Influence of wetting properties on the hydrodynamic boundary condition at a fluid-solid interface

TL;DR

This study uses molecular dynamics simulations to show that the wetting properties of a solid surface significantly influence the slip boundary condition of a fluid, with large slip lengths observed for high contact angles.

Contribution

It provides a quantitative relation between contact angle and slip length, supported by molecular dynamics simulations and theoretical derivation.

Findings

Large slip lengths (>30 molecular diameters) at high contact angles.

Good agreement between theoretical predictions and simulation results.

Implications for transport in nanoporous media under nonwetting conditions.

Abstract

It is well known that, at a macroscopic level, the boundary condition for a viscous fluid at a solid wall is one of "no-slip". The liquid velocity field vanishes at a fixed solid boundary. In this paper, we consider the special case of a liquid that partially wets the solid, i.e. a drop of liquid in equilibrium with its vapor on the solid substrate has a finite contact angle. Using extensive Non-Equilibrium Molecular Dynamics (NEMD) simulations, we show that when the contact angle is large enough, the boundary condition can drastically differ (at a microscopic level) from a "no-slip" condition. Slipping lengths exceeding 30 molecular diameters are obtained for a contact angle of 140 degrees, characteristic of Mercury on glass. On the basis of a Kubo expression for $\delta$, we derive an expression for the slipping length in terms of equilibrium quantities of the system. The predicted behaviour is in very good agreement with the numerical results for the slipping length obtained in the NEMD simulations. The existence of large slipping lentgh may have important implications for the transport properties in nanoporous media under such "nonwetting" conditions.