Microscopic studies of static and dynamic contact angles

Molecular dynamics simulations are used to test macroscopic theories for static and dynamic contact angles. Young's equation is verified by comparing observed static contact angles to angles calculated from the independently measured surface tensions between phases. Laplace's relation between the interfacial curvature and pressure is also checked. Both equations agree with simulation results within statistical errors. Hydrodynamic theories of dynamic contact angles are less well defined because they produce diverging stresses at the contact line between the solid and fluid interfaces if the usual no-slip boundary condition is assumed. Our simulations show that slip occurs within about two molecular diameters of the contact line, and that local hydrodynamics breaks down in the slip region. The slip results from large tangential stresses along the solid wall. A surprising result is that changes in the boundary condition for single-fluid flow at molecular scales produce dramatic changes in the dynamic contact angle.