Non-Contact Friction

Non-contact friction can be artificially enhanced. Usually for two bodies in relative motion to feel friction the respective surface atoms have to be in contact. There is a type of friction, however, which can act between two surfaces not actually in contact. This dilute friction is attributed to the van der Waals force, a common but weak attractive force which arises when an atom or molecule spontaneously develops a dipole moment (that is, although it is neutral, a small region of net negative charge can develop, offset slightly from a comparable positive region) owing to a thermal fluctuation (related to the random motion of the electrons and ions) or a quantum fluctuation (the very positions of the particles varies from moment to moment owing to the uncertainty relations built into quantum reality). This short-lived polarity can in turn induce a dipole moment in a neighboring atom or molecule, some distance away. A new study of van der Waals friction by Alexander Volokitin and Bo Persson at the Institut fur Festkorperforschung (Julich, Germany) accounts for recent odd friction experiments conducted with STM probes. The theory holds that van der Waals friction can be greatly enhanced (by up to a factor of ten million at a separation of 10 angstroms in comparison with the case of good conductors with clean surfaces) by adsorbing certain molecules onto one or both of the surfaces. This increases the resonant electromagnetic force (which can be viewed as the tunneling of photons) between the objects, especially if they are made of the same material. The adsorbate atoms can be thought of as tiny antennas, one acting as an emitter and one as a receiver; when the two antennas are in tune the electromagnetic interaction between them will be greatly enhanced (see figure).

A better understanding of this kind of non-contact friction will, at the fundamental level, help physicists to study the quantum behavior of atoms at surfaces and, at the level of applications, to prepare "brakes" for micromachines where large friction is not needed. (Volokitin and Persson, Physical Review Letters, 5 September 2003)