# Large-Scale Fabrication of Graded Convex Structure for Superhydrophobic Coating Inspired by Nature
## 1 Introduction  
The superhydrophobic phenomenon usually refers to the phenomenon of water droplets falling onto the surface of a sample with contact angles of 150\({}^{\circ}\) or more and rolling angles of less than 10\({}^{\circ}\). In the natural environment, there are numerous superhydrophobic phenomena, such as the leaves of plants [1, 2, 3, 4], the wings of butterflies, and the feet of insects [5], among which the surfaces of lotus leaves are the most widely known. According to the surface-structure characteristics of the lotus-leaf effect, other researchers have summarized two necessary conditions to prepare a superhydrophobic surface [6, 7, 8]: a micro-nano double-rough structure on the surface of the material, which is modified with low-surface-energy substances. Based on the above principles, the biomimetic design of the superhydrophobic surface inspired by superhydrophobic structures in nature has considerable value and is expected to be popularized and widely used in self-cleaning [9, 10, 11, 12], antifouling [13, 14] and water-oil separation [15, 16, 17] and other industries.  
In previous studies, typical methods for preparing superhydrophobic surfaces have included the etching method [18, 19], electrospinninging [20, 21], the sol-gel method [22, 23], and physical- and chemical-deposition methods [24, 25]. [PERSON] et al. [26] combined submicron silica particles with different diameters in the form of covalent bonds, and then modified them with polydimethylsiloxane. Through the impregnation method, a durable and chemically stable superhydrophobic fabric was prepared, which had excellent oil-water-separation performance. [PERSON] [27] thermally induced the formation of polydopamine (Pdop) particles on a copper mesh or sponge (MF), and after modification with octadecylamine (ODA), a metal mesh with superhydrophobic/superlipophilic properties was obtained. However,complicated operating procedures and expensive equipment have greatly hindered the introduction of products into the market. To this end, some improved technologies have been developed in order to produce superhydrophobic surfaces, among which spraying is a means to achieve large-scale production. [PERSON] et al. (2018) sprayed an ethanol suspension of nanocellulose filaments beforehand. The surface of the wood treated with the adhesive was modified by chemical-vapor precipitation to obtain a superhydrophobic coating of nano-cellulose filaments. However, the micro-nano structure and the durability of low-surface-energy substances are key factors that determine whether the spraying method can be widely used to prepare superhydrophobic coatings and market them.  
In this study, a uniform superhydrophobic coating that mimics the surface of the lotus leaves with a micro-nano structure was constructed through effective spraying. As demonstrated in the mechanism diagram 1, polydivinylbenzene (PDVB) of different particle sizes were prepared by a hydrothermal reaction combined with ultrasonic treatment, then mixed in different proportions and sprayed onto the surface of the substrate to which the adhesive had been pre-attached. The surface imitated the micro-nano convex structure of the lotus-leaf surface, which not only significantly improved the surface roughness of the substrate, but also created a large amount of air gaps between the water droplets and the coating surface, resulting in superhydrophobic properties. Moreover, the micro-nano structure composed of countless small particles has good abrasion resistance. When the outermost nano-sized balls are worn out, new nano-sized balls are exposed, so the superhydrophobic properties can be preserved. The method is simple, economical, and scalable, and the superhydrophobic coating shows considerable application prospects for water-oil separation.