The mechanics of nanometre-scale molecular contacts

The adhesive interactions and friction-load relationships have been investigated in liquid mixtures. For hydrogen bond-forming monolayers in acetone-heptane mixtures a linear friction-load relationship is observed at compositions that yield extensive surface solvation. As the concentration of the hydrogen bond acceptor in the liquid medium is reduced, non-linear friction-load relationships are observed that may be modelled using DMT mechanics. These observations are rationalized by assuming, as others have previously suggested, that the friction force is the sum of a load-dependent term and a shear term. The load-dependent term is found to be invariant with the adhesion force and represents energy dissipation in molecular ploughing (conformational disruption of the surface). The shear term results from adhesive interactions between the probe and the surface and correlates closely with the free energy of interaction between the hydrogen bonding functional groups. A non-linear friction-load relationship is normal, with linearity representing a limiting form of behaviour where unusually weak adhesion occurs (e.g. a highly solvated surface). Observations of the solvent-dependence of the pull-off force and the surface shear strength enable the prediction of thermodynamic properties of the interacting functional groups.