Friction and Adhesion mediated by supramolecular host-guest complexes

TL;DR

This study uses dynamic Monte Carlo simulations to explore how supramolecular host-guest complexes influence friction and adhesion, revealing complex behaviors like velocity-dependent forces, plateaus, and novel anti-ageing effects.

Contribution

The paper introduces a detailed model linking molecular complex dynamics to macroscopic friction and adhesion phenomena, including novel anti-ageing effects.

Findings

Pull-off force varies with unloading rate due to bond breakage dynamics.

Plateaus in retraction force are caused by tip geometry.

Friction can remain constant over a range of sliding velocities.

Abstract

The adhesive and frictional response of an AFM tip connected to a substrate through supramolecular host-guest complexes is investigated by dynamic Monte Carlo simulations. The variation of the pull-off force with the unloading rate recently observed in experiments is here unraveled by evidencing a simultaneous (progressive) break of the bonds at fast (slow) rates. The model reveals the origin of the observed plateaus in the retraction force as a function of tip-surface distance, showing that they ensue from the tip geometrical features. In lateral sliding, the model exhibits a wide range of dynamic behaviors ranging from smooth sliding to stick-slip at different velocities, with the average friction force determined by the characteristic formation/rupture rates of the complexes. In particular, it is shown that for some molecular complexes friction can become almost constant over a wide range of velocities. Also, we show the possibility to exploit ageing effect through slide-hold-slide experiments, in order to infer the characteristic formation rate. Finally, our model predicts a novel "anti-ageing" effect which is characterized by a decrease of static friction force with the hold time. Such effect is explained in terms of enhancement of adhesion during sliding, especially observed at high driving velocities.