Friction is an old, yet still very relevant topic thanks to its importance in our everyday lives. The development of nanotechnology has enabled researchers to look into the origins of friction at the atomic level for the first time and much work has also gone into controlling friction by engineering materials at the nanoscale.

The present work is inspired by research conducted by Dr Kim’s and Dr Park’s teams at KAIST in the Republic of Korea. These scientists found that fluorinating graphene increases friction on the carbon material at the nanoscale. However, it was extremely difficult to identify the atomic level origin of this friction in experiments.

Atomic roughness increases friction

Researchers from the University of Akron, Purdue University and the University of California, Merced, have now employed molecular dynamics simulations to model these previous experimental measurements. The simulations, which capture the dynamics of each atom, allowed them to correlate the friction behaviour with atomic level information. The results reveal that it is atomic roughness – rather than variations in rigidity or adhesion – that produces the increased friction observed.

After chemically modifying the graphene surface with hydrogen atoms, its two-dimensional structure evolves into a three-dimensional one with much more surface roughness. Even though roughness decreases the real contact area, and thus adhesion properties, it causes so-called atomic interlock at the interface and so ultimately leads to increased friction on the material.

The discovery of this new mechanism also challenges a widely accepted belief that friction is proportional to contact area at the nanoscale. The results show that, while contact quantity and real contact area play an important role in atomic friction, interlock at interfaces is another hitherto unsuspected factor.

More information can be found in the journal Nanotechnology 24 375701.

Further reading

Nanowires lead to strong friction (Jan 2009)
Deformation of nano-asperities captured on video (Dec 2012)
Nanomachines could benefit from superlubricity (Apr 2012)