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

    Year: 2021

    Journal: Langmuir, Volume 37, JAN 12, page 124–131

    Authors: Castro, Lucas D. C.; Larocca, Nelson M.; Pessan, Luiz A.

    Organizations: Sao Paulo Research Foundation (FAPESP)Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP) [2015/20052-0]; Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior.Brasil (CAPES)Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior (CAPES) [001, PNPD20131474 -33001014004P9]; Complex Materials group-ETH Zurich

    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 (Phi(crit)) required to stabilize the solid-liquid-air interface; thus, a fully wetted Wenzel state is invariably observed with water contact angles (WCAs) similar to 130 degrees. 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 similar to 167 degrees. 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.
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