Analysis of contact area between water and irregular fibrous surface for prediction of wettability

Abstract

A characterization method, which allows visual observation of the surface area that is wet by a liquid on a roughened surface, called the solid fraction (f_s) in the Cassie–Baxter model was developed. To this end, superhydrophobic polystyrene (PS) webs (contact angles ∼170°, sliding angles ∼3°) were fabricated via electrospinning and subsequent coating with perfluorodecyltrichlorosilane (PFDTS). The theoretical solid fraction of the Cassie–Baxter model, f_s, was calculated from the equation. Microscopic images of the PS webs were converted into black and white binary images after adjusting the gray-scale of the images to match the bright area fraction with the theoretical f_s. For visual observation of the actual solid fraction, a drop of an aqueous solution with a hydrophobic fluorescent dye was rolled on the surface. Traces of the dye on the PS surface were analyzed by fluorescence microscopy to measure the actual solid fraction (f^dye_s) that was wet by the aqueous solution. The f^dye_s value corresponded well with the theoretical f_s for the webs with superhydrophobic characteristics.