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

TL;DR

This study uses molecular dynamics simulations to explore how nanoscale surface textures influence slip behavior and flow structure in fluids, revealing anisotropic effects and the relationship between slip velocity and interfacial diffusion.

Contribution

It provides new insights into the impact of both periodic and random nanoscale textures on slip length and flow anisotropy, aligning MD results with hydrodynamic models.

Findings

Effective slip length depends on pattern size and wettability.

Transverse flow profiles vary with stripe orientation.

Slip velocity correlates with interfacial diffusion coefficient.

Abstract

The influence of periodic and random surface textures on the flow structure and effective slip length in Newtonian fluids is investigated by molecular dynamics (MD) simulations. We consider a situation where the typical pattern size is smaller than the channel height and the local boundary conditions at wetting and nonwetting regions are characterized by finite slip lengths. In case of anisotropic patterns, transverse flow profiles are reported for flows over alternating stripes of different wettability when the shear flow direction is misaligned with respect to the stripe orientation. The angular dependence of the effective slip length obtained from MD simulations is in good agreement with hydrodynamic predictions provided that the stripe width is larger than several molecular diameters. We found that the longitudinal component of the slip velocity along the shear flow direction is proportional to the interfacial diffusion coefficient of fluid monomers in that direction at equilibrium. In case of random textures, the effective slip length and the diffusion coefficient of fluid monomers in the first layer near the heterogeneous surface depend sensitively on the total area of wetting regions.