Hydrophobicity Evolution on Rough Surfaces

Hydrophobicity
is abundant in nature and obtainable in industrial
applications by roughening hydrophobic surfaces and engineering micropatterns.
Classical wetting theory explains how surface roughness can enhance
water repellency, assuming a droplet to have a flat bottom on top
of micropatterned surfaces. However, in reality, a droplet can partially
penetrate into micropatterns to form a round-bottom shape. Here, we
systematically investigate the evolution of evaporating droplets on
micropatterned surfaces with X-ray microscopy combined with three-dimensional
finite element analyses and propose a theory that explains the wetting
transition with gradually increasing penetration depth. We show that
the penetrated state with a round bottom is inevitable for a droplet
smaller than the micropattern-dependent critical size. Our finding
reveals a more complete picture of hydrophobicity involving the partially
penetrated state and its role in the wetting state transition and
can be applied to understand the stability of water repellency of
rough hydrophobic surfaces.