Effect of interfacial liquid structuring on the coherence length in nanolubrication

The degree of interfacial structuring of n-hexadecane and octamethylcyclotetrasiloxane (OMCTS) was measured within a nanometer boundary regime to silicon surfaces. Boundary-layer effects on lubricating sliding (in terms of a thermodynamic stress activation parameter) and the layer thickness were determined by scanning force microscopy. A 2.0+/-0.3 nm thick, entropically cooled layer was found for n-hexadecane. Measurements on spherically shaped OMCTS molecules exhibited only an interfacial "monolayer," and identified the molecular shape of n-hexadecane responsible for augmented interfacial structuring. Interfacial liquid structuring was found to reduce friction.