Organic Molecular Films under Shear Forces:  Fluid and Solid Langmuir Monolayers

Abstract

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 µm. 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 µm/s. In contrast, for the fluid STA monolayers the friction forces behave nonmonotonically with a maximum value around 0.2 µm/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.

The effect of chain density on the frictional behavior of surfaces modified with alkylsiloxanes and immersed in n-alcohols

“…These maxima shifted to lower sliding velocity with increasing n and increasing applied normal load. These maxima were similar to maxima in the friction force reported under dry conditions for surface forces apparatus 14 and LFM 13,15 studies of monolayers of amorphous dialkylammonium surfactants, as well as for LFM studies of LangmuirBlodgett films of stearic acid in the liquid expanded state 16 and monolayers of unsymmetrical n-dialkyl sulfides on gold. 17 The behavior of the maxima that we observed for SAMs of OTS in n-alcohols was consistent with a model of the energy dissipation in the film based on the characteristic relaxation time of the alkyl chains that is analogous to viscoelastic behavior in bulk polymer systems.…”

“…Decreases in chain density act to decrease even in the absence of solvent, 16,17 but in liquid environments, and u max also may depend on how the chain density or compressibility affects the extent of solvent partitioning, or plasticization, when the monolayer is compressed. A thermodynamic analysis of the solubility of the solvent in the monolayer as a function of pressure provides insight about the driving force for exclusion of solvent from the compressed region.…”

Friction Modifier Additives

“…In AFM/LFM a sphere or sharp tip is rubbed against a substrate to provide a reasonably close analogue of a high pressure sliding asperity contact. Most work has used very smooth mica or silicon wafer substrates and examined three types of surface film; deposited Langmuir-Blodgett films [75], grafted alkylthiol and siloxane monolayers [76][77] and films of OFMs adsorbed from hydrocarbon solution [78]. Clearly the last of these is most relevant to understanding OFMs but the first two provide bettercharacterised, close-packed monolayers.…”

The characteristics of phosphorylated 3-aminopropyltriethoxysilane self-assembled nanometre thin films

“…The self-assembled monolayer (SAM) has gained growing interest over the past years because it has advantageous characteristics of well-defined structure, strong head group-substrate binding and dense packing of hydrocarbon chains. Indeed SAM considerably reduces friction and adhesion and has found its use in various MEMS devices [7][8][9][10][11].…”