## 3 Results and Discussion  
### 3.1. Wettability Test Results  
Figure 2 reflects the changes in the contact angle of water droplets on the surface of the samples etched by the corrosive solution for 5, 10, 15, and 20 h after being placed in the indoor atmosphere for different times. The sample that has been corroded for 5 h is kept in a room for 15 days, and the surface is still hydrophilic. As soon as the water drop contacts the surface of the sample, it immediately penetrates into the microstructure with  
Figure 1: The cylindrical array structure. (**a**) Model; (**b**) printed entity; (**c**) SEM image magnified 100 times; (**d**) SEM image magnified 500 times. the contact angle of 0\({}^{\circ}\). For the samples corroded for 10 h, after 9 days in the room, the contact angle has increased slightly. By the 15 th day, the contact angle has increased to 15.3\({}^{\circ}\), and the surface is still hydrophilic, unable to achieve the transition from hydrophilic to superhydrophobic state. For the samples corroded for 15 h, the contact angle of the water droplets increased greatly on the 3 rd day indoors, reaching 122.4\({}^{\circ}\), and on the 7 th day, the contact angle of the water droplets reached 152.8\({}^{\circ}\), realizing the transition from hydrophilic surface to superhydrophobic surface. For the samples corroded for 20 h, the contact angle of water droplets on the surface increased to 90.2\({}^{\circ}\) after being placed indoors for 5 days, realizing the transition from hydrophilic to hydrophobic. As the placement time continued to extend, by the 15 th day, the surface contact angle increased to 144.2\({}^{\circ}\), and the hydrophobic effect is obviously improved. This phenomenon of extreme change in the wettability of the sample surface after being placed in the air for a period of time is not an isolated case after consulting a large number of documents. [PERSON] et al. have used laser irradiation to obtain micro-nano composite structures on metal surfaces [28; 29; 30]. These surfaces are initially hydrophilic, but after being placed in the air for a period of time, they become a superhydrophobic surface. The explanation for this phenomenon is generally believed that this time-dependent wetting state transition is mainly due to the adsorption of organic matter in the air on the surface of the microstructure, thereby reducing the surface energy. In addition, a suitable micro-nano layered structure is also a necessary condition for this transformation. For the samples prepared in this experiment, we used the field emission scanning electron microscope and X-ray photoelectron spectrometer to analyze the surface morphology and surface chemical composition of the microstructure to verify whether the cause of this experimental phenomenon is consistent with the mainstream interpretation. Figure 3 shows the surface microstructure morphology of the 3D-printed sample etched by the etching solution for different times. Figure 3a-d correspond to the surface morphology of the sample after etching for 5 h, 10 h, 15 h, and 20 h, respectively. For the sample that has been etched for 5 h, in Figure 3a2, it can be seen that there are still some papillary structures on the sidewall of the cylinder, which have not been completely corroded by the etching solution, and as shown in Figure 3a3,a4, the surface morphology is further enlarged. It can be seen that the top surface of the cylinder is relatively smooth, and the nanostructures are not prominent. For the sample etched for 10 h, in Figure 3b2, there is no papillary structure on the side wall of the cylinder. Enlarging the image further, as shown in Figure 3b3,b4, there are faint nano-sized creases on the top of the cylinder. For the sample corroded for 15 h, from Figure 3c3, we can clearly see many _bumpy phenomena_. Further magnifying, as shown in Figure 3c4, it can be seen that these phenomena are  
Figure 2: The change curve of the contact angle of 3D-printed + corroded samples with time. actually formed by nano-sized layered structures.