Scale-Dependent Friction-Coverage Relations and Nonlocal Dissipation in Surfactant Monolayers.

Area of Science:

• Tribology
• Materials Science
• Nanotechnology

Background:

• Organic friction modifiers (OFMs) are crucial lubricant additives for reducing friction and wear.
• Macroscale friction typically decreases with increasing OFM coverage, contrasting with recent nanoscale observations.

Purpose of the Study:

• To investigate the origin of opposing friction trends observed at macroscale and nanoscale.
• To elucidate the role of molecular plowing in nanoscale friction with OFM monolayers.

Main Methods:

• Nonequilibrium molecular dynamics (NEMD) simulations.
• Studied kinetic friction between stearamide OFM monolayers and silica AFM tips of varying radii.
• Analyzed OFM coverages on iron oxide surfaces.

Main Results:

• Nanoscale friction showed a nonmonotonic dependence on OFM coverage, with maxima at intermediate coverages, unlike macroscale trends.
• Molecular plowing by the AFM tip was identified as the cause of increased friction at the nanoscale.
• Friction was found to be insensitive to sliding velocity, suggesting instability-driven origins.

Conclusions:

• The nonmonotonic friction-coverage relationship arises from a balance between confinement and packing density effects on OFM monolayer stiffness.
• Friction is primarily caused by the plowing of OFM monolayers by the AFM tip's leading edge.
• Thermal dissipation is concentrated at the trailing edge of the tip.