Drag reduction for viscous laminar flow on spray-coated non-wetting surfaces

We estimate the effective Navier-slip length for flow over a spray-fabricated liquid-repellent surface which supports a composite solid-air-liquid interface or 'Cassie-Baxter' state. The morphology of the coated substrate consists of randomly distributed corpuscular microstructures which encapsulate a film of trapped air (or 'plastron') upon contact with liquid. The reduction in viscous skin friction due to the plastron is evaluated using torque measurements in a parallel plate rheometer resulting in a measured slip length of bslip approximate to 39 mu m, comparable to the mean periodicity of the microstructure evaluated from confocal fluorescence microscopy. The introduction of a large primary length-scale using dual-textured spray-coated meshes increases the magnitude of the effective slip length to values in the range 94 mu m <= b(slip) <= 213 mu m depending on the geometric features of the mesh. The wetted solid fractions on each mesh are calculated from free surface simulations on model sinusoidal mesh geometries. The trend in measured values of b(slip) with the mesh periodicity L and the computed wetted solid-fraction r phi(s) are found to be consistent with existing analytic predictions.1183