Thermal Friction Enhancement in Zwitterionic Monolayers

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.

Automated summary

We introduce a model for zwitterionic monolayers and study their tribological response to changes in load, velocity, and temperature using molecular-dynamics simulations. The model shows different motion regimes depending on temperature and velocity. Friction increases remarkably with temperature due to the formation and rupture of transient bonds between sliding layers caused by thermal fluctuations of molecular orientations. Comparing with simulations of charge-free systems highlights the effect of molecular charges. These findings are important for nanoscale rheology and tribology experiments on locally-charged lubricated systems.