## 4 Discussion
Figure 21: Summary of the IWT test campaign results found on uncoated (**a–e**) and coated (**f–j**) surfaces of the test article. Increase in exposure time at low temperature has the effect to increase the accumulated ice, as expected, and no differences can be observed in the shape and appearance of ice accreted on coated and uncoated surfaces (Figure 21e_j_). The major advantage in applying a SHC can be highlighted at a higher LWC and higher velocity, where the nucleation is more difficult and slower for SHC with respect to the uncoated surface [31,32]. At a higher LWC, the isolated ice droplets disappear and, in their place, several ice rivulets appear alongside the internal part of the nacelle lip (Figure 21b,g). The presence of rivulets on both uncoated and coated surfaces is due to the increased amount of impacted liquid water, which would only freeze partially, while the remaining water would coalesce into rivulets and run back over the downstream surface [27]. Figure 21b,g highlight a prominent difference in the length of rivulets frozen on coated and uncoated surfaces ascribed once again to the low wettability of the applied SHC, which allows the liquid water to slide off before freezing, thereby reducing the length of the frozen rivulets. At the impingement line, the impacting water quickly freezes, accreting as spherical-like ice regardless of the wettability of the surface. This is because at the impingement line, no helpful forces are present to take advantage from the low wettability of the SHC and allow water to slide off. On the other hand, at a higher velocity, the high-speed water droplets colliding with the test article surface do not get enough time to freeze. Incoming airflow pushes these droplets further away from the stagnation line which leads, on the uncoated surface, to the accumulation of these droplets along upper and lower sides of the stagnation line as thin and narrow runback rivulets. In contrast to the uncoated surface, the high-speed water droplets for the coated surface slip away rapidly without freezing at all downstream the stagnation line (Figure 21d,i), and so reducing de facto the length of the accreted compact ice Lci, leaving the area upper the stagnation line free of ice. A direct comparison between the present and the previous [11] IWT test campaign can be made only between the test performed at higher velocity, namely at \(-5\)\({}^{\circ}\)C, v = 95 m/s, LWC = 0.3 g/m\({}^{3}\), and exposure time = 140 s, and the previous one carried out at \(-6\)\({}^{\circ}\)C, v = 95 m/s, LWC = 0.3 g/m\({}^{3}\), and exposure time = 137 s (test 3 in [11]). Ascribed to the different geometrical configuration of the two test articles, many differences can be observed for the ice accumulated on both the uncoated and coated surfaces of the test article. In fact, instead of thin and narrow runback rivulets, in the previous work, a typical dry time ice accretion can be observed on the uncoated surface, with ice droplets which become less dense and isolated on the coated one. On the contrary, in the present work, the isolated frozen water droplets can be seen only at the inner left and right sides of the test article (Figure 16h). Figure 22 displays the percentage change of the ice thickness and the impingement length calculated for each test conditions as ratios between the mean values of the differences between measurements taken on coated and uncoated configurations and the mean values of those acquired on the uncoated configuration, showing the effect of the main investigated parameters (LWC, T, exposure time and airspeed). Results highlight a reduction in the ice thickness accreted on the coated surfaces with respect to the uncoated one across the entire spectrum of test conditions investigated. The maximum ice thickness reduction, i.e., \(-49\%\), which was recorded during the tests at \(-12\)\({}^{\circ}\)C, matched to an increase in the impingement length of 0.5%.