The notion of operating machinery without friction is, of course, fiction – except, possibly, in the realm of quantum field theory. When it comes to the very very small (nanoscale) and the way materials behave at the quantum level, the rulebook we use at human scale has to be re-written. Think about it, what would be the benefit of operating machines (albeit very small machines) without friction? They might run forever, or more likely, they’ll be extremely efficient. Among many other things, they could be tiny pumps in artificial organs, or nanoscale devices used for levitation. New research at the Department of Energy Ames Laboratory (Ames, Iowa, USA) has opened a gate, just a theoretical crack, on a potential path toward exploiting what is known as a ‘repulsive Casimir effect.’

The Casimir effect was named after Dutch physicist Hendrik Casimir, who postulated its existence in 1948. Using quantum theory, Casimir predicted that energy should exist even in a vacuum, which can give rise to forces acting on the bodies brought into close proximity of each other. For the simple case of two parallel plates, he postulated that the energy density inside the gap should decrease as the size of the gap decreases, also meaning work must be done to pull the plates apart. Alternatively, an attractive force that pushes the plates closer together can be said to exist.

Casimir forces observed experimentally in nature have almost always been attractive and have rendered nanoscale and microscale machines inoperable by causing their moving parts to permanently stick together. This has been a long-standing problem that scientists working on such devices have struggled to overcome.

Since 1954 a number of experiments have succeeded in measuring the Casimir effect, at least the attraction force part of it. This has established its ‘scientific reality.’ The repulsion force, however, is more problematical. In this case experiments with various materials, notably a demonstration by a research team at Harvard using gold-plated nanoparticles and silica immersed in bromobenzene, have shown a Casimir-based levitation effect. Perhaps the Ames Lab’s biggest contribution to the ongoing research is the discovery of the effect in chiral metamaterials.

…this new discovery demonstrates that a repulsive Casimir effect is possible using chiral metamaterials. Chiral materials share an interesting characteristic: their molecular structure prevents them from being superimposed over a reverse copy of themselves, in the same way a human hand cannot fit perfectly atop a reverse image of itself. Chiral materials are fairly common in nature. The sugar molecule (sucrose) is one example. However, natural chiral materials are incapable of producing a repulsive Casimir effect that is strong enough to be of practical use. For that reason, the group turned its attention to chiral metamaterials, so named because they do not exist in nature and must instead be made in the lab.
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The chiral metamaterials the researchers focused on have a unique geometric structure that enabled them to change the nature of energy waves, such as those located in the gap between the two closely positioned plates, causing those waves to exert a repulsive Casimir force.

[Source: Ames Laboratory]

These results are theoretical, more explicitly, they were obtained by running mathematical models, which predict the results. This is, of course, a long way from practical and repeatable demonstration, much less from actual application. Nevertheless, as we learn more about once almost mystical quantum effects, this is an area of research likely to have major impact within the next decade or two.