Friction is ubiquitous, ignoring superlubricity. But, what about at tuning friction at the nanoscale, might that be possible? Researchers at Massachusetts Institute of Technology think so, they have developed an experimental technique to simulate friction between surfaces at the near-atomic level that allows them to observe how individual atoms rub up against each other at an interface. Moreover, they have shown how they can manipulate the arrangement of atoms at the surfaces and tune the degree of friction observed even switching off friction by adjusting the interatomic distances.
MIT's Vladan Vuletic and colleagues point out that make friction tuneable could smooth the route to nanomachines and delay wear and tear at the molecular level. "There's a big effort to understand friction and control it, because it's one of the limiting factors for nanomachines," he explains. Unfortunately, there has been scant progress in its control at any scale. "What is new in our system is, for the first time on the atomic scale, we can see this transition from friction to superlubricity," he adds.
Working with Alexei Bylinskii and Dorian Gangloff, the team experimented with an optical lattice generated by two interacting laser beams and an ion crystal of hot ytterbium atoms cooled rapidly to close to absolute zero with an additional laser beam. The ions can be manipulated with an electric field to stretch and squeeze it, altering the inter-ionic separation.
The team measured the interaction with the optical lattice at different ionic separations and found that when the spacing matched that of the optical lattice, they observed maximum friction. If the atoms are spaced so that they each occupies a potential trough in the optical lattice, then moving them all together involves a juddering release of pent up frictional energy.
However, when the ionic spacing is mismatched relative to the optical lattice, there is no friction. The ion crystal does not adhere to the peaks and troughs of the optical lattice but slides in a fluid way across the "surface". As the ion crystal is pulled across the optical lattice, one ion may slide partially down a peak, releasing enough stress to allow a second ion to climb from a trough...pulling a third ion and so on.
"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," explains Vuletic. Gangloff adds that the same technique described in their paper in the journal Science might also be useful for studying and controlling biological components, such as protein motors.
"The next steps for us are to take these experiments into the quantum domain, where the atoms cannot only hop over the barrier through thermal activation, but also tunnel through the barrier," Vuletic told Materials Today. "This regime is virtually unexplored, both theoretically and experimentally. We are also trying to relate our friction results to the so-called Aubry transition between a sliding and a pinned phase, where it has been predicted theoretically that (for infinitely long chains) the arrangement of the ions relative to the optical lattice forms a fractal structure."
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the bestselling science book "Deceived Wisdom".