  In this Letter, the falling of super-hydrophobic spheres is investigated
experimentally at low Reynolds numbers. In particular, we show that
super-hydrophobic coatings become ineffective at reducing drag unlike predicted
by theoretical and numerical approaches. A time scale analysis describing both
Marangoni-induced stresses and air/liquid interface deformation shows that
these mechanisms are unlikely to account for the slippage effect disappearance
observed in our study. Instead, we propose a simple model based on an isotropic
porous medium approach, derived to account for losses induced by the motion of
the gas encapsulated around the sphere. The key parameter of this mechanism is
found to be the surface tortuosity, whose range estimated from microscopic
surface imaging corroborate those predicted by our scaling analysis and
previous studies.
