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Lubricants 2018, 6(3), 64; https://doi.org/10.3390/lubricants6030064

Molecular Mechanisms Underlying Lubrication by Ionic Liquids: Activated Slip and Flow

University of Illinois at Urbana-Champaign, 205 N Matthews Avenue, Urbana, IL 61801, USA
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Received: 19 June 2018 / Revised: 13 July 2018 / Accepted: 17 July 2018 / Published: 20 July 2018
(This article belongs to the Special Issue Ionic Liquids: Friction and Lubrication Mechanisms)
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Abstract

The present study provides molecular insight into the mechanisms underlying energy dissipation and lubrication of a smooth contact lubricated by an ionic liquid. We have performed normal and lateral force measurements with a surface forces apparatus and by colloidal probe atomic force microscopy on the following model systems: 1-ethyl-3-methyl imidazolium bis-(trifluoro-methylsulfonyl) imide, in dry state and in equilibrium with ambient (humid) air; the surface was either bare mica or functionalized with a polymer brush. The velocity-dependence of the friction force reveals two different regimes of lubrication, boundary-film lubrication, with distinct characteristics for each model system, and fluid-film lubrication above a transition velocity V. The underlying mechanisms of energy dissipation are evaluated with molecular models for stress-activated slip and flow, respectively. The stress-activated slip assumes that two boundary layers (composed of ions/water strongly adsorbed to the surface) slide past each other; the dynamics of interionic interactions at the slip plane and the strength of the interaction dictate the change in friction -decreasing, increasing or remaining constant- with velocity in the boundary-film lubrication regime. Above a transition velocity V, friction monotonically increases with velocity in the three model systems. Here, multiple layers of ions slide past each other (“flow”) under a shear stress and friction depends on a shear-activation volume that is significantly affected by confinement. The proposed friction model provides a molecular perspective of the lubrication of smooth contacts by ionic liquids and allows identifying the physical parameters that control friction. View Full-Text
Keywords: ionic liquids; friction; lubrication mechanisms; surface forces apparatus; atomic force microscopy ionic liquids; friction; lubrication mechanisms; surface forces apparatus; atomic force microscopy
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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Han, M.; Espinosa-Marzal, R.M. Molecular Mechanisms Underlying Lubrication by Ionic Liquids: Activated Slip and Flow. Lubricants 2018, 6, 64.

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