Preparation and electrowetting transitions on superhydrophobic/ hydrophilic bi-layer structures

A simple method of preparing porous superhydrophobic materials using glass fiber materials, where hydrophobicity is provided by a variety of coatings such as self-assembled alkyl-silane monolayers and fluoropolymers such as Teflon is presented. Fibrous structures of the filter material provide for the modulation of ‘‘surface roughness’’ on the microand nano-scale, required for achieving a superhydrophobic state, with advancing contact angle of water on such surfaces close to 150 degrees. Such superhydrophobic structures are effective at separating wateroctane mixtures by allowing only low-surface-tension component to go through the thickness of the material, while repelling the water (high-surface-tension component) and preventing it from permeating through the material. In addition, a bi-layer structure that combines a superhydrophobic surface with a highly hydrophilic bulk material is described. It is formed by subjecting superhydrophobic fiber material to a brief oxygen plasma treatment to remove the hydrophobic coating from one side of the material, whereas the opposite side is protected during treatment and remains superhydrophobic. Tunable properties of the superhydrophobic fiber material are demonstrated using electrowetting with PEDOT–PSS conductive polymer core, parylene as a dielectric and Teflon as a hydrophobic coating. Applicability of such bi-layer materials to microfluidic and energy storage micro-devices is discussed.