Thermally activated effects in nanofriction

The influence of thermal activation on the motion of a nanotip sliding on a flat surface is discussed. In a dry environment thermal vibrations may induce the tip to jump from an equilibrium position to the next one along its path. This effect leads to a logarithmic increase of friction with the sliding velocity at very low speeds (nu < 10 mum s(-1)). At higher speeds thermal activation plays a minor role, and the friction versus velocity curve ends with a plateau. A new analytical formula is discussed, which explains both the increase and the stabilization of friction with velocity. In a humid environment, the situation is complicated by water capillaries, which form between tip and surface, if this is rough. These bridges act as an obstacle for thermally activated jumps. Depending on the wettability of the surface, different behaviours are observed.