They simulate the behavior of nanoparticles moving along a surface and which change their form due to wear. This approximately corresponds to the processes involved in polishing a metal surface with sandpaper. The simulation takes into account all interactions among the atoms and their temporal progression due to wear. As it turned out, the effective contact area between the nano-objects comprise the decisive factor. It serves as the benchmark for the number of atoms which interact with each other, and thus cause friction.
Due to the fact that the effective contact area increases in proportion to the load i.e. the strength with which the particle is pressed on the surface, there is a linear connection between the load and friction, something which da Vinci also once observed on macroscopic objects. According to the researchers, this conclusion now makes it possible to also understand complicated cases which cannot be explained as the simple connection between the load and frictional force. Here they are referring to atomic force microscopy.