Easy-to-Clean Superhydrophobic Coatings Based on Sol-Gel Technology: A Critical Review

Superhydrophobicity (SH) is defined by a water contact angle (CA) higher than 150 degrees and sliding angle (SA) of less than 5 degrees. SH is a desirable property for a wide range of applications, from the aerospace industry to basic commodities. SH is obtained by a combination of two main principles: low surface free energy and surface roughness. While implementing the proper chemistry and the right hierarchical micro-nano structure, numerous methods can be used to fabricate SH coatings and surfaces, among them being the sol-gel technology. Sol-gel technology is a widely-used fabrication method since it requires only mild conditions. A hybrid sol-gel, composed of inorganic silica and organically modified silanes, forms into a three-dimensional gel network. It is a synergetic hybrid which possesses both silica network's properties such as hardness, and organic phase's properties like flexibility and good adhesion. Considering the potential applications of SH coatings, the science and technology communities encounter the challenge to fabricate a durable, transparent superhydrophobic coating which can be manufactured on an industrial scale. Furthermore, since roughness is an essential condition for obtaining SH, transparency may be reduced. Therefore, fine-tuning is necessary in order to achieve both SH and transparency. The following review is dedicated to SH coatings fabricated using the sol-gel method. Along with a comprehensive review of the sol-gel process itself, the effects of various precursors, nanoparticles and fabrication techniques on the coating properties are discussed. Water repellency, mechanical properties and adhesion can be tailored by incorporating organo-functionalized silanes and silica nanoparticles. Adhesion is obtained by creating a covalent bond between the coating and the substrate under thermal or radiation curing. Furthermore, adding the organic component to the mixture as a precursor rather than as a post-silylation agent contributes to the required surface roughness.