Apparent slip arising from Stokes shear flow over a bidimensional patterned surface

A mathematical model is presented for the problem of apparent slip arising from Stokes shear flow over a composite surface featuring mixed boundary conditions on the microscale. The surface can be composed of a bidimensional array of solid areas placed on an otherwise no-shear surface corresponding to an envelope over the tops of posts, or no-shear areas placed on an otherwise solid surface corresponding to an envelope over the tops of holes. Posts and holes of circular or square cross section, and solid areas of no-slip or partial-slip types are studied. Following some previously proposed scaling laws, the effective slip length is expressed as a certain function of the solid fraction for some specific cases. More refined equations based on linear regression of the computed results are obtained for these cases. Amounts of slippage arising from these bidimensional patterns are compared with those from the one-dimensional patterns of grooves/grates. It is also shown that a larger slip length can result from an arrangement where the pitch is larger in the spanwise direction than in the streamwise direction.