For the first time, researchers in the US have made friction almost completely vanish between two surfaces at the nanoscale. The discovery paves the way for engineering surfaces that can slide past each other with virtually no resistance, and could hugely advance the development of nanomachines.
Ordinarily, friction exists wherever two surfaces meet - whether that's car tyres on a road or a protein flowing through a blood stream. But there's a phenomenon known as 'superlubricity' where friction almost entirely disappears, and this is what scientists have now managed to recreate by carefully tuning the spacing of individual atoms on a surface.
The most direct application of this research is the creation of longer-lasting nanomachines made out of single molecules, which currently are worn down by friction far more rapidly than larger objects because they have so few atoms to lose. But if scientists could one day work out how to control superlubricity on a larger scale, it could make pretty much everything – from cars, trains, powerlines and computers - more efficient.
"There's a big effort to understand friction and control it, because it's one of the limiting factors for nanomachines, but there has been relatively little progress in actually controlling friction at any scale," lead researcher Vladan Vuletic from the Massachusetts Institute of Technology (MIT) in the US told Jennifer Chu from the MIT News Office. "What is new in our system is, for the first time on the atomic scale, we can see this transition from friction to superlubricity."
To do this, the team simulated friction at the nanoscale by engineering two special surfaces: an optical lattice with peaks and troughs of electric potential, sort of like a tiny egg carton; and an ion crystal made up of charged atoms held in place using specific voltages and something known as the Coulomb force.
The researchers then adjusted these voltages to push and pull the ion crystal across the lattice, and also adjust the spacing of its atoms.
What they found was that, when the atoms in the ion crystal were spaced out at the same distance as the peaks and troughs of the optical lattice, they had the most friction, like interlocking Lego bricks getting stuck together and then ripped apart, Chu explains.
But when the team changed the spacing of the ion crystal so that the atoms weren't matched up with the optical lattice, the friction almost entirely disappeared.
"What we can do is adjust at will the distance between the atoms to either be matched to the optical lattice for maximum friction, or mismatched for no friction," said Vuletic. The research has been published in Science.
This knowledge could help them to engineer nanomachines that aren't constantly worn down by friction, and could also help them to control proteins and other biological components.
It's important to note that superlubricity doesn't mean that friction disappears entirely, but the team managed to reduce friction down to a tiny friction coefficient of ~0.004, which means that there is significantly less resistance than between most surfaces – in contrast, dry concrete and rubber sliding past each other have a friction coefficient of around 1.
Tobias Schaetz, a physicist at the University of Freiburg in Germany, who wasn't involved in the research, told Chu that the results were a "clear breakthrough" in gaining insight into "otherwise inaccessible fundamental physics".
"The applications and related impact of their novel method propels a huge variety of research fields investigating effects relevant from raft tectonics down to biological systems and motor proteins," said Schaetz. "Just imagine a nanomachine where we could control friction to enhance contact for traction, or mitigate drag on demand."
The possibilities are pretty exciting.