Subnanometer Interfacial Hydrodynamics: Spatially Resolved Viscosity and Surface Friction.

For an accurate description of nanofluidic systems, it is crucial to account for the transport properties of liquids at surfaces on subnanometer scales, where the finite range of surface-liquid interactions implies both spatially extended surface-liquid friction and modified interfacial viscosity. This is accounted for via generalized, position-dependent friction-coefficient and interfacial viscosity profiles, which enable the accurate description of interfacial flow at the nanoscale using the Stokes equation. Such profiles are extracted from nonequilibrium molecular dynamics simulations of water on polar, nonpolar, fluorinated, and unfluorinated alkane and alcohol self-assembled monolayers spanning a wide range of wetting characteristics. The Navier friction coefficient, interfacial viscosity excess, and depletion length are found to be interrelated through power laws and to scale exponentially with the work of adhesion. Our framework establishes a foundation for describing subnanometer interfacial fluid flow with implications for electrokinetics, biophysics, and nanofluidics.