Orders of Magnitude Changes in the Friction of an Ionic Liquid on Carbonaceous Surfaces

N Voeltzel and N Fillot and P Vergne and L Joly, JOURNAL OF PHYSICAL CHEMISTRY C, 122, 2145-2154 (2018).

DOI: 10.1021/acs.jpcc.7b10173

The fast development of ionic liquids as new lubricants and carbon-based coatings as performant tribological surfaces calls for the characterization of their frictional and interfacial hydrodynamic behavior. Here we use molecular dynamics simulations to explore the response under shear of an ionic liquid confined between various carbon- based surfaces and an iron oxide surface for comparison. We show that extremely low fluid friction and giant hydrodynamic slippage can be obtained on graphite and to a lesser extent on diamond, but that friction on amorphous carbon surfaces is comparable to that on iron oxide. We relate these differences to the atom-scale roughness of the surfaces. In particular, although amorphous carbon surfaces are apolar, their nanometric roughness is enough to generate a fluid friction comparable to that of the extremely smooth but polar iron oxide surface. We also show that, at high shear rates, seemingly small differences in viscosity and interfacial friction can result in a significant change of the slip length. We finally discuss the consequences of the ultralow fluid friction that we observed on the macroscopic behavior of lubricated contacts.

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