  Analytical expressions for the flow field as well as for the effective slip
length of a shear flow over a surface with periodic rectangular grooves are
derived. The primary fluid is in the Cassie state with the grooves being filled
with a secondary immiscible fluid. The coupling of both fluids is reflected in
a locally varying slip distribution along the fluid-fluid interface, which
models the effect of the secondary fluid on the outer flow. The obtained
closed-form analytical expressions for the flow field and effective slip length
of the primary fluid explicitly contain the influence of the viscosities of the
two fluids as well as the magnitude of the local slip, which is a function of
the surface geometry. They agree well with results from numerical computations
of the full geometry. The analytical expressions allow investigating the
influence of the viscous stresses inside the secondary fluid for arbitrary
geometries of the rectangular grooves. For classic superhydrophobic surfaces,
the deviations in the effective slip length compared to the case of inviscid
gas flow are are pointed out. Another important finding with respect to an
accurate modeling of flow over microstructured surfaces is that the local slip
length of a grooved surface is anisotropic.
