MIT Physicists Simulate Friction at the Nanoscale

MIT physicists have devised an experimental method to simulate friction at the nanoscale. They directly examined separate atoms at the interface of two surfaces and altered their arrangement by tuning the quantity of friction between the surfaces. They altered the atomic spacing on one surface and noticed a point at which friction vanished.

Friction occurs whenever two surfaces rub against each other. Even the flowing of proteins in the bloodstream causes some friction. However, there are a few cases when friction disappears, a phenomenon called “superlubricity.” Here, the surfaces just glide over one other with no resistance at all.

Vladan Vuletic, the Lester Wolfe Professor of Physics at MIT, states that tuning friction could help in creating nanomachines such as tiny robots, constructed from parts of a single molecule in size. He adds that friction may be higher at the nanoscale, or in other words, could create wear and tear on nano-motors at a faster rate than at larger scales.

“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,” Vuletic says. “What is new in our system is, for the first time on the atomic scale, we can see this transition from friction to superlubricity.”

Vuletic, along with graduate students Alexei Bylinskii and Dorian Gangloff, publish their results in the journal Science.

Friction was created at the nanoscale by designing two surfaces, an optical lattice and an ion crystal, and arranging them in contact with one other.

The optical lattice was produced by two laser beams traveling in opposite directions, the fields of which together form a sinusoidal periodic pattern in a single dimension. The optical lattice resembles an egg carton, with the troughs signifying a minimum electric potential, and the peaks signifying a maximum. The atoms are attracted to areas with minimum potential (trough area) when they pass such an electric field.

The ion crystal is a charged atomic grid created by Vuletic to analyze the effects of friction, atom by atom. The researchers applied light to either ionize or charge neutral ytterbium atoms rising from a tiny heated oven. The atoms were then cooled down with more laser light to a temperature immediately above absolute zero.

Using voltages applied to metallic surfaces in very close proximity, it is possible to trap charged atoms. When positively charged, the atoms begin to repel each other due to the Coulomb force. The repulsion successfully maintains the atoms at a distance from each other, such that they form lattice-or crystal-like surfaces. The MIT physicists applied the same forces used for trapping the atoms to pull and push the ion crystal over the lattice, and to squeeze and stretch the ion crystal, in a motion similar to an accordion, to modify the atomic spacing.

They observed that the two surfaces underwent maximum friction, similar to two complementary Lego bricks, when atoms in the ion crystal were normally spaced at intervals equaling the optical lattice spacing.

It was found that when the atomic spacing is such that each atom occupies a trough in the optical lattice, if complete ion crystal is shifted across the optical lattice, initially the atoms tend to adhere to the troughs of the lattice. This occurs due to their tendency to be attracted to a lower electric potential, and because of the Coulomb forces that cause the atoms to repel.

However, when a certain level of force is used, the ion crystal abruptly slips, as the atoms jointly move to the next trough.

“It’s like an earthquake,” Vuletic says. “There’s force building up, and then there’s suddenly a catastrophic release of energy.”

The team continued stretching and squeezing the ion crystal in order to influence the arrangement of atoms. They found that if the atom spacing did not match that of the optical lattice, friction between the two surfaces disappeared.

In this situation, the crystal is inclined not to stick, and abruptly slips, and continues to move smoothly across the optical lattice, similar to a caterpillar’s movement across a surface.

For example, in arrangements wherein certain atoms are in troughs, certain stoms in peaks, and other atoms in between troughs and peaks, when the ion crystal is transferred across the optical lattice, one atom may move down a peak providing a little stress for another atom to move up a trough, which may help pull another atom 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,” Vuletic says.

Gangloff adds that the team’s method can be used in other areas such as for controlling proteins, molecules, and other biological parts.

Melanie Gonick/MIT (with computer simulations from Alexei Bylinkskii)

“In the biological domain, there are various molecules and atoms in contact with one another, sliding along like biomolecular motors, as a result of friction or lack of friction,” Gangloff says. “So this intuition for how to arrange atoms so as to minimize or maximize friction could be applied.”

Tobias Schaetz, a professor of physics at the University of Freiburg in Germany, sees the results as a “clear breakthrough” in gaining insight into “otherwise inaccessible fundamental physics.” The method can be used in numerous areas, from the nanoscale to the macroscale, he added.

“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,” says Schaetz, who was not involved in the research. “Just imagine a nanomachine where we could control friction to enhance contact for traction, or mitigate drag on demand.”

This research endeavor was funded in part by the National Science Foundation and the National Science and Engineering Research Council of Canada.