  In this contribution we study wetting and nucleation of vapor bubbles on
nanodecorated surfaces via free energy molecular dynamics simulations. The
results shed light on the stability of superhydrophobicity in submerged
surfaces with nanoscale corrugations. The re-entrant geometry of the cavities
under investigation is capable of sustaining a confined vapor phase within the
surface roughness (Cassie state) both for hydrophobic and hydrophilic
combinations of liquid and solid. The atomistic system is of nanometric size;
on this scale thermally activated events can play an important role ultimately
determining the lifetime of the Cassie state. Such superhydrophobic state can
break down by full wetting of the texture at large pressures (Cassie-Wenzel
transition) or by nucleating a vapor bubble at negative pressures (cavitation).
Specialized rare event techniques show that several pathways for wetting and
cavitation are possible, due to the complex surface geometry. The related free
energy barriers are of the order of 100 k_B T and vary with pressure. The
atomistic results are found to be in semi-quantitative accord with macroscopic
capillarity theory. However, the latter is not capable of capturing the density
fluctuations, which determine the destabilization of the confined liquid phase
at negative pressures (liquid spinodal).
