Thermal Friction Enhancement in Zwitterionic Monolayers

TL;DR

This study uses molecular dynamics simulations to explore how zwitterionic monolayers exhibit increased friction at higher temperatures due to transient bond formation and rupture, with implications for nanoscale tribology.

Contribution

It introduces a molecular model for zwitterionic monolayers and analyzes their tribological behavior under various conditions, highlighting the role of molecular charges and thermal fluctuations.

Findings

Friction increases with temperature due to transient bond dynamics.

Different motion regimes depend on temperature and sliding velocity.

Charge-free systems show contrasting tribological responses.

Abstract

We introduce a model for zwitterionic monolayers and investigate its tribological response to changes in applied load, sliding velocity, and temperature by means of molecular-dynamics simulations. The proposed model exhibits different regimes of motion depending on temperature and sliding velocity. We find a remarkable increase of friction with temperature, which we attribute to the formation and rupture of transient bonds between individual molecules of opposite sliding layers, triggered by the out-of-plane thermal fluctuations of the molecules' orientations. To highlight the effect of the molecular charges, we compare these results with analogous simulations for the charge-free system. These findings are expected to be relevant to nanoscale rheology and tribology experiments of locally-charged lubricated systems such as, e.g., experiments performed on zwitterionic monolayers, phospholipid micelles, or confined polymeric brushes in a surface force apparatus.