# Review of Hygroscopic Coating on Aluminum Fin Surface of Air Conditioning Heat Exchanger
## 3 Hydrophobic/Hydrophilic Coated Heat Exchangers
### Kinds of Surface Wettability  
As were demonstrated in the previous studies [70], a simple hydrophobic or hydrophilic surface could be obtained by the surface substrate's chemical modification or topographical modification. High droplet nucleation rates have been reported on hydrophilic surfaces, and the hydrophobic surface revealed a better heat transfer performance [61]. And in the air condition heat exchanger, the surface of the condensing fins is required to have both efficient droplet mobility and high nucleation ability to enhance dehumidification capacity. Therefore, the hybrid surface has been introduced, and the different patterned model would be discussed as follows:  
Cactus model: Inspired by cactus, which captures water molecules by the hydrophilic needles and is transported by the hydrophobic spine. [PERSON] et al. [71] forced an array of hydrophilic needles through a robust superhydrophobic polymer film to prepare a superhydrophobic-hydrophilic surface (Figure 7). Due to the difference in temperature and wettability, condensation occurs preferentially on the needle surface. As the droplet grows, the droplet on the needle maintains the Cassie state and does not wet the superhydrophobic surface below. They observed that the water condensation rates of the hybrid surface were twice that of a single superhydrophobic surface and four times that of a flat copper surface. [PERSON] et al. [72] prepared a cactus model surface by combining mechanical perforating and template replica technology; the condensation water droplets grow on the hydrophilic needle and then moving down onto the hydrophobic. Their design opens up a new avenue to adsorb water molecules efficiently and may be potentially useful for indoor dehumidification field. [PERSON] et al. [73] fabricated the hybrid surfaces of hydrophilic-superhydrophobic (HS). And the experimental tests have been carried out, and the results show that when there is a large amount of non-condensable gas, the surface has high condensation heat transfer performance, but it does not show excellent performance for pure steam condensation. In addition, they concluded that relative humidity is one of the main factors affecting the condensation heat transfer coefficient. Grass model: Inspired by grass, which captures water molecules by the hydrophilic leaves and is transported by the hydrophobic stems. [PERSON] et al. [74] laid super hydrophilic tracks (filmwise condensation domain) on a hydrophilic surface (Figure 8). This design facilitates the timely drainage of capillary-driven condensate, reduces the residence time of the condensate droplets on the hydrophilic surface, increases the heat exchange area where the air is in direct contact with the surface, and enhances the heat transfer coefficient of the surface. And the experimental results show that this composite metal surface can increase the heat transfer performance by 30%. Figure 7: Condensation on a hybrid surface [71]. [PERSON] et al. [75] laid the super-hydrophilic tracks of the bio-inspired wettability patterned on the hydrophilic surface. When the droplets generated on the hydrophilic surface grow up, they will be sucked away by the super-hydrophilic tracks, forming a water film. Driven by the capillary force, the condensate can be drained in time on the super-hydrophilic tracks, which effectively enhances the condensation heat transfer performance of the surface. They demonstrated that in a non-condensable vapor environment, the performance of the patterned surface was improved by 19% compared with the normal hydrophilic surface. [PERSON] et al. [76] studied the flow and condensation heat transfer of pure steam on different wettability surfaces, including hydrophobic copper, hydrophilic copper, and two surfaces with a combination of hydrophilic and hydrophobic patterns (Figure 9). They found that the condensation heat transfer coefficient on the surface of the hybrid pattern is an order of magnitude higher than that of a single hydrophilic or hydrophobic surface. [PERSON] et al.