Origins of hydration lubrication

TLDR

Hydration shells around charged molecules form subnanometre layers that can dramatically reduce friction, a mechanism invoked to explain the low sliding friction in healthy hips and knees, yet its detailed contribution remains poorly understood. The study directly measures the energy dissipation in sheared hydration shells to quantify their lubricating role. By trapping hydrated ions in a 0.4–1 nm gap between atomically smooth charged surfaces during sliding, the authors separate friction dissipation modes and identify viscous losses in subnanometre hydration shells. The results clarify the origins of hydration lubrication, underscoring its relevance for aqueous boundary lubricants and biolubrication.

Abstract

Why is friction in healthy hips and knees so low? Hydration lubrication, according to which hydration shells surrounding charges act as lubricating elements in boundary layers (including those coating cartilage in joints), has been invoked to account for the extremely low sliding friction between surfaces in aqueous media, but not well understood. Here we report the direct determination of energy dissipation within such sheared hydration shells. By trapping hydrated ions in a 0.4–1 nm gap between atomically smooth charged surfaces as they slide past each other, we are able to separate the dissipation modes of the friction and, in particular, identify the viscous losses in the subnanometre hydration shells. Our results shed light on the origins of hydration lubrication, with potential implications both for aqueous boundary lubricants and for biolubrication. Subnanometre-thick hydration layers can greatly reduce the friction between two sliding objects, an effect termed hydration lubrication. Here, Ma et al.determine the frictional dissipation in such layers, which can account for the observed lubricating action of hydrated ions or zwitterions.