Does Rotational Melting Make Molecular Crystal Surfaces More Slippery?

TL;DR

This study uses molecular dynamics simulations to investigate how the rotational phase transition in molecular crystals affects surface friction and adhesion, revealing modest frictional changes but significant adhesion drops linked to molecular alignment.

Contribution

It provides new insights into the effects of rotational melting on surface friction and adhesion, highlighting the importance of molecular orientation during sliding at the transition temperature.

Findings

Friction drops modestly (~20%) across the transition at fixed orientation.

Adhesion and friction decrease by a factor of about 2 when the flake misaligns.

Alignment changes during pull-off are entropy-driven and significant for understanding sliding behavior.

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.