Influence of velocity in nanoscale friction processes

Force-microscopy images of boric acid crystals were obtained experimentally and simulated with the use of a two-dimensional mechanical model. An analysis of the stick and slip movement of the microscope tip shows that the energy-dissipation mechanism is strongly influenced by the non-linear dynamics of the sliding system. The contributions of stick and viscous forces on the energy dissipation (or friction forces) are studied as a function of the relative scanning velocity. At low relative velocities, the stick forces are shown to be responsible for the energy dissipation. This energy is velocity-dependent, due to the coupling between the two degrees of freedom of the sliding system. As the scanning velocity increases the stick forces are damped; the viscous force is then predominant in the energy-dissipation process.