  A full characterization of the water flow past a silicon superhydrophobic
surface with longitudinal micro-grooves enclosed in a microfluidic device is
presented. Fluorescence microscopy images of the flow seeded with fluorescent
passive tracers were digitally processed to measure both the velocity field and
the position and shape of the liquid-air interfaces at the superhydrophobic
surface. The simultaneous access to the meniscus and velocity profiles allows
us to put under a strict test the no-shear boundary condition at the liquid-air
interface. Surprisingly, our measurements show that air pockets in the surface
cavities can sustain non-zero interfacial shear stresses, thereby hampering the
friction reduction capabilities of the surface. The effects of the meniscus
position and shape as well as of the liquid-air interfacial friction on the
surface performances are separately assessed and quantified.
