The lubrication properties of nano-confined liquids control many natural and industrial processes. The diffusion of molecules and ions at interfaces, the functioning of our joints, and the efficiency of engines all hinge on the behaviour of a nano-confined lubricant. It is estimated that 25% of the energy consumed worldwide is lost to friction leaving significant scope for improvement.<br/>Part of the problem comes from our lack of understanding of lubricated friction at molecular level, especially near surface singularities such as chemical and topographical defects.<br/>Here, we use Atomic Force Microscopy to build a functional picture of two model lubricated systems down from single molecule details and up to molecular group effects (~100 nm). In aqueous systems, we show that the hydrogen bond network created by nanoconfined water at the interface with solids induces a glassy-like behaviour that dominates lubrication. Single metal ions act as lubricants by reducing the elasticity of the nano-confined liquid and the magnitude of the hydrodynamic friction. The measured friction force can be quantitatively described by a thermally activated process whereby the moving ions break the hydrogen bonds with surrounding water. This picture breaks down at high salt concentrations due to ion-ion interaction and jamming effects.<br/>In contrast, organic lubricants, such as squalane, affect lubrication in an indirect manner, through their conformational relaxation dynamics. This in turn depends on the proximity of local surface singularities.<br/>The experimental results, complemented by molecular dynamics simulations, offer a molecular-level basis for the understanding of tribological problems on multiple spatial scales.

Lubricated friction at surface nano-defects

2019 Fall National Meeting

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