# Parameter Study for the Ice Adhesion Centrifuge Test
## 4 Discussion  
In this study, different test parameter set-ups for the ice adhesion centrifuge test were assessed in combination with durable reference materials, representing different surface properties with regard to the roughness and wettability. With regard to the effects  
Figure 9: Surface free energy (SFE) and ice adhesion strength data for different ice formation types. of ice types, this study showed that the ice adhesion strength is lower for impact ice compared to static ice. This was also reported by [PERSON] and [PERSON] in tensile ice adhesion tests [24]. One explanation for this finding is that ice accreted at a high velocity (in our study, 95 m/s) contains more air inclusions due to quasi-instant freezing of the droplets by impacting the surface or ice that has already been built at the test surface. The ice appears opaque, and the trapped air may reduce the contact area with the surface. Reducing the impact energy of the water (in our study, 60 and 40 m/s, static) leads to ice with less air inclusions and more bonding options to the surface, increasing the ice adhesion. Moreover, from a material perspective (at least for polymeric materials), we can postulate that a slower freezing process leads to increased adhesion. At a sub-molecular level, polar groups in the polymer matrix may become oriented to the interface and promote interactions with the water molecules. [PERSON] and [PERSON] identified further possible explanations for the finding of higher adhesion strength for static (mold) ice, including stress differences frozen into the ice, larger grains, and, for rough surfaces, weaker stress concentrations at the interface due to a lack of penetration into surface structures [1]. Due to the complexity of ice formation processes, there are certainly overlapping parameters that cancel each other out or intensify the observed effects. This also applies to the effects of drag forces on the different ice shapes (impact vs. static ice) during the centrifuge test. This paper does not further address these aspects but includes a study that was conducted with regard to the effects of ice types on ice adhesion measurements using a centrifuge test. [PERSON] et al. identified for different ice types the following ranking (increasing ice adhesion): precipitation (hard rim) ice 5.6 m/s > impact ice 15 m/s > static ice. They concluded that the ice adhesion strength inversely correlated with the apparent density of the ice type [8]. The same trend was also reported in a related study, but it was less significant [25]. These contrary results emphasize the need to further improve the understanding of ice formation processes to improve test systems and identify crucial parameters for technical applications. Considering the topic \"surface properties\" in the overall discussion, the complexity further increases. As summarized in Table 1, different studies have already addressed correlation aspects. The diversity of different correlation findings is now understandable, as the ice type used has a significant impact on the test results and the material ranking with regard to their ability to reduce the ice adhesion. However, roughness is a crucial surface parameter and can increase the ice adhesion strength significantly, as it was observed in this study for the primer coating material (R\({}_{\text{a}}\) = 1.5 \(\upmu\)m), only delivering cohesive ice failures or no ice shedding at all. This finding is in agreement with other centrifuge test-related studies for static ice [17], impact ice at 25 m/s [15], and rotor tests [7; 14]. Nevertheless, the complexity of the surface roughness is higher than the expressed parameter R\({}_{\text{a}}\) and R\({}_{\text{z}}\), especially for superhydrophobic surfaces. This is to be addressed in future studies. For the expression of the wettability, this study used the surface free energy (SFE) as the determining parameter. The higher this value, the better the wettability of a liquid (water) at the solid surface due to available bonding options.