  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.

Materials Science;Other Condensed Matter