Hydration friction in nano-confinement: from bulk via interfacial to dry friction
Prof. Roland Netz, Freie Universität Berlin
The viscous properties of confined water are relevant for colloidal jamming, surface drainage, lubrication of joints and nanofluidics. Experiments measuring the friction between surfaces separated by thin water layers suggest nano-confined water to differ drastically from bulk, but a microscopic picture of water-mediated nanoscale friction is missing. We study the shear friction between hydrophilic surfaces by non-equilibrium molecular dynamics simulations. With decreasing water film thickness we find three consecutive friction regimes: For thick films friction is governed by bulk water viscosity. At intermediate separations of about a nanometer the highly viscous interfacial water layers dominate and increase the surface friction, while at the transition to the dry friction limit interfacial slip sets in. We propose a confinement-dependent model that accounts for the additive friction contributions from bulk-like water, interfacial water layers and interfacial slip.