# A Facile Method to Prepare Superhydrophobic Coatings for Various Substrates
## 3 Results  
### Preparation and Characterization of the Superhydrophobic Coatings  
The wettability of the composite coatings influenced with the SiO\({}_{2}\) content was shown in Figure 1. The mass fraction of CNTs in each of these coatings was fixed at 30%. With increasing SiO\({}_{2}\) nanoparticles content, the WCA of the composite coating improved gradually. At the same time, the contact angle hysteresis of the composite coating decreased gradually. When the total mass of the nanoparticles accounted for 75% of the coatings and the mass ratio of CNTs to SiO\({}_{2}\) was 2:3, the superhydrophobic composite coatings with a WCA of 156.8\({}^{\circ}\) and contact angle hysteresis of 3.6\({}^{\circ}\) were fabricated through mixing fluorine resin and two kinds of nanoparticles of CNTs and SiO\({}_{2}\). In fact, the hydrophobic property of the composite coating could be improved slightly by further improving the amount of SiO\({}_{2}\) nanoparticles used. However, it was not suitable to further increase the SiO\({}_{2}\) nanoparticles content considering the surface state and mechanical strength of the composite coatings. In this study, the optimal formula is that the mass ratio of CNTs to SiO\({}_{2}\) is 2:3, and the loading of nanoparticles is 75 wt. %. Unless otherwise stated, the following superhydrophobic composite coating refers to the coating in this formulation. Many methods for preparing superhydrophobic surfaces are based on specific substrates. The dependence on substrates is due to the need for substrates to provide all or part of the roughness. For example, the superhydrophobicity of metal surfaces can be achieved by chemical etching and post-hydrophobic molecular modification [34]. If the superhydrophobicity of a surface does not depend on the substrate, then the superhydrophobic coatings can be used more widely. In this study, the superhydrophobic coatings can be used for various substrates. The superhydrophobic composite coatings coated on cloth, board, steel plate, and PVC plate are shown in Figure 2. In Figure 2, the superhydrophobic composite coating is suitable for a variety of substrates. The superhydrophobic composite coatings were dipped into three aqueous solutions for three days to investigate their chemical stability. The photographs of the experiment are displayed in Figure 3. The superhydrophobic composite coating could still maintain its hydrophobicity after being soaked in HCl (pH = 2), NaOH (pH = 12) and 3.5 wt. % NaCl solution for three days. This result shows that the superhydrophobic composite coating has good chemical stability. The surface topography of the composite coatings are displayed in Figure 4. The CNTs are clearly visible in Figure 4. In Figure 4, to the composite of silica nanoparticles and fluorine resin, a lesser degree of surface rough ness of the composite coating can be observed. The mass fraction of silica nanoparticles is \(75\%\). The surface roughness of the composite coating was significantly improved with an increase in the use of CNTs. Compared to adding CNTs alone, the composite application of silica nanoparticles and CNTs can further improve the hydrophobicity of the coating and reduce the contact angle hysteresis. The difference between the three images (Figure 4a-c) is not obvious because the silica nanoparticles have little change on the surface morphology of the composite coatings. This may be because silica nanoparticles have good compatibility with fluorine resin due to the large number of alkyl groups on its surface. Therefore, the silica nanoparticles have little change on the surface morphology of the composite coatings. Figure 1: Relation between weight ratio of SiO\({}_{2}\), water contact angles, and contact angle hysteresis. The mass fraction of CNTs in each of these coatings was 30%. The inserts were 5 \(\upmu\)L water drop profiles on the surface of the composite coatings. Figure 2: The superhydrophobic composite coatings coated on cloth (**a**), board (**b**), steel plate (**c**), and PVC plate (**d**).