  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.
