Interfacial Water at Hydrophobic and Hydrophilic Surfaces: Slip, Viscosity, and Diffusion

TLDR

For hydrophobic surfaces under shearing, the hydrodynamic boundary condition involves finite surface slip. The study examines the dynamics and structure of water at hydrophobic and hydrophilic diamond surfaces using non‑equilibrium molecular dynamics simulations. The authors employ non‑equilibrium molecular dynamics simulations and analyze the influence of the Lennard‑Jones cutoff length on interfacial properties. The study finds that slip length on hydrophobic surfaces is highly sensitive to interaction strength and roughness, with scaling relations to contact angle and depletion layer thickness, while inert gas only modestly increases slip; on hydrophilic surfaces slip vanishes but a thin high‑viscosity layer forms whose thickness and viscosity depend on polar group density, and water dynamics shift from diffusive on hydrophobic to transient binding on hydrophilic interfaces.

Abstract

The dynamics and structure of water at hydrophobic and hydrophilic diamond surfaces is examined via non-equilibrium Molecular Dynamics simulations. For hydrophobic surfaces under shearing conditions, the general hydrodynamic boundary condition involves a finite surface slip. The value of the slip length depends sensitively on the surface water interaction strength and the surface roughness; heuristic scaling relations between slip length, contact angle, and depletion layer thickness are proposed. Inert gas in the aqueous phase exhibits pronounced surface activity but only mildly increases the slip length. On polar hydrophilic surfaces, in contrast, slip is absent, but the water viscosity is found to be increased within a thin surface layer. The viscosity and the thickness of this surface layer depend on the density of polar surface groups. The dynamics of single water molecules in the surface layer exhibits a similar distinction: on hydrophobic surfaces the dynamics is purely diffusive, while close to a hydrophilic surface transient binding or trapping of water molecules over times of the order of hundreds of picoseconds occurs. We also discuss in detail the effect of the Lennard−Jones cutoff length on the interfacial properties.