Friction and adhesion mediated by supramolecular host-guest complexes

Area of Science:

• Surface science
• Nanotechnology
• Computational chemistry

Background:

• Atomic Force Microscopy (AFM) is crucial for probing nanoscale interactions.
• Supramolecular host-guest complexes offer tunable adhesive and frictional properties.
• Understanding the dynamic response of these complexes is key for advanced nanotechnology.

Purpose of the Study:

• To investigate the adhesive and frictional behavior of an AFM tip interacting with a substrate via supramolecular host-guest complexes.
• To elucidate the mechanisms behind experimentally observed unloading rate dependencies and retraction force plateaus.
• To explore the dynamic friction regimes and novel phenomena like aging and anti-aging effects.

Main Methods:

• Dynamic Monte Carlo simulations were employed to model the AFM tip-substrate system.
• The simulations focused on the breaking of bonds within host-guest complexes during tip retraction and sliding.
• Analysis included varying unloading rates, tip geometry, sliding velocities, and hold times.

Main Results:

• The model explains the variation of pull-off force with unloading rate by differentiating bond breaking mechanisms (simultaneous vs. progressive).
• Tip geometrical features were identified as the origin of plateaus in retraction force curves.
• A wide range of friction dynamics, from smooth sliding to stick-slip, was observed, with friction dependent on complex formation/rupture rates.
• A novel 'anti-aging' effect, where static friction decreases with hold time, was predicted and explained by enhanced adhesion at high velocities.

Conclusions:

• The study provides a detailed molecular-level understanding of AFM tip adhesion and friction governed by supramolecular interactions.
• The findings offer insights into controlling nanoscale friction and adhesion through molecular design and experimental conditions.
• The predicted anti-aging effect presents a new avenue for manipulating interfacial forces and inferring molecular dynamics.