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.
Highlights
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, has been invoked to account for the extremely low sliding friction between surfaces in aqueous media, but not well understood
The concept of hydration lubrication has been invoked to account for the extremely low sliding friction observed at high pressures between charged surfaces in high-salt solution[3,4,5], or when coated by surfactants[6,7,8], liposomes[9] or hydrated polymer brushes10–12— boundary layers resembling those at articular cartilage surfaces[12,13]
The present study overcomes these limitations by trapping hydrated ions in the gap between molecularly smooth surfaces, and measuring the friction between them as they slide at pressures and shear rates one to two orders of magnitude higher than previously achieved[3]
Summary
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. The present study overcomes these limitations by trapping hydrated ions in the gap between molecularly smooth surfaces, and measuring the friction between them as they slide at pressures and shear rates one to two orders of magnitude higher than previously achieved[3] This allows a clear separation of different dissipation regimes, and enables to isolate the dissipation arising from shear of the subnanometre hydration shells themselves, which are intrinsic to all hydration lubrication processes, from any system-dependent dissipation. Our results reveal that viscous dissipation within hydration shells—for the ions examined—is some 250-fold larger than the viscosity either of bulk water or of confined non-hydration water This sheds strong light on the origins of hydration lubrication, with implications both for aqueous boundary lubricants and for lubrication in biological systems
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