AVS 53rd International Symposium
    Nanometer-scale Science and Technology Wednesday Sessions
       Session NS+SS+TF-WeM

Paper NS+SS+TF-WeM5
Influence of the Solvent Environment on the Contact Mechanics of Tip-Sample Interactions in Friction Force Microscopy of Self Assembled Monolayers

Wednesday, November 15, 2006, 9:20 am, Room 2016

Session: Nanotribology and Nanomechanics
Presenter: T.J. Colburn, University of Sheffield, UK
Authors: T.J. Colburn, University of Sheffield, UK
G.J. Leggett, University of Sheffield, UK
Correspondent: Click to Email
The application of friction force microscopy (FFM) to the characterisation of surface composition and nanoscale tribological phenomena requires an adequate understanding of the tip-sample contact mechanics. We present new data that show that the properties of the liquid medium influence not only the strength of the frictional interaction in FFM, but also the nature of the contact mechanics model that describes the tip-sample interaction. FFM measurements have been carried out on self assembled monolayers of dodecanethiol (C@sub 10@CH3) and mercaptoundecanoic acid (C@sub 10@COOH) in a variety of liquid media using tips functionalised with alkanethiols (chemical force microscopy). In perfluorodecalin, a liquid with a low dielectric constant, the friction-load relationship fits the behaviour predicted using the Johnson-Kendall-Roberts model for like pairs of interacting molecules, and the Derajuin-Muller-Toporov model for unlike molecules. In contrast, measurements in ethanol, a liquid with a larger dielectric constant, obey Amontons' law (i.e. the friction force is linearly proportional to the load). These findings suggest that single asperity contact mechanics are observed in media with low dielectric constants, where dispersion forces are very strong and frictional interactions are adhesion-controlled. In media with large dielectric constants, sliding is not adhesion-controlled and linear friction-load behaviour is observed.