## 4 Discussion  
Pictures of ice accreted on the uncoated and coated surfaces acquired after tests can be meaningful and useful to understand the different icing dynamics. A summary of the IWT test campaign results is shown in Figure 21.  
Figure 19: Images of the coated surface of the test article after IWT test campaign; left side of the test article (**a**); middle side of test article (**b**,**c**). Figure 20: Wettability test performed on the test article after the IWT test campaign (**a**–**d**). It appears that one difference raised during the IWT tests between coated and uncoated surfaces is the presence of isolated ice droplets observed especially on coated surfaces and in tests 1-2 (Figures 9, 12 and 21a,f,c,h). The presence of isolated frozen water droplets can be ascribed to the low wettability and consequently the low rolling angles [30] of coated surfaces, so the water droplets impacting the coated surface quickly bounce on it and/or slide off. Thus, depending on the test conditions, they take a longer path before freezing or not freeze at all. From a microscopic perspective, this evidence could be explained with the classical nucleation theory [31; 32], according to which the larger the contact angle is, the larger the free energy barrier \(\Delta\)G for the formation of ice nucleus and the smaller the nucleation rate J, so the nucleation becomes more difficult and slower. The small contact area of the applied SHC and the dynamic behavior of water droplets reduce the probability of nucleation, thus reducing the probability of ice on the surface. As a matter of fact, the isolated ice droplets observed on the coated surface are located at a bigger distance from the impingement area with respect to the uncoated one, thereby increasing de facto the transition area (see for instance Figure 9 and scheme in Figure 3c). Another remarkable finding is that the isolated ice droplets accreted on the coated surface have a spherical shape and larger dimensions than those frozen on the uncoated surface. The shape of the isolated ice droplets on the coated surface can be ascribed to the low wettability of SHC at low temperature (Figure 6), so the surface also retains the Cassie-Baxter state at low temperature, whereas the larger dimensions could be explained by imagining that the fewer water droplets frozen on the coated surface become ice nucleation points, causing the other incoming droplets to freeze on them, thus increasing their dimensions. The high-humidity environment of test conditions [33] and some surface imperfections could be the reason why there is a transition from the Cassie-Baxter to the Wenzel state with penetration and condensation into the surface cavities [33]. Therefore, these microdroplets nucleate and grow, becoming anchoring points for further incoming water droplets. The authors observed the same behavior in other previous IWT tests [11] performed on two-wing profiles NACA0015 tested at temperatures ranging between \(-3\,^{\circ}\)C and \(-23\,^{\circ}\)C, speed 50 and 95 m/s, and LWC 0.3 and 0.6 g/m\({}^{3}\). Here, the isolated and spherical water droplets were observed in all tests performed at temperatures higher than \(-12\,^{\circ}\)C.  
At low temperature, sparse ice droplets appear on both coated and uncoated surfaces, displaying a typical dry rime ice accretion [27]. Once again, the presence of isolated ice droplets which reduce the density of the transition area can be observed especially on the coated surface, but compared to the baseline tests at \(-5\,^{\circ}\)C, at \(-12\,^{\circ}\)C, this phenomenon is largely reduced and the larger isolated ice droplets are relatively absent (Figure 21a,f,c,h). This is most likely because the lower temperature allows the water droplets to freeze immediately upon impact with the surface, so their sliding off is hugely reduced. This evidence agrees with studies from [PERSON] et al. [34], according to which the frosting time reduces as the temperature reduces.