch physicist Hendrik Casimir, who first proposed it in 1948 – the force is usually attractive because, when two parallel plate conductors in vacuum are placed a short distance apart, only a discrete set of quantum fluctuations can exist in the gap between them. The set of permitted vacuum fluctuations outside the gap is effectively unlimited, however. Therefore, the vacuum fluctuations on the backs of the plates exert more pressure than the fluctuations in the gap, pushing the plates together. This can be troublesome in nanotechnology, causing nanoparticles to clump together, for example.

In the 1950s, researchers predicted that repulsive Casimir forces could arise if the vacuum were replaced by a fluid and one of the two materials was switched for material with a lower refractive index than the fluid. It was confirmed experimentally in 2009.

Tuneable combinations

Purely repulsive forces are not much more use than purely attractive ones — but what would be really useful is the ability to create tuneable combinations of attractive and repulsive forces that could hold a particle with no energy input.

In 2010 at the Massachusetts Institute of Technology (MIT), Alejandro Rodriguez and associates proposed a scheme for obtaining such “Casimir equilibria”. “You need fluid, and you need some kind of coating [on one of the surfaces],” he explains. “Since then, there have been a whole host of different predictions for how this could be proved using different materials or different topologies, but the crux is using the underlying physics of Casimir forces in fluids to superintend a stable equilibrium.”

In the new research, Xiang Zhang of the University of California, Berkeley, and colleagues have shown a solid proof for this effect, it was for the first time in history. They coat a gold plate in Teflon and, above this, they suspend a nanoscale gold flake in ethanol.

The Teflon has a lower refractive ratio than the ethanol, so the Casimir force between the gold flake and the Teflon is repulsive. The Casimir interaction between the gold flake and the gold plate is attractive, however – and much stronger than the repulsive interactions between the gold and the Teflon.

These are tiny, tiny forces, so measuring this is a triumph of optical metrology

                                                                                                                                     - Alejandro Rodriguez -

When the gold flake is really close to the Teflon-coated gold plate, the relative contrast between the distance to the Teflon surface and the distance to the gold surface underneath is vital, so the repulsion from the Teflon dominates and the gold flake is pushed away. As the flake travels further away, nevertheless, the relative distances to the gold and Teflon become similar, until eventually, the attraction becomes dominant. The flake remains at the equilibrium point at which repulsion and attraction are perfectly balanced. The exact location of this point above the surface increases with the thickness of the Teflon coating.

Zhang believes that the system could potentially find various technological applications. “If you have a magnetic slider moving at meters per second speeds against an atomically flat surface in a computer hard drive, then the Casimir force can push them together and cause the drive to crash,” he says. “One of the main causes of computer crashes is stiction, and there are many other examples of such failure in nanomechanical devices that they study in Silicon Valley. If there were a way to do frictionless bearings, that would be very desirable.”

Rodriguez is touched by the result. “This force depends on quantum fluctuations happening at every frequency from zero to ultraviolet,” he notes. “It wasn’t clear until very recently that you could do much with this other than get an attractive force, so the really interesting part is the demonstration that this very complicated force can be harnessed and understood to create something as simple as a stable equilibrium”. He adds, “These are tiny, tiny forces, so measuring this is a triumph of optical metrology and the agreement with theory is frankly surprising.” Whether the practical complexities will permit technological applications, he states, remains to be seen.