title: Impact of air and water vapor environments on the hydrophobicity of surfaces.
abstract: OBJECTIVE
  Droplet wettability and mobility play an important role in dropwise condensation heat transfer. Heat exchangers and heat pipes operate at liquid-vapor saturation. We hypothesize that the wetting behavior of liquid water on microstructures surrounded by pure water vapor differs from that for water droplets in air.
  
  
  METHODS
  The static and dynamic contact angles and contact angle hysteresis of water droplets were measured in air and pure water vapor environments inside a pressure vessel. Pressures ranged from 60 to 1000 mbar, with corresponding saturation temperatures between 36 and 100°C. The wetting behavior was studied on four hydrophobic surfaces: flat Teflon-coated, micropillars, micro-scale meshes, and nanoparticle-coated with hierarchical micro- and nanoscale roughness.
  
  
  RESULTS
  Static advancing contact angles are 9° lower in the water vapor environment than in air on a flat surface. One explanation for this reduction in contact angles is water vapor adsorption to the Teflon. On microstructured surfaces, the vapor environment has little effect on the static contact angles. In all cases, variations in pressure and temperature do not influence the wettability and mobility of the water droplets. In most cases, advancing contact angles increase and contact angle hysteresis decreases when the droplets are sliding or rolling down an inclined surface.
authors.affiliation: Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, 1206 W. Green St., Urbana, IL 61801, USA. Electronic address: weisens2@illinois.edu.
authors.forename: Patricia B
authors.initials: PB
authors.lastname: Weisensee
authors.affiliation: Department of Chemistry, University of Illinois at Urbana-Champaign, 600 S. Mathews Av., Urbana, IL 61801, USA. Electronic address: neelaka2@illinois.edu.
authors.forename: Nitin K
authors.initials: NK
authors.lastname: Neelakantan
authors.affiliation: Department of Chemistry, University of Illinois at Urbana-Champaign, 600 S. Mathews Av., Urbana, IL 61801, USA. Electronic address: ksuslick@illinois.edu.
authors.forename: Kenneth S
authors.initials: KS
authors.lastname: Suslick
authors.affiliation: Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, 1206 W. Green St., Urbana, IL 61801, USA. Electronic address: a-jacobi@illinois.edu.
authors.forename: Anthony M
authors.initials: AM
authors.lastname: Jacobi
authors.affiliation: Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, 1206 W. Green St., Urbana, IL 61801, USA. Electronic address: wpk@illinois.edu.
authors.forename: William P
authors.initials: WP
authors.lastname: King
keywords: Droplet mobility; Dynamic contact angle; Nanoparticle spray; Sliding angle; Spreading pressure; Superhydrophobic; Teflon; Vapor adsorption; Wetting; Zinc Oxide (ZnO)