M ay 2 00 6 Non - contact friction between nanostructures

We calculate the van der Waals friction between two semi-infinite solids in normal relative motion and find a drastic difference in comparison with the parallel relative motion. The case of the good conductors is investigated in details both within the local optic approximation , and using a non-local optic dielectric approach. We show that the friction may increase by many order of magnitude when the surfaces are covered by adsorbates, or can support low-frequency surface plasmons. In this case the friction is determined by resonant photon tunneling between adsorbate vibrational modes, or surface plasmon modes. The theory is compared to atomic force microscope experimental data. 1 Introduction A great deal of attention has been devoted to non-contact friction between nanostructures, including, for example, the frictional drag force between two-dimensional quantum wells [1, 2, 3] , and the friction force between an atomic force microscope tip and a substrate [4, 5, 6, 7, 8]. In non-contact friction the bodies are separated by a potential barrier thick enough to prevent electrons or other particles with a finite rest mass 1