Friction on the micro-scale depends on surface orientation

We meet friction forces in our everyday life; energy loss or wear of materials due to friction are causes of large financial losses. A quest for better understanding friction, which may help reducing the energy demands of our society, therefore counts among the research priorities in material sciences.

Friction is defined as a resistive force acting between two macroscopic bodies which are in mutual contact while moving with respect to each other. The origin of research on friction dates back to Leonardo da Vinci. In 18th century, C. A. Coulomb found a simple law which states that the friction force can be determined as a product of the normal (load) force exerted between the two bodies and the so called coefficient of friction, which is a material-specific quantity. In spite of more than three centuries of concentrated research, we are still far from complete understanding of the physical mechanisms that give rise to friction.

Results of recent research indicate that macroscopic friction substantially depends on the surface atomic structure of two touching objects. We can describe friction in terms of creating, stretching, and breaking of thousands of atomic contacts. One of the keys to understanding the friction mechanism on the atomic level is an ability to measure lateral forces (i.e. force components lying in the plane of the touching surfaces) between individual atoms in the contact area. Implementing such a measurement, however, constitutes a non-trivial task.

Fig. 1: Schematic rendering of the Si (100) with its characteristic dimer-row reconstruction in two different orientations (parallel and perpendicular) with respect to the lateral oscillations of the scanning tip. The oscillating tip in this setup directly probes lateral forces with atomic-scale resolution.

In a paper we have published in Physical Review Letters in collaboration with our colleagues from the University of Regensburg, we introduced a new method of detecting atomic-scale lateral forces using a modified atomic force microscope. This achievement opens up new vistas for the research of friction. The significance of this work is highlighted by the decision of the journal editor to list it as Editor's Suggestion and to solicit a related Viewpoint article. The paper was also reported in Physics Today.

Fig. 2: Results of an experiment (topography and plots related to lateral force detection) show lateral forces measured either in directions perpendicular (on the left) and parallel (on the right) to the silicon dimers. Ball-and-stick atomic models of the surface are added for reference (yellow balls represent silicon, white balls hydrogen). Lateral forces measured during scans along the blue or red line shown the corresponding topography image are plotted in the lower part of the figure; solid lines shows the experimental results while the dots correspond to simulations.

Owing to the new setup of the microscope, lateral forces between two bodies can be accurately determined. As a proof of concept, we have measured lateral forces on a hydrogen-passivated silicon surface with atomic resolution. The measurement has clearly demonstrated directional dependence of the forces. That means that friction between two macroscopic objects depends on the mutual orientation of atomic patterns on the two touching surfaces. Moreover, our numerical simulations designed to model the interaction between the scanning tip of the microscope and the silicon surface give an excellent agreement with the measured data. This allows us to determine the origin of the directional dependence exhibited by the lateral forces (and consequently by friction). The key differences are seen in exciting vibrational degrees of freedom, so called 'rocking-mode' vibrations (vibrational modes in which the two silicon atoms that form one dimer on the reconstructed silicon surface oscillate in mutually opposite phases normal to the surface plane; see an illustration. These results show that the surface atomic structure and its vibrational spectrum play an essential role in directional dependence of friction.

Contact: Pavel Jelínek