Resonance Shear Measurement on Nano-Confined Liquids and Friction Analysis

The resonance shear measurement (RSM), which we developed, can evaluate the rheological and tribological properties of confined liquids at surface separation distances (D) from μm (practically liquids are in the bulk state) to nm thicknesses. Thus, it is especially useful for studying the boundary lubrication for which the lubricant layer becomes in the nm level thickness and the solid surfaces are supposed to be partially in contact. In this article, we describe the principle and advantages of RSM, and review our recent RSM studies on tribology of nano-confined liquids. Four phenyl ether lubricant oils with different bulk viscosity confined between mica surfaces, and two ionic liquids ([C4 mim] [NTf2]) and [C4mim] [BF4]) showed significant increase in viscosity and the their magnitude relation became reverse when the gap became in nanometer thick. Friction of hydrogel (double network gel) and silica sphere was dominated by the elasticity of the deformed gel-silica interface.