title: Filamentary superhydrophobic Teflon surfaces: Moderate apparent contact angle but superior air-retaining properties.
abstract: OBJECTIVE
  Micro-scale textured Teflon surfaces, resulting from plasma etching modification, show extremely high water contact angle values and fairly good resistance to water penetration when hit by water drops at medium-high speed. This behavior is more pronounced when these surfaces present denser and smaller micrometric reliefs. Tailoring the top of these reliefs with a structure which further stabilizes the air may further increase resistance to wetting (water penetration) under static and dynamic conditions.
  
  
  METHODS
  Conditions of the oxygen fed plasma were tuned in order to explore the possibility of obtaining differently topped structures on the surface of the polymer. Scanning Electron Microscopy (SEM) was used to explore topography and X-ray Photoelectron Spectroscopy (XPS) to assess chemical similarity of the modified surfaces. Beside the usual advancing and receding water contact angle (WCA) measurements, surfaces were subjected to high speed impacting drops and immersion in water.
  
  
  RESULTS
  At milder, i.e. shorter time and lower input power, plasma conditions formation of peculiar filaments is observed on the top of the sculpted reliefs. Filamentary topped surfaces result in a lower WCA than the spherical ones, appearing in this sense less superhydrophobic. However, these surfaces give rise to the formation of a more pronounced air layer when placed underwater. Further, when hit by water drops falling at medium/high speed, they show a higher resistance to water penetration and a sensitively lower surface-liquid contact time. The contact time is as low as previously observed only on heated solids. This behavior may be ascribed to the cavities formed beneath the filaments which, similarly with the salvinia leaf structures, require a surplus of pressure to be filled by water. Also, it suggests a different concept of superhydrophobicity, which cannot be expected on the basis of the conventional water contact angle characterization.
authors: Department of Mechanics, Mathematics and Management, Politecnico di Bari, v.le Japigia 182, 70126 Bari, Italy. Electronic address: rosa.dimundo@poliba.it.
authors: Rosa
authors: R
authors: Di Mundo
authors: Department of Mechanics, Mathematics and Management, Politecnico di Bari, v.le Japigia 182, 70126 Bari, Italy.
authors: Francesco
authors: F
authors: Bottiglione
authors: CNR-NANOTEC, via Orabona 4, 70126 Bari, Italy.
authors: Fabio
authors: F
authors: Palumbo
authors: Department of Civil and Environmental Engineering and Chemistry, Politecnico di Bari, via Orabona 4, 70126 Bari, Italy.
authors: Michele
authors: M
authors: Notarnicola
authors: Department of Mechanics, Mathematics and Management, Politecnico di Bari, v.le Japigia 182, 70126 Bari, Italy; CNR - Institute for Photonics and Nanotechnologies U.O.S. Bari, Department of Physics "M. Merlin", via Amendola 173, I-70126 Bari, Italy.
authors: Giuseppe
authors: G
authors: Carbone
keywords: Air retaining surface; Contact time; Drop impact; Dynamic contact angle; Polytetrafluoroethylene; Superhydrophobicity; Teflon; Underwater surface