# Fabrication of an Anti-Icing Aluminum Alloy Surface by Combining Wet Etching and Laser Machining
## 3 Results and Discussion
### Dynamic Water Repellency Experiment  
Figure 7 shows the process of a droplet impacting the surface of four different samples. After impact, the droplet moved laterally until it was fully unfolded, at which point the radius was at its maximum. Then, the droplets gradually contracted and elongated vertically, and finally bounced and separated. The process can be summarized as impact-contact-expansion-contraction-bounce. The two parts from impact to expansion were determined by the volume and release velocity of the droplet, while the expansion part was mainly determined by the hydrophobic properties of the sample. Figure 7a,b show that both HS and NS can bounce after impact, indicating that the contact time between the droplets and the sample was short. The droplet touched the sample surface at 3 ms, bounced at 12 ms, and the dynamic contact time was less than 9 ms. However, the droplets did not bounce off the surface of MS and SS, as shown in Figure 7c,d. Starting from the 9 ms, MS repeated two cycles of spread and contraction, but still did not bounce and instead adhered to the sample's surface. The droplet directly adhered to the SS surface after colliding with it. To understand the relationship between the surface wettability and the solid-liquid contact time of droplet impact, the contact angles of four samples were measured. The contact angle hysteresis (the difference between forward contact angle and receding contact angle) of HS and NS samples were 5\({}^{\circ}\) and 7\({}^{\circ}\), respectively, showing good dynamic hydrophobicity. The contact angle hysteresis of MS and SS samples were 48\({}^{\circ}\) and 55\({}^{\circ}\), respectively. The MS sample had good static hydrophobicity (contact angle 142\({}^{\circ}\)), but poor dynamic hydrophobicity. Figure 8 shows the static contact angles, advanced contact angle, and receding contact angle of all samples. Figure 7: Schematic diagram of droplet bouncing process on the sample surface (**a**) droplets on the surface of HS; (**b**) droplets on the surface of NS; (**c**) droplets on the surface of MS; (**d**) droplets on the surface of SS. The above phenomenon indicates that the droplet that can bounce on the sample's surface is mainly determined by the size of the dynamic contact angle and the hysteresis of the contact angle. In contrast, the static contact angle enables the droplet to go through multiple cycles of spreading and contracting, and cannot directly affect the contact time between the droplet and the sample. Figure 9 shows the relationship between droplet contact time and sample surface wettability. HS and NS samples had the shortest solid-liquid contact time. In addition, the static hydrophobicity (HS had a static contact angle of 151\({}^{\circ}\), and NS 145\({}^{\circ}\)) and dynamic hydrophobicity (hysteresis of contact angle HS: 5\({}^{\circ}\), NS: 7\({}^{\circ}\)) were the best. The liquid droplets on samples MS and SS had an order of magnitude of solid-liquid contact time, and both adhered to the surface without bouncing. Meanwhile, MS had better static hydrophobicity and poor dynamic hydrophobicity, while SS had poor static and dynamic hydrophobicity. As the static contact angle decreased, the contact time of the droplet impact sample increased. Therefore, the contact  
Figure 8: Measurement results of contact angle of four samples. Figure 9: Relationship between contact angle and impact contact time of samples. time of the droplet impact on the sample surface was not positively correlated with the static contact angle, but its trend was consistent with the hysteresis of contact angle. It indicated that the contact time of droplet impinging on the sample surface was mainly determined by dynamic wettability, such as the hysteresis of contact angle, but not by the static contact angle. As the droplets hit the surface of the HS sample at a certain speed, the droplets could not exclude the air captured inside the MS and retain the Cassie wetting state.