Frictional fronts: from atomic to tectonic scales

What happens when we make an object slide on a surface? Friction will prevent motion until the force we apply is strong enough. Friction is important in everyday life, allowing us to walk without slipping, and has far reaching technological implications for the functioning of mechanical devices and relevance for geological phenomena such as avalanches and earthquakes. The static friction force, as described by Amontons and Coulomb centuries ago, is proportional to the normal load and independent of the apparent contact area and can thus be described with a single parameter. Recent work showed, however, that the picture is more complicated. At the macroscopic level, frictional slip is ruled by the propagation of a shear crack breaking contact interface so that the process is similar to fracture.

Researchers from the Center for Complexity and Biosystems of the University of Milan decided to investigate friction at the atomic scale. To this end, they simulated the motion of an aluminium block pushed over a copper substrate. This is an idealized atomic scale system, with a perfect crystalline block moving over a perfectly ordered surface. As in macroscopic samples, if the lateral force applied to the block is not strong enough to overcome the friction due to the interaction between aluminium and copper atoms at the interface, then the block does not move. When the lateral force reaches a given threshold, the block instead slides over the substrate. Between the stable and moving phase, CC&B researchers identified a precursory activity, associated with the propagation and arrest of shear fronts on the contact interface, that is similar to the one observed in macroscopic objects in contact, or even at geological scales when tectonic plates are sliding against each other. In this nanoscale system, however, fronts display specific atomic-scale features due to the periodicity of the two crystals in contact and can not be described by macroscopic continuum mechanics.

The results of this study clearly shows that dynamic front propagation arises already at the atomic scales and shed light on the connections between nanoscale and macroscopic friction. The work is published in the recent issue of the Journal of Physical Chemistry Letters and is supported by ERC project SIZEFFECTS whose goal is to investigate how materials deforms at different length-scales.

Atomic-Scale Front Propagation at the Onset of Frictional Sliding
Silvia Bonfanti, Alessandro Taloni, Carlotta Negri, Alessandro L. Sellerio, Nicola Manini, and Stefano Zapperi
J. Phys. Chem. Lett., 2017, 8, pp 5438–5443
http://pubs.acs.org/doi/10.1021/acs.jpclett.7b02414