  We report an experimental investigation of the wake of free falling
super-hydrophobic spheres. The mutual interaction between the air layer
(plastron) encapsulating the super-hydrophobic spheres and the flow is
emphasised by studying the hydrodynamic performances. It is found that the air
plastron adapts its shape to the flow-induced stresses which compete with the
surface tension. This competition is characterised by introducing the Weber
number $\mathcal{W}e$, whilst the plastron deformation is estimated via the
aspect ratio $\chi$. While noticeable distortions are locally observed, the
plastron becomes more and more spherical in average (i.e. $\chi \rightarrow 1$)
as far as $\mathcal{W}e$ increases. In comparison to the reference spheres,
high deformation of the air plastron plastron (oblate shape) leads to lift and
drag increase, whereas low deformation (spherical shape) yields lift and drag
mitigation. Accordingly, taking into account the plastron deformation provides
an attractive way to explain the somehow discordant results reported in other
studies at comparable Reynolds numbers. If confirmed by additional studies, our
findings would imply that plastron compliance and its feedback on the flow,
which are currently neglected in most theoretical works and numerical
simulations, must be accounted for to design super-hydrophobic surfaces and/or
predict their performances.
