Physicists in the US have worked out a way of making the Casimir force repulsive or attractive depending on the size of the gap between two objects. This creates a potential energy minimum with no actual energy input – which the researchers believe it could be useful in the creation and operation of a range of nanomechanical systems.
The Casimir effect is a bizarre 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 normally 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 were switched for a material with lower refractive index than the fluid. This was confirmed experimentally in 2009.
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, Alejandro Rodriguez and colleagues proposed a scheme for obtaining such “Casimir equilibria”. “You need a 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 demonstrated using different materials or different topologies, but the crux is using the underlying physics of Casimir forces in fluids to engineer a stable equilibrium.”
In the new research, Xiang Zhang of the University of California, Berkeley and colleagues have demonstrated this effect for the first time. They coat a gold plate in Teflon and, above this, they suspend a nanoscale gold flake in ethanol.
The Teflon has a lower refractive index 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 metrologyAlejandro Rodriguez
When the gold flake is very close to the Teflon-coated gold plate, the relative difference between the distance to the Teflon surface and the distance to the gold surface underneath is significant, so the repulsion from the Teflon dominates and the gold flake is pushed away. As the flake moves further away, however, 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.
The Casimir effect: a force from nothing
Zhang believes that the system could potentially find various technological applications. “If you have a magnetic slider moving at metres 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 impressed by result. “This force depends on quantum fluctuations happening at every frequency from zero to ultraviolet,” he says. “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 says, remains to be seen.
The research is described in Science.