The durability of superhydrophobic films

Superhydrophobic sol–gels, spin-coated on a glass substrate, were synthesized by varying the hexamethyldisilazane/tetraethoxysilane ratio (Ratio) from 1 to 2. A simple heat treatment at 200 °C for 2 h, an additional prolonged heat treatment at 200 °C for 24 h, and an additional prolonged electric field aging for 24 h with specimens connected to the positive or negative electrodes were performed. The specimens were subjected to a water impact test (WIT) for 2 h, and water contact angle (WCA) and sliding angle (SA) were measured before and after WIT. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy were employed to analyze the variations of elements and functional groups in the hydrophobic film before and after the WIT. The variations of area ratio of XPS deconvolution peaks showed that during the WIT more SiO2 groups than OSi(CH3)3 groups were washed away in the hydrophobic film. Molecular simulation provided a complete image showing the molecules constituting the surface of the hydrophobic film before and after WIT. The additional heat and electric field treatment induced SiOH network and vertical arrays of hydroxyl chains, respectively, which exerted viscous forces on the water drops but also mitigated the impact strength of the water column. Hence, before WIT, the specimen with a simple heat treatment at 200 °C for 2 h exhibited the highest WCA and lowest SA. After the WIT at a Ratio of 1.6, 1.8, and 2, SA increased in the following order: additional 24-h heat-treated specimen < electric field aging negative electrode specimen < electric field aging positive electrode specimen < simple heat-treated specimen.