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 good conductors is investigated in detail both within the local optic approximation and using a nonlocal optic dielectric approach. We show that the friction may increase by many orders 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.

• Received 16 January 2003

DOI:https://doi.org/10.1103/PhysRevB.68.155420