# Large-Scale Fabrication of Graded Convex Structure for Superhydrophobic Coating Inspired by Nature
## 3 Results and Discussion
### Oil\(-\)Water Separation Capability of the Superhydrophobic Coating  
Due to the excellent superhydrophobic capability of P40-P4-P0.4 coating, it is not only hydrophobic but also lipophilic. Therefore, P40-P4-P0.4-treated paper has high potential in oil-water-separation applications. The water and oil wettability of the P40-P4-P0.4-coated superhydrophobic filter paper initially proved its potential (Figure 10a,b). As can be seen, the surface of the filter paper has superhydrophobic and superlipophilic properties, and the water droplets and oil droplets on the filter paper were immediately absorbed, but  
Figure 8: Micromorphology of the P40–P4-P0.4 superhydrophobic coating before and after (**a**,**c**) 100 attrition cycles and (**b**,**d**) 96 h UV-light irradiation, the inset is the topography of the superhydrophobic surface before wear at higher magnification.  
Figure 9: Self-cleaning capability of (**a**) P40–P4–P0.4 superhydrophobic glass slide and (**b**) glass slide.  
only the oil droplets were absorbed by the superhydrophobic filter paper coated with P\({}_{40}\)-P\({}_{4}\)-P\({}_{0.4}\), leaving the water droplets remaining as a spheres on the surface. In addition, both the filter paper and the superhydrophobic filter paper coated with P\({}_{40}\)-P\({}_{4}\)-P\({}_{0.4}\) were immersed in a glass beaker containing n-hexane that was dyed with Sudan red and water dyed with methylene blue in order to study the oil-water-separation ability. Obviously, the n-hexane was completely absorbed by the superhydrophobic filter paper coated with P\({}_{40}\)-P\({}_{4}\)-P\({}_{0.4}\), and there was no apparent oil remaining in the collection container (Figure 10c,d), proving that the superhydrophobic filter paper coated with P\({}_{40}\)-P\({}_{4}\)-P\({}_{0.4}\) showed excellent hydrophobicity and lipophilicity. The oil-water mixture was poured into a solvent filter equipped with a P\({}_{40}\)-P\({}_{4}\)-P\({}_{0.4}\)-coated superhydrophobic cellulose filter paper (Figure 10e-g). The results show that only the red oil reached the flask through the superhydrophobic filter coated with P\({}_{40}\)-P\({}_{4}\)-P\({}_{0.4}\), but the blue-dyed water was thoroughly prohibited and stayed in the funnel. After separation, there was no obvious blue water remaining in the collection container and no red oil in the funnel, which confirmed that the superhydrophobic filter coated with P\({}_{40}\)-P\({}_{4}\)-P\({}_{0.4}\) had excellent oil-water-separation performance.  
In Figure 11a, after 50 cycles of separation, the separation capability declined but maintained above 85%, indicating an excellent separation capability of the prepared P\({}_{40}\)-P\({}_{4}\)-P\({}_{0.4}\) superhydrophobic filter paper in oil-water separation applications. Moreover, the CA of the P\({}_{40}\)-P\({}_{4}\)-P\({}_{0.4}\)-coated superhydrophobic filter paper after multiple separation cycles of oil-water mixtures was tested in order to prove the recyclability and reusability of oil-water  
Figure 10: Wettability of (**a**) the paper and (**b**) the P\({}_{40}\)–P\({}_{4}\)–P\({}_{0.4}\)-coated superhydrophobic paper. (**c,d**) the process of superhydrophobic filter paper adsorbing oil from water. (**e–g**) oil–water-separation tests of the P\({}_{40}\)–P\({}_{4}\)–P\({}_{0.4}\)-coated superhydrophobic filter.  
separation. The CA of the superhydrophobic filter paper coated with P\({}_{40}\)-P\({}_{4}\)-P\({}_{0.4}\) remained at 145\({}^{\circ}\). The above separation cycle (Figure 11b) confirms the strong oil-water-separation capability of the superhydrophobic filter paper coated with P\({}_{40}\)-P\({}_{4}\)-P\({}_{0.4}\).