Noncontact friction

Can be enhanced by photon tunneling. Usually, two bodies in relative motion feel friction when the respective surface atoms are in contact. In contrast, noncontact friction is similar to the van der Waals force, a common but weak attractive force that arises when an atom or molecule spontaneously develops an electric dipole moment due to a thermal or quantum fluctuation. The short-lived atomic polarity can induce a dipole moment in a neighboring atom or molecule some distance away. A new study of van der Waals friction by theorists Alexander Volokitin and Bo Persson of the Institute of Solid-State Research at Germany’s Research Center Jülich found that when surfaces are separated by about 1 nm, the van der Waals friction can be greatly enhanced in three ways. First, they found that the friction increases a 100-fold or more when two clean conducting surfaces move toward or away from each other rather than in parallel relative motion. The other two effects involve resonant photon tunneling. For semiconductor surfaces, photon tunneling can generate surface plasmons whose interaction can increase the noncontact friction by several orders of magnitude. And resonant tunneling between vibrational modes of adsorbate atoms on otherwise clean conducting surfaces can enhance the van der Waals friction over that of the clean surfaces alone. The adsorbate atoms act like tiny antennas: When the emitters and receivers are in tune, their electromagnetic interaction is greatly enhanced. The researchers’ calculation showed a seven-orders-of-magnitude enhancement for this effect, consistent with previously unexplainable experimental results with scanning tunneling microscope probes. (A. I. Volokitin, B. N. J. Persson, Phys. Rev. Lett. 91 , 106101, 2003 http://dx.doi.org/10.1103/PhysRevLett.91.106101 ).