Sphere-on-cone microstructures on Teflon surface: Repulsive behavior against impacting water droplets

Highlights: • Micro-scale relieves with a sphere on the top are generated on Teflon surface. • Modified surfaces strongly repel vertically impacting water droplets. • Sphere parameters affect the critical pressure for water penetration. • Drop pinning reversibility depends on the entity of the de-wetting cycle. Teflon (polytetrafluoroethylene) surface has been modified with a single step oxygen-fed plasma process resulting in the formation of peculiar "sphere-on-cone" micro-scale relieves. Though presenting some irregularity and random distribution, surfaces with a steep variation of topography can be obtained by properly tuning plasma process parameters. We show that these surfaces exhibit similar superhydrophobic properties in terms of water contact angle but, the ability to withstand vertically impacting water droplets falling at medium/high impacting speeds can be sensitively different. In particular, high repulsive properties characterize surfaces with denser and smaller relieves. In the other cases the occurrence of pinning events results in longer time-of-contact and a shorter time-of-flight of the drop. Interestingly, the impact pressure values at which pinning transitions have been observed are consistent with critical pressures for penetration calculated by modeling the surface as an array of spheres. When critical pressure is exceeded, and water penetration is expected, reversible or non reversible pinning can be explained on the basis of the wetting/de-wetting cycle within the sphere-on-cone's layer.