The superhydrophobic property is inspired by nature. In the 1990s, with the advent of electron microscopes, more detailed studies on the structure of materials became possible. Earlier in the decade, two German scientists studied 200 species of hydrophobic plants, including the lotus, and discovered the self-cleaning properties of the plant's leaves [1, 2]. Their studies reported that the lotus leaf had a very rough surface structure in which protrusions and depressions (including cell wall structures and cuticle folds) with a height of about 3 to 10 µm coated with hydrophobic nanoparticles. Such a structure (presence of nanoscale bumps on rough microstructures) is called a hierarchical structure. In addition to the hierarchical structure, the lotus leaf hydrophobic is also related to its low surface free energy. In fact, the combination of surface roughness and low surface free energy has caused this property in lotus flowers because the hierarchical structure (protrusions and depressions) reduces the area of contact with water and hydrophobic nanoparticles reduce surface free energy. As a result, water cannot wet this type of surface, and therefore water droplets on the surface become almost spherical and, in their path, they remove impurities from the surface [3]. Therefore, the superhydrophobic surface is inspired by nature due to attractive properties such as waterproofing, self-cleaning and reduction of biological powder [4–6], and also the strong potential for application in a wide range of industries, including automotive, construction, food packaging and textiles [7], has attracted much scientific and technological attention in the field of research and development of hydrophobic coatings. Such surfaces usually show a water contact angle of more than 150° and a rolling angle of less than 10° [8, 9]. Most preparation of such gripping surfaces focuses on generating a nano/microscale hierarchical roughness on low-energy surfaces or coating low-energy materials on nano/microstructural surfaces [10–12]. One of the methods to reduce surface energy is using silicates (polydimethylsiloxane) [13, 14], fluorocarbons (polytetrafluoroethylene) [15], organic materials such as alkylene, polycarbonate and polyamide [16–18] and inorganic materials such TiO2 [19]. Another efficient method to make a superhydrophobic surface is applying biopolymers directly to the surface [20]. For example, Polyethylene glycol and 1,1,2,2- tetrahydro perfluorodecyl acrylate, among others, have been grafted to cellulose to induce hydrophobicity [21].
Using nano dimension particles to enhance the surface roughness of a hydrophobic surface should be feasible. ZnO nanoparticles have been primarily studied lately due to their many notable physical and chemical properties. However, an appropriate surfactant is essential to obtain a good dispersion and weak interfacial adhesion [22, 23]. Surfactants, including reactive functional groups such as silane coupling agents [24] and polymers such as fluorinated polysiloxane, can improve the hydrophobic properties of ZnO [22, 25].
Silica nanoparticles have been widely used to produce films with controlled roughness using various methods such as rotating coatings, spray coatings, etc., and the superhydrophobic properties are achieved by surface modification by fluoroalkyl silane compounds [26, 27]. Bhagat et al. Showed that silica-based superhydrophobic coatings could be applied to stainless steel using a practical and cost-effective method such as the immersion method [28]. Fluoroalkyltrimethoxysilane (FAS) molecules often perform modification of surfaces with silanol groups [29, 30]. This function combines with roughness and creates hydrophobicity. Silica nanoparticles with a diameter of approximately 10 nanometers are obtained by SiCl4 pyrolysis in the presence of oxygen and hydrogen gas. Silica nanoparticles agglomerate at high temperatures, which increases the grain-to-mass ratio (greater than 100 m2g− 1), which can be adjusted by the amount of agglomeration [31].
Polydimethylsiloxane (PDMS) is a reasonable possibility for the expansion of composite coating films, as its essential chain ((Si-O-Si) n) has excellent bond energy and significant bond angle, which supplies beneficent thermal stability and elasticity, in addition to hydrophobic characteristics [32].
ZnO is structurally similar to silica due to its Zn-O-Zn bond, so it is expected to be easily dispersed into the SiO2 matrix and established the ZnO-SiO2 nanocomposite [33–35]. The different methods were applied to synthesize superhydrophobic ZnO-SiO2 nanocomposites such as hydrothermal, sono chemical, co-precipitation, and spray pyrolysis. However, the main challenge of these methods is the phenomenon of agglomeration. The high surface energy of ZnO is one of the essential factors causes of agglomeration [36–39].
In this work, we have used the new method for synthesizing SiO2, polysiloxane, ZnO-SiO2 and ZnO-SiO2@Polysiloxane nanocomposites. For this purpose, tetraethyl orthosilicate, zinc acetate dehydrated, and vinyl trimethoxy silane were used as precursors receptively. After heat treatment, the zinc acetate complex can be decomposed to ZnO nano crystallites and reinforced by the SiO2 superhydrophobic. These films can adhere to the substrate through Zn-O-Si linkages. It is also noteworthy that the synthesized ZnO can be homogeneously distributed within the SiO2 matrix. Another innovative idea employed in this work is the synthesis of ZnO-SiO2@Polysiloxane nanocomposite. It is expected that the hydrophobic properties ZnO-SiO2@Polysiloxane will increase significantly compare with ZnO-SiO2 nanocomposite. The nanocomposites were characterized by FTIR, XRD, TGA, DLS, SEM, AFM, and water repellence abilities were investigated by WCA analyses.