# Fabrication of an Anti-Icing Aluminum Alloy Surface by Combining Wet Etching and Laser Machining
## 2 Experiment
### Experiments Processing
#### 2.2.3. Icing Delay, Ice Adhesion, and Dynamic Water Repellency Experiments  
* Icing delay experiment:  
Water droplets impact aircraft parts at various angles and in a wide range during flight, therefore it is necessary to consider the freezing time of droplets adhering to the surface. It is important to mention that prolonged icing time on the surface is beneficial for the aircraft to shake off liquid droplets during flight, and can be vaporized before the supercooled water droplets freeze through the auxiliary deicing system. This greatly relieves the pressure of the aircraft de-icing system. The icing delay experiment was carried out for this reason. The icing delay experiment was carried out on the smooth surface (SS), nanostructure surface (NS), microstructure surface (MS), and hierarchical structure (HS), the surfaces of which were all fluorinated. The icing delay test device is shown in Figure 4a. The anti-icing performance was evaluated by comparing the droplet freezing time on different surfaces at \(-10\)\({}^{\circ}\)C. To carry out these experiments, three different sites were selected on material surfaces, and five values of icing delay time were recorded from each area, with the mean value taken. During the test, the relative humidity was 50\(-\)60%, and the dew point was about 13.9-16.7\({}^{\circ}\).
* Ice adhesion experiment:  
By delaying the freezing time of the droplets on the aircraft's surface, more time can be taken for dealing with droplets which are at risk of freezing. The strength of droplet adhesion on the surface determines the difficulty of dealing with these hidden dangers. Therefore, the adhesion strength of the bornel surface is very important for anti-icing and de-icing. The ice adhesion experiments were set to test the adhesion ability of different sample surfaces. The samples used in the experiment include SS, MS, NS, and HS. Initially, ice cubes of 10 mm \(\times\) 10 mm \(\times\) 10 mm were prepared and kept together with the sample in a refrigerator at a certain temperature for 24 h to obtain cubes of ice that adhered to the surface of the samples. Later on, the push-pull digital dynamometer was used to push the ice onto the surface of the sample. The computer was used to monitor the absolute value  
Figure 3: Surface topography of each structure. (**a**) The SEM of the microstructure; (**b**) the SEM of the nanostructure; (**c,d**) the SEM of hierarchical structure; (**e**) 3D morphology of the HS surface; (**f**) height profile of the HS surface.  
in real-time, and plotted to record the peak values, and took the average value. The icing adhesion test device is shown in Figure 4b.  
Dynamic water repellency experiment:  
The ice formed when supercooled water droplets hit the surface of an aircraft and freeze is called frost ice, commonly forming at low temperatures (0 \({}^{\circ}\)C to \(-\)40 \({}^{\circ}\)C). Due to the small size of the impacted droplet, the freezing time is short, and the near-spherical droplets do not form a film of water on the aircraft's surface, but freeze directly in that shape. The type of frost ice is relatively regular, from the cross-section of a wedge shape, thus it is also called wedge ice. Although this type of icing has a small range and fragile texture, it still cannot be ignored, since it has a large impact on aircraft. The high-speed camera of the contact angle measuring instrument was used to record the dynamic contact process of a droplet hitting the sample surface, as well as observe droplets bouncing on different surfaces. The samples used in the experiment include SS, MS, NS, and HS. The droplet volume was 5 \(\mu\)L, the release height was 5 mm, the release speed was 5 \(\mu\)L/s, and the frame number of the high-speed camera was 350 FPS.