Research news

Ballistic Nanofriction

Roberto Guerra, Ugo Tartaglino, Andrea Vanossi*, and Erio Tosatti*

Very few people in the nanofriction community have worried so far about what happens when the sliding is really high speed, a “ballistic” regime which nonetheless appears of great potential interest. Thorough a molecular dynamics simulation study, researchers from AFRI and ACOF CRPs have begun to unravel the elemental mechanisms at play in the frictional dynamics of fast sliding nano-objects deposited on crystalline surfaces. The results appear of relevance to the fundamental understanding of ballistic nanofriction as well as to potential applications in nanosystems such as, e.g., nanoelectromechanical systems and nanomotors increasingly involving large speeds of the moving interfaces.

Figure: Simulated gold cluster on graphite substrate. A truncated octahedron Au459 cluster sliding with its 36-atom (111) facet (area 177Å2) over a mobile graphite substrate. In-registry (I) and out-of-registry (II) geometries for the cluster’s (111) contact facet and the graphite substrate.

By simulating the motion of drifting and of kicked Au clusters on graphite -a workhorse system where some experimental and theoretical facts are known – the study demonstrates and characterizes the new ballistic nanofriction regime at high speed, quite separate and different from drift at low speed. The temperature dependence of the cluster slip distance and time, measuring friction, is opposite in these two regimes, consistent with theory. Crucial to both regimes is the interplay of rotations and translations, shown to be correlated in slow drift but becoming anticorrelated in fast sliding. Despite these differences, the velocity dependence of ballistic friction turns out to be, like drift, viscous.

These results have recently appeared in Nature Materials 9, 634 (2010); they are highlighted by A. Schirmeisen’s paper “Nanofriction: Surfing on graphite waves” in news & views (Nature Materials 9, 615 (2010)).

*E-mail to: A. Vanossi or E. Tosatti