Researchers have uncovered a new type of friction that happens only among nanoparticles, according to a paper published in Nature Materials this week. In these experiments, gold clusters sliding across a graphite surface experience what the researchers have termed "ballistic nanofriction," which caused them to move in a way that is entirely unlike that of larger sliding objects. Manufacturers will likely have to account for the effect as nanomechanical devices become more common.

Many experiments have studied the relationship between sliding objects and the kinds of forces they experience due to friction. Scientists have found that a macroscopic disc moving across a surface will slide and spin at the same time, and both kinds of motion will also stop at the same time. In the nanoscale regime, this relationship is turned on its head.

The researchers slid tiny clusters of gold across a smooth, unblemished graphite surface. They found that, during a fast slide, the gold clusters were unaffected by drift or the effects of Brownian motion, both of which are normally significant at the nanoscale. The authors termed this fast motion "ballistic."

But the clusters' motion wasn't entirely undisturbed: when they hit thermal bumps in the graphite, some of their energy would be transferred to rotational motion, and the particles would be sent spinning away. Likewise, if the gold clusters were initially spinning, but moving at slow translational speeds, an alignment with the graphite would transfer most of the motion from spinning to sliding, causing a sudden acceleration of the linear motion. This is nothing like you would see with a sliding macroscopic disc.

The researchers note that the behavior only applies to tiny bits of matter that have been propelled along in some way—slow-moving particles overwhelmingly feel the effects of Brownian motion and diffusion. Still, nanosystems with fast-moving parts, even if they move over tiny distances, can be expected to experience this new type of friction.

Nature Materials, 2010. DOI: 10.1038/NMAT2798 (About DOIs).