Influence of wetting properties on hydrodynamic boundary conditions at a fluid/solid interface

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 fie ld vanishes at a fixed solid boundary. In this paper, we consider the speci al case of a liquid that partially wets the solid, i.e., a drop of liquid i n equilibrium with its vapor on the solid substrate has a finite contact an gle. Using extensive non-equilibrium molecular dynamics (NEMD) simulations, we show that when the contact angle is large enough, the boundary conditio n can drastically differ (at a microscopic level) from a "no-slip" conditio n. Slipping lengths exceeding 30 molecular diameters are obtained for a con tact 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 l ength in terms of equilibrium quantities of the system. The predicted behav iour is in very good agreement with the numerical results for the slipping length obtained in the NEMD simulations. The existence of large slipping le ngth may have important implications for the transport properties in nanopo rous media under such "nonwetting" conditions.