Large Slip Effect at a Nonwetting Fluid-Solid Interface

TLDR

In macroscopic flows, viscous fluids satisfy a no‑slip condition, but for partially wetting liquids with a finite contact angle the velocity field vanishes at the solid boundary. Molecular dynamics simulations reveal that when the contact angle is sufficiently large, the microscopic boundary condition deviates markedly from no‑slip. For a 140° contact angle, characteristic of mercury on glass, slip lengths exceeding 30 molecular diameters were observed, suggesting significant effects on 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. 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\ifmmode^\circ\else\textdegree\fi{}, characteristic of mercury on glass. This finding may have important implications for the transport properties in nanoporous media under such ``nonwetting'' conditions.