Atomic friction effects

Friction is encountered both in everyday life and in technical processes where its influence can be either positive (i.e. when it enables wheel drive) or negative (e.g. as a cause for wear). Atomic friction, as encountered in applications such as nanomachines or biomolecules, however, is very difficult to access, so that little is known about it. Contrary to macroscopic objects, which are atomically speaking rough-textured and only touch each other where irregularities occur, in the universe of miniature objects, smooth surfaces are superposed. It is therefore necessary to take the contact area into account within the scope of model computations. Such models predict new, fascinating phenomena – such as superlubricity – where static friction is nearly entirely absent.

To measure friction exactly, a powerful instrument already exists, namely the friction force microscope. In contrast, the dynamics of two frictional systems cannot be observed directly; instead, it requires a work-around in the form of a model system consisting of elements which behave as similarly as possible. Such a system has been developed at the QUEST Institute at PTB, in collaboration with the University of Sydney.

The core piece of this system consists of ytterbium ions that are trapped in an ion trap and are cooled down to a few millikelvins by means of lasers until they form a two-dimensional crystal composed of two superposed ion chains. These ion chains are coupled by the Coulomb interaction. If the ions are irradiated with laser light whose frequency is close to their resonant frequency, they start fluorescing and can be moved in a targeted way by means of light pressure. Using high-resolution imaging optics, the individual atomic particles can be observed in their motion.

If the periodicity of the chain arrangement is broken by a topological defect, a fascinating multiparticle effect occurs, leading to a phase transition at which static friction ceases to exist (see diagram). This so-called Aubry transition was predicted as early as in the 1980s but could not be experimentally measured for another 30 years. It has now been possible to observe atomic chains rubbing against each other for the first time with atomicscale resolution.

The dynamics of the ion chains is comparable to that of molecule chains such as those occurring in DNA. In these chains, defects may cause the proteins to break apart. This new physical model system is generally suited for investigations of the complex, nonlinear dynamics of friction in one-, two- or three-dimensional systems with atomic-scale resolution. Further cooling levels will allow transport phenomena occurring in the quantum universe to be investigated.