Does Rotational Melting Make Molecular Crystal Surfaces More Slippery?

Abstract

The surface of a crystal made of roughly spherical molecules exposes, above its bulk rotational phase transition at T= T${}_r$, a carpet of freely rotating molecules, possibly functioning as "nanobearings" in sliding friction. We explored by extensive molecular dynamics simulations the frictional and adhesion changes experienced by a sliding C${}_{60}$ flake on the surface of the prototype system C${}_{60}$ fullerite. At fixed flake orientation both quantities exhibit only a modest frictional drop of order 20% across the transition. However, adhesion and friction drop by a factor of $\sim$ 2 as the flake breaks its perfect angular alignment with the C${}_{60}$ surface lattice suggesting an entropy-driven aligned-misaligned switch during pull-off at T${}_r$. The results can be of relevance for sliding Kr islands, where very little frictional differences were observed at T${}_r$, but also to the sliding of C${}_{60}$ -coated tip, where a remarkable factor $\sim$ 2 drop has been reported.

References in corpus (2)

• Melting and nonmelting of solid surfaces and nanosystems
• Optimal Energy Dissipation in Sliding Friction Simulations