Study on the wetting behavior and theoretical models of polydimethylsiloxane/silica coating

The hydrophobic coatings were successfully fabricated through spraying via mixing the hydrophobic silica (SiO2) and the cross-linked polydimethylsiloxane (PDMS) which was cured by tetraethoxysilane (TEOS) under the catalysis of dibutyltin dilaurate (DBTDL). The effects of SiO2 content on the surface morphology and wettability, as well as the water at different temperatures on the hydrophobic behavior were investigated. When the mass ratio of SiO2 to PDMS-TEOS is 0.3, the micromorphology of coating shows random micro/nanostructure and the water contact angle (WCA) of the coating reaches 153.4° with a sliding angle (SA) lower than 5°. However, with the increase of temperature of water droplet over 50 °C, the WCA falls below 130.4° and the SA significantly increases to nearly 180°, which implies that the state of water droplet on superhydrophobic surface has changed from Cassie–Baxter (CB) model to Wenzel model. Meanwhile, on the basis of the variant WAC of water at different temperatures on the same surfaces, a revised model is proposed to explain the state of water droplet on the hydrophobic surface. Thus, the effective way to increase the WCA is to capture more air in the grooves. Finally, based on the models, the relationship between hydrophobicity and superhydrophobicity is explained.