Ion Adsorption and Hydration Superlubricity

Friction and wear are accompanied by many aspects of production and life, from the micro- to the macroscale. Humans have been committed to reducing friction for thousands of years, saving energy and resources, protecting the environment, and improving service life. Aqueous lubricants play key roles in many mechanical, technical, medical, physiological, and biological applications, particularly over the past twenty years or so, their role in boundary lubrication has been widely studied. The role of hydrated ions in aqueous superlubricity has long been known. The structure of hydrated ion layers adsorbed on solid surfaces that determine the lubricating properties of aqueous lubricants is thought to be invariable in hydration lubrication. However, we prove that the ion surface coverage dictates the roughness of the hydration layer and its lubricating properties, especially under subnanometer confinement. In this study, we systematically evaluate the adsorption and lubrication properties of monovalent, divalent, and trivalent ions at microscale and macroscale conditions through using a surface force apparatus (SFA) and a universal mechanical tribometer (UMT), respectively. The hydration layer structure is characterized by an atomic force microscope (AFM), streaming current measurements and molecular dynamic (MD) simulations. Our results shed light on the origins of boundary superlubricity, with potential applications both for marine engineering and biolubrication.