Universal wetting transition of an evaporating water droplet on hydrophobic micro- and nano-structures.

Water-repellent, rough surfaces have a remarkable and beneficial wetting property: when a water droplet comes in contact with a small fraction of the solid, both liquid-solid adhesion and hydrodynamic drag are reduced. As a prominent example from nature, the lotus leaf-comprised of a wax-like material with micro- and nano-scaled roughness-has recently inspired numerous syntheses of superhydrophobic substrates. Due to the diverse applications of superhydrophobicity, much research has been devoted to the fabrication and investigations of hydrophobic micro-structures using established micro-fabrication techniques. However, wetting transitions remain relatively little explored. During evaporation, a water droplet undergoes a wetting transition from a (low-frictional) partial to (adhesive) complete contact with the solid, destroying the superhydrophobicity and the self-cleaning properties of the slippery surface. Here, we experimentally examine the wetting transition of a drying droplet on hydrophobic nano-structures, a previously unexplored regime. In addition, using a theoretical analysis we found a universal criterion of this wetting transition that is characterized by a critical contact angle. Different from previous results showing different critical droplet sizes, our results show a universal, geometrically-dependent, critical contact angle, which agrees well with various data for both hydrophobic micro- and nano-structures.