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

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 (fdyes) that was wet by the aqueous solution. The fdyes value corresponded well with the theoretical f_s for the webs with superhydrophobic characteristics.