Theoretical investigations on the superhydrophobicity of intrinsic hydrophilic surfaces with overhang microstructures

Abstract

It has been accepted generally that it is necessary to obtain the so-called surface superhydrophobicity on intrinsically hydrophobic materials. However, recent experiments have indicated that it could be possible to prepare superhydrophobic surfaces on intrinsically hydrophilic materials by creating adequate roughness. In this work, such a strategy for surface superhydrophobicity on hydrophilic materials with an intrinsic contact angle less than 90° was demonstrated thermodynamically based on a proposed 2-D analytical model. In particular, different (trapezoidal, vertical and inverse-trapezoidal) microstructures were employed to analyze their wetting states such as composite and noncomposite and superhydrophobic behavior as well as the previous corresponding experimental observations. Based on the thermodynamic calculations, it was demonstrated that for an overhang microstructure, intrinsic contact angle, which was restricted by the sidewall angle of micropillars, was not an independent parameter to affect superhydrophobicity. Furthermore, an overhang microstructure was critical to realize the transition from hydrophilicity to superhydrophobicity, and for such a transition, the sidewall angle should be less than the intrinsic contact angle where a positive free energy barrier could support the liquid/vapor interfaces and separate the Wenzel and Cassie states on such hydrophilic surfaces. Most importantly, it was found that for such hydrophilic surfaces, generally, the free energy of the noncomposite or Wenzel states were lower than that of the composite or Cassie states for those trapezoidal, vertical and inverse-trapezoidal microstructures, implying that once a noncomposite state was formed, it can hardly become a composite state, or in other words, even if superhydrophobic behavior was possible, it could be temporary or unstable.