Effect of Structural Features on the Superhydrophobicity of SiO₂-Based Coatings

A detailed correlation between topographical features and wettability of chemically modified coatings based on silica nanoparticles (SiO₂) was performed. In this study, hierarchical structures were prepared by the layer-by-layer (LbL) technique using two different approaches: random roughened surfaces were obtained by exploring stacking defects spontaneously arisen after 15, 30, and 45 assembly cycles of 22 nm SiO₂, and a particular structure, commonly known as raspberry-like, was obtained by depositing 22 nm SiO₂ over the first deposited 400 nm SiO₂. As an intrinsic attribute of the assembly process, the average slope of random roughened surfaces seems to be constant and virtually independent of the number of deposited layers. Additionally, the local slopes are always lower than a critical value (Φ_crit) required to stabilize the solid-liquid-air interface; thus, a fully wetted Wenzel state is invariably observed with water contact angles (WCAs) ∼130°. On the other hand, since the local slopes of the raspberry-like structure follow a nearly spherical curvature, small SiO₂ can stabilize the solid-liquid-air interface by increasing the local contact angle and avoid the deep penetration of water into the surface asperities, resulting in a WCA ∼167°. The results also suggest that nanoroughness might also play an important role in the pinning effect of the solid-liquid-air contact line, favoring the maintenance of superhydrophobicity of raspberry-like surfaces.