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 degrees, characteristic of mercury on glass. This finding may have important implications for the transport properties in nanoporous media under such "nonwetting" conditions. [S0031-9007(99)09329-1].