Large Slip Effect at a Nonwetting 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 field vanishes at a fixed solid boundary. 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 molecular dynamics 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^, characteristic of mercury on glass. This finding may have important implications for the transport properties in nanoporous media under such ``nonwetting'' conditions.