Solvation Forces and Contact Mechanics at the Nanometer Scale in Molecular Liquids

Abstract

Solvation forces and contact mechanics is studied on a graphite surface using conducting atomic force microscope (C-AFM) in various liquids. The nanoscale contact follows continuum models for the tip interacting with both graphite and an ordered solid-like monolayer of linear alkane (hexadecane) or branched alkane (squalane). The solid-like behavior of the squalane monolayer was confirmed by scanning tunneling microscopy (STM). The squeezing of a disordered monolayer (2,2,4,4,6,8,8-Heptamethylnonane (HMN)) does not follow continuum mechanics models due to the trapping of confined molecules, as indicated in a recent simulation. Solvation forces on a self-assembled monolayer (SAM) surface are also studied to understand the effects of surrounding fluids on measured contact resistance. The results show solvation layering of liquids but solvation does not affect the resistance of the SAM. The resistance does change in different liquids because the mechanical response of the SAM changes depending on the fluid.