Confined Liquids Studied by Resonance Shear Measurement: Molecular Mechanism of Lubrication.

Confined liquids present intriguing phenomena not only for foundational research but also for various engineering applications, particularly in tribology. When liquids are confined in a nanospace between solid substrates, they exhibit unique properties different from those of the bulk state due to altered molecular packing and motion restrictions and/or molecular interaction with the substrate surfaces. It has profound implications in the study of lubrication, especially in boundary lubrication where energy efficient low-viscosity lubricants typically lead to high friction and wear. Recent findings suggest that some lubricant molecules can persist between substrates, with their effective viscosity increasing dramatically depending on the molecular structure. Resonance shear measurement (RSM) with a surface forces apparatus (SFA) has become an important tool for the molecular level analysis of confined liquids. This review examines research utilizing RSM on various confined liquids, such as interfacial water, ionic liquids, and lubricant oils, to understand their lubrication mechanism. It discusses the impact of alkali ion hydration and hydrogen bonding on interfacial water properties and how different anions paired with the same cations in ionic liquids can lead to different ion packing at the interface, ultimately affecting their lubrication properties. A novel feature of boundary lubrication is presented based on the properties of confined oil lubricants. These findings, supported by molecular simulation and X-ray diffraction, underscore the significance of confined liquids in both scientific inquiry and the engineering of more efficient and energy-saving lubrication systems, paving the way for molecular design of future lubricants.