## 3 Results  
### Superhydrophobic Coating Application  
Since paint adheres to rougher surfaces more strongly than to smooth surfaces because of the increased contact area and an interlocking effect, a specific roughness of the test article substrate was required. A previous work [24] established the best roughness at about 3-4 \(\upmu\)m achieved through sand blasting. Figure 4a shows the M28 PZL vehicle highlighted in the red circle of the nacelle test article, whereas Figure 4b,c display the nacelle lip-skin before and after the sand blasting useful for achieving the required Ra = 3.2 \(\upmu\)m \(\pm\) 0.2. Figure 4d shows the nacelle lip-skin after the application of the CIRA SHC [19; 20], having a thickness of 32 \(\upmu\)m \(\pm\) 6 and a roughness Ra = 1.6 \(\upmu\)m \(\pm\) 0.3.  
The test article surface after the sand blasting treatment has a uniform sponge-like morphology (Figure 5a,b) which was preserved after the application of the coating (Figure 5c). This morphology is helpful to guarantee a good adhesion of the coating due to the mechanical interlocking occurring around the interphase substrate-coating. Moreover, the sponge-like morphology preserved after the application of the coating is useful to trap air and contribute to superhydrophobicity [25].  
Figure 4: M28 PZL vehicle (**a**) and nacelle test article before (**b**) and after (**c**) the sand blasting treatment, and after the application of the superhydrophobic coating (**d**).  
Figure 3: Measurements of thickness measurement (**a**) and of the impingement length (**b**); schematic draw of the accreted compact ice Lci and isolated droplets (**c**).  
Figure 6 shows pictures of water droplets on flat samples, representative of the nacelle test article having the same roughness, at 25 \({}^{\circ}\)C and \(-27\)\({}^{\circ}\)C [24]. Measurements of water contact angles performed at 25 \({}^{\circ}\)C give values of 44\({}^{\circ}\) and 164\({}^{\circ}\) on the uncoated and coated surfaces, respectively (Figure 6a,c). These values seem to be preserved at \(-27\)\({}^{\circ}\)C (Figure 6b,d). Wettability measurements have also been performed on the test article before and after the coating application using the set-up in Figure 7a. The test confirmed the measured CA on the uncoated surface; however, due to the low wettability of the applied SHC, it has been impossible to perform the measurements of the wettability on the coated surface since water droplets bounce or roll away from the 3D-coated surface, thereby making the acquisition of the static water contact angle impossible. Therefore, on coated surfaces, the quality of the coating application and its corresponding low wettability were assessed through the slipping and/or the bouncing off of water droplets impacting the coated surface, as shown in Figure 7b,c, where the movement of the water droplet is highlighted by a red arrow.  
Figure 5: Set-up for the optical microscopy acquisitions (**a**); microscope images at 140\(\times\) of the lip-skin nacelle surface before (**b**) and after (**c**) the coating’s application.  
Figure 6: Wettability tests of the coated flat sample representative of the test article performed at 25 \({}^{\circ}\)C (**a**) and \(-27\)\({}^{\circ}\)C (**b**); wettability of reference performed at 25 \({}^{\circ}\)C (**c**) and at \(-27\)\({}^{\circ}\)C (**d**) [24].  
Figure 7: Wettability test carried out on the test article before (**a**) and after (**b**,**c**) the application of the SHC. The red arrow points at the movement of the water droplet.