Molecular diffusion and slip boundary conditions at smooth surfaces with periodic and random nanoscale textures

摘要

The influence of periodic and random surface textures on the flow structureand effective slip length in Newtonian fluids is investigated by moleculardynamics (MD) simulations. We consider a situation where the typical patternsize is smaller than the channel height and the local boundary conditions atwetting and nonwetting regions are characterized by finite slip lengths. Incase of anisotropic patterns, transverse flow profiles are reported for flowsover alternating stripes of different wettability when the shear flow directionis misaligned with respect to the stripe orientation. The angular dependence ofthe effective slip length obtained from MD simulations is in good agreementwith hydrodynamic predictions provided that the stripe width is larger thanseveral molecular diameters. We found that the longitudinal component of theslip velocity along the shear flow direction is proportional to the interfacialdiffusion coefficient of fluid monomers in that direction at equilibrium. Incase of random textures, the effective slip length and the diffusioncoefficient of fluid monomers in the first layer near the heterogeneous surfacedepend sensitively on the total area of wetting regions.