Organic molecular films under shear forces: Fluid and solid Langmuir monolayers

Scanning probe microscope observations of monolayers of a classic boundary lubricant, stearic acid (STA), reveal long-range dynamics of wear and reconstruction of monomolecular films under the shear forces caused by the sliding tip. The STA monolayer in a fluid state displays a flow of material from the worn area and its redistribution resulting in multilayer formation within the range of 80 mu m. Surface diffusion of mobile organic material is responsible for the observed long-range effects of the local shear stresses produced within the contact area. Solid and fluid monolayers have very different velocity dependencies of the friction forces. For solid monolayers, we observe a monotonic increase of the friction forces with velocity rising from 0.02 to 1000 mu m/s. In contrast, for the fluid STA monolayers the friction forces behave nonmonotonically with a maximum value around 0.2 mu m/s. We observe significant compression of the STA monolayers under the tip reaching 35% of initial thickness before the fatal damaging. The observed compression can be related to the collective tilting of the molecules under normal loads due to a formation of gauche conformers in alkyl chains. Estimated Young's modulus is in the range of 0.2-0.7 GPa for very small deformations (<3% of thickness compression). However, the elastic modulus decreases sharply to tens of MPa at higher deformations.