Nano Lett. 14, 6872–6877 (2014)

Friction occurs when two surfaces slide against each other and is a phenomenon that can be observed at both small and large scales. The origin of friction is, however, microscopic and ultimately depends on the interaction between the atoms in the two surfaces. Understanding friction of water on different surfaces is important for the development of a range of applications, including water desalination and purification. Experiments are typically incapable of providing the required atomistic level information, and for this reason, simulations are often used. Angelos Michaelides and colleagues at University College London and the Université de Lyon now report using ab initio molecular dynamics simulations to study the friction of a water layer on graphene and on boron nitride, and find that the friction coefficient on boron nitride is three time larger than on graphene.

This observation is at first sight surprising because the structure of the water layers simulated on the two surfaces is very similar. A closer inspection, however, reveals that the surface energy is much more corrugated with boron nitride. In particular, the researchers calculated the difference in energy between the most stable position of a water molecule on the atomic structure of the two surfaces and its most unstable position. They found that with boron nitride the difference is 2.6 times larger than with graphene, which is comparable to the ratio between friction coefficients.