Researchers from Basel and Tel Aviv found that friction varies with velocity in concrete graphene structures on platinum surfaces, violating Coulomb’s law, which states that friction is velocity independent in the macrocosm.
Materials made from individual atomic layers are highly valued for their low friction properties, which are useful for reducing friction on hard drives or moving parts of satellites or space telescopes. Graphene, which consists of a single layer of carbon atoms arranged in a honeycomb pattern, is a prime example and is being tested for its potential as a lubricating layer. Previous research has shown that the graphene strip can glide across a gold surface with almost no friction.
Great results with a rough surface
If graphene is applied to a platinum surface, it significantly affects the measured frictional forces. Now physicists from the Universities of Basel and Tel Aviv have reported in a journal. Nano Letters , in which case the friction depends on the speed at which the tip of the atomic force microscope (AFM; see box) moves. surface This finding is surprising because, according to Coulomb’s law as it applies in the macro world, friction is independent of velocity.
When combined with a platinum substrate, graphene no longer forms a hexagonal honeycomb structure consisting only of carbon atoms, but instead creates superstructures known as Moiré superstructures. Then the surface is no longer perfectly flat and has a certain degree of roughness.
“If we move the AFM tip at low speed over this slightly corrugated surface, we measure a weak and almost constant frictional force,” explains Professor Ernst Mayer, from the Swiss Institute of Nanoscience and the Faculty of Physics at the University of Basel. First author Dr. “Over a certain threshold, however, friction increases with the speed of the AFM tip,” adds Yimin Song. “The larger the moire superstructure, the lower the threshold at which friction becomes velocity dependent.”
The researchers found that there is more friction on the ridges of moiré superstructures during tip movement. These ridges are elastically deformed by the pushing end before they relax again when the pressure is high enough. This effect results in higher frictional forces that increase with tip speed. Simulations and an analytical model confirm the experimental results obtained by this international research group.
Source: Port Altele
