  Using Molecular Dynamics simulations of a polymer liquid flowing past flat
and patterned surfaces, we investigate the influence of corrugation,
wettability and pressure on slippage and friction at the solid-liquid
interface. For one-dimensional, rectangular grooves, we observe a gradual
crossover between the Wenzel state, where the liquid fills the grooves, and the
Cassie state, where the corrugation supports the liquid and the grooves are
filled with vapor. Using two independent flow set-ups, we characterize the
near-surface flow by the slip length, $\delta$, and the position,
$z_\textrm{h}$, at which viscous and frictional stresses are balanced according
to Navier's partial slip boundary condition. This hydrodynamic boundary
position depends on the pressure inside the channel and may be located above
the corrugated surface. In the Cassie state, we observe that the edges of the
corrugation contribute to the friction.
