# Review of Hygroscopic Coating on Aluminum Fin Surface of Air Conditioning Heat Exchanger
## 3 Hydrophobic/Hydrophilic Coated Heat Exchangers
### Kinds of Surface Wettability
[77] prepared hybrid-patterned surfaces, and compared them with a single hydrophilic surface of filmwise condensation, and they showed a significant enhancement in heat flux and condensation heat transfer coefficient. They reported that the surface with \(\beta\) and \(\alpha\) zones are 0.6 mm and 0.3 mm, respectively, which have improved heat transfer performance by 480% and 180%, respectively, compared with the filmwise and dropwise condensation surface. The \(\beta\)-regions are where the droplets nucleate. After the droplets grow up, they will be absorbed by the \(\alpha\)-regions. Under the action of gravity, the condensed water can be quickly drained along the \(\alpha\)-regions (Figure 10). Figure 8: Periodically patterned samples for condensation experiments [74]. Figure 9: Schematic detailing hydrophobic/hydrophilic patterned surfaces [76]. Namib desert beetle model: Inspired by the Namib desert beetle, which captures water molecules by wax-free hydrophilic pumps and is transported by the waxy hydrophobic surround. [PERSON] et al. [78] deposited the polytetrafluoroethylene nanoparticles prepared by femtosecond laser onto the superhydrophobic copper mesh, and the copper mesh was coated on the hydrophilic copper sheet to prepare the hybrid superhydrophobic-hydrophilic surface, as shown in Figure 11, and soaked in 1M NaOH, 1M HCl, and 10 wt% NaCl solution for 2 h, respectively, the surface has excellent corrosion resistance. [PERSON] et al. [79] prepared a hydrophobic/hydrophilic cooperative Janus system (Figure 12), which could reduce the re-evaporation rate of harvested water molecules, and a thinner collection surface boundary layer helps improve absorption efficiency. Figure 11: The preparation procedure for the hybrid superhydrophobic-hydrophilic surfaces [78]. Figure 10: Concept of droplet migration mechanism between two different wettability regions [77]. [PERSON] et al. [80] proposed a simple method to create a superhydrophilic micropattern on a superhydrophobic surface by printing a lipid ethanol solution onto the porous superhydrophobic surface. [PERSON] et al. [81] prepared various hydrophilic and hydrophobic patterned surfaces by using a simple two-step plasmachemical method. And it was found that the surface is suitable for water microcondensation. The water droplets formed on the hydrophobic surface will flow to the hydrophilic region, and combined with droplets already present on the hydrophilic region, then continue to grow on the surface, and drop down after reaching the critical point of gravity. [PERSON] et al. [82] mimicked the beetle's back to prepare a hybrid surface, which compared with the single hydrophobic surface, the heat transfer coefficient is increased by 63%. [PERSON] et al. [83] used poly-water/2-propanol solution to deposit a series of hydrophilic spots on the superhydrophobic surface to form a mixed wettability structure. [PERSON] et al. [84] used inkjet printing technology to prepare superhydrophilic micropatterns on superhydrophobic surfaces. Compared with uniform superhydrophobic and superhydrophobic surfaces, the water collection efficiency was improved. It could be concluded from the above research that the hybrid superhydrophobic-hydrophilic surface has excellent dehumidification and heat transfer performance, but it is a microscopic structure, the preparation technology is difficult, and the preparation conditions are demanding. Most of the research still stays in the laboratory stage and cannot be used in large-scale production on the market. Future development needs to reduce the technical difficulty of generating high-performance wettability surfaces, low production costs, and meet the market demand for large-scale production.