Ionic Liquid Nanotribology:Stiction Suppression andSurface Induced Shear Thinning.

The friction and adhesion between pairs of materials(silica, alumina,and polytetrafluoroethylene) have been studied and interpreted interms of the long-ranged interactions present. In ambient laboratoryair, the interactions are dominated by van der Waals attraction andstrong adhesion leading to significant frictional forces. In the presenceof the ionic liquid (IL) ethylammonium nitrate (EAN) the van der Waalsinteraction is suppressed and the attractive/adhesive interactionswhich lead to “stiction” are removed, resulting in anat least a 10-fold reduction in the friction force at large appliedloads. The friction coefficient for each system was determined; coefficientsobtained in air were significantly larger than those obtained in thepresence of EAN (which ranged between 0.1 and 0.25), and variationin the friction coefficients between systems was correlated with changesin surface roughness. As the viscosity of ILs can be relatively high,which has implications for the lubricating properties, the hydrodynamicforces between the surfaces have therefore also been studied. Thelinear increase in repulsive force with speed, expected from hydrodynamicinteractions, is clearly observed, and these forces further inhibitthe potential for stiction. Remarkably, the viscosity extracted fromthe data is dramatically reduced compared to the bulk value, indicativeof a surface ordering effect which significantly reduces viscous losses. [ABSTRACT FROM AUTHOR]