ChemRxiv
These are preliminary reports that have not been peer-reviewed. They should not be regarded as conclusive, guide clinical practice/health-related behavior, or be reported in news media as established information. For more information, please see our FAQs.
AFM_of_OFMs-final.pdf (7.08 MB)
0/0

Scale-Dependent Friction-Coverage Relations and Non-Local Dissipation in Surfactant Monolayers

preprint
submitted on 09.04.2020 and posted on 10.04.2020 by Hongyu Gao, James Ewen, Remco Hartkamp, Martin H. Müser, Daniele Dini
Surfactant molecules, known as organic friction modifiers (OFMs), are added to lubricants to reduce friction and wear between sliding surfaces. In macroscale experiments, friction generally decreases as the coverage of OFM molecules on the sliding surfaces increases. However, recent nanoscale experiments with sharp atomic force microscopy (AFM) tips have shown increasing friction. To elucidate the origin of these opposite trends, we use nonequilibrium molecular dynamics (NEMD) simulations and study kinetic friction between OFM monolayers and an indenting nanoscale asperity. For this purpose, we study various coverages of stearamide OFMs on iron oxide surfaces and silica AFM tips with different radii of curvature. For our small tip radii, the friction coefficient and indentation depth both have a non-monotonic dependence on OFM surface coverage, with maxima occurring at intermediate coverage. This suggests that friction is dominated by plowing. We rationalise the non-monotonic relations through a competition of two effects (confinement and packing density) that varying the surface coverage has on the effective stiffness of the OFM monolayers. We also show that kinetic friction is not very sensitive to the sliding velocity in the range studied, indicating that it originates from instabilities. Indeed, while friction predominately originates from the plowing action of the monolayers by the leading edge of the tip, thermal dissipation is mostly localised in molecules towards the trailing edge of the tip.

Funding

Alexander von Humboldt Foundation

Engineering and Physical Sciences Research Council (EPSRC)

History

Email Address of Submitting Author

j.ewen@imperial.ac.uk

Institution

Imperial College London

Country

United Kingdom

ORCID For Submitting Author

0000-0001-5110-6970

Declaration of Conflict of Interest

None

Version Notes

Submitted version

Exports

Logo branding

Exports