Thermally activated phenomena in nanoscopic sliding friction

The role of thermal activation for a nanoasperity sliding on a flat surface with low speed is discussed. If the asperity slides in a vacuum, thermal vibrations may excite slipping from an equilibrium position to the next one along the sliding path. This effect is reduced if the velocity increases and leads to a logarithmic increase of friction with the sliding velocity. This is not always the case in a humid environment, where the formation of capillaries between tip and surface acts as an obstacle for thermally activated jumps. Depending on the wettability of the surface, different behaviors are observed.