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