Durable Superhydrophobic Surfaces via Spontaneous Wrinkling of Teflon AF

We report the fabrication of both single-scale and hierarchical
superhydrophobic surfaces, created by exploiting the spontaneous wrinkling
of a rigid Teflon AF film on two types of shrinkable plastic substrates.
Sub-100 nm to micrometric wrinkles were reproducibly generated by
this simple process, with remarkable control over the size and hierarchy.
Hierarchical Teflon AF wrinkled surfaces showed extremely high water
repellence (contact angle 172°) and very low contact angle hysteresis
(2°), resulting in droplets rolling off the surface at tilt angles
lower than 5°. The wrinkling process intimately binds the Teflon
AF layer with its substrate, making these surfaces mechanically robust,
as revealed by macroscale and nanoscale wear tests: hardness values
were close to that of commercial optical lenses and aluminum films,
resistance to scratch was comparable to commercial hydrophobic coatings,
and damage by extensive sonication did not significantly affect water
repellence. By this fabrication method the size of the wrinkles can
be reproducibly tuned from the nanoscale to the microscale, across
the whole surface in one step; the fabrication procedure is extremely
rapid, requiring only 2 min of thermal annealing to produce the desired
topography, and uses inexpensive materials. The very low roll-off
angles achieved in the hierarchical surfaces offer a potentially up-scalable
alternative as self-cleaning and drag-reducing coatings.