  In microfluidics, varying wetting properties, expressed in terms of the local
slip length, can be used to influence the flow of a liquid through a device. We
study flow past surfaces on which the slip length is modulated in stripes. We
find that the effective slip length for such a flow can be expressed as a
function of the individual slip lengths on the stripes. The angle dependence of
the effective slip is in excellent agreement with a recent theory describing
the slip length as a tensorial quantity. This tensorial nature allows to induce
a transverse flow, which can be used in micro mixers to drive a vortex. In our
simulations of a flow through a square channel with patterned surfaces we see a
homogeneous rotation about the direction of the flow. We investigate the
influence of patterns of cosine shaped varying local slip on the flow field
depending on the orientation of the pattern and find the largest effective slip
length for periods of stripes parallel to the flow direction.
