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Bases of Chemical Force Microscopy by Friction: Energetics and Dynamics of Wearless Friction between Organic Monolayers in Terms of Chemical and Physical Properties of Molecules

Molecular dynamics and simple phenomenological simulations are used to model the friction between two ordered monolayers of alkyl chains bound at their ends, i.e., pinned atoms, to rigid substrates. We could previously interpret the origin of friction differences in organic monolayer systems by comparing these two simulations. In the present work, insight given by such comparisons is described in detail. In particular, the molecular mechanism of energy dissipation, the origin of friction force microscopic image, the chain length dependence, the packing density (tilt angle) dependence, and anisotropy of friction due to sliding direction are discussed by taking into account the effects of the molecular properties on two simplified potentials of the monolayer films. One potential is due to the lateral collective deformation of a film, and the other is a corrugated interfacial potential between films facing each other. This interpretation of molecular lubrication mechanisms in terms of the chemical and physical properties of molecules using the simple phenomenological model can be extended widely to understand the bases of chemical force microscopy by friction force microscopy.