On the possibility of superhydrophobic behavior for hydrophilic materials.

It has been recognized well that it is necessary to achieve superhydrophobic surfaces on intrinsically hydrophobic materials. However, recently experiments have demonstrated that it is possible to fabricate superhydrophobic surfaces on intrinsically hydrophilic materials by creating adequate roughness. In this study, such a possibility for superhydrophobicity on a hydrophilic surface with an intrinsic contact angle (CA) of 80 degrees, with a comparison to a hydrophobic surface with an intrinsic CA of 120 degrees, is thermodynamically analyzed using a pillared microtexture. Based on the calculations of free energy (FE) and free energy barrier (FEB), it is found that for such hydrophilic materials, generally, the FE for noncomposite or Wenzel's state is lower than that composite or Cassie's state for various geometrical wetting systems. Furthermore, even if pillar height or roughness is adequately large, it is hard to realize superhydrophobic behavior because of the surface wicking resulted from its special FE state. In addition, due to the negative FEB of the noncomposite state, there is no transition between noncomposite and composite states no matter how surface geometry varies. The above results also indicates that once noncomposite state is formed, it can hardly be become composite state, or in other words, even if superhydrophobic behavior is possible, it could be temporary and unstable. The present theoretical investigation therefore keeps a reservation on the practicability of superhydrophobic surfaces built on hydrophilic materials.