# Durability of Superamphiphobic Polyester Fabrics in Simulated Aerodynamic Icing Conditions
## 3 Results and Discussion
### Airfoil Configuration  
The wetting properties of superamphiphobic fabrics measured following exposure to a warm cloud of droplets at airspeeds up to 120 m/s on a NACA 0012 profile are plotted in Figure 5a. The water contact angle remained constant within an experimental accuracy range of 20-100 m/s. At the highest airspeed, 120 m/s, the contact angle on the suction side and leading edge decreased sharply to 130\({}^{\circ}\) whereas the pressure side continued on the flat trend to 160\({}^{\circ}\). Within experimental accuracy, the water roll-off angle was almost constant until 100 m/s, after which complete pinning (represented by 90\({}^{\circ}\)) was observed. The roll-off angle slightly increased near 100 m/s. The sudden decrease in contact angle and increase in roll-of angle indicate damage to the coating on the fabric, mainly on the suction side and the stagnation line of the airfoil. Further evidence of this damage is given by the SEM image of Figure 5c. The coating was finally damaged after exposure to 120 m/s but for a cumulative duration of 50 min, since a single piece of fabric was used for testing the progressive airspeed exposure. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Configuration**} & **Total Air** & **Velocity** & **Angle of** & **Duration of** & **Total Duration of** & **Volume Flow** \\  & **Temperature** (\({}^{\circ}\)C) & **(m/s)** & **Attack** (\({}^{\circ}\)) & **Cycle (min)** & **Exposure (min)** & **Rate Water** \\ \hline \multirow{3}{*}{Airfoil} & +20 & 20-120 & \multirow{3}{*}{0 \(\pm\) 2} & \multirow{3}{*}{10} & 50 & \multirow{3}{*}{10} \\  & +20 & 95 & & & 40 & \\ \cline{1-1}  & \(-\)3 & 50 & & & 40 & \\ \cline{1-1} \cline{3-6} \multirow{2}{*}{Flat Plate} & +12.5 & \multirow{2}{*}{90-115} & \(20\pm 1\) & \multirow{2}{*}{3-10} & 10 & 10 \\ \cline{1-1}  & \(-\)5 & & & 45 \(\pm\) 1 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: List of conditions used for icing wind tunnel durability testing. The angle of attack of the airfoil was meant to be at 0\({}^{\circ}\) (Figure 11a), however, contact angle measurements presented in Figure 5a suggest that tests were done at a non-zero angle of attack. The pressure-side boundary layer and suction-side boundary layer create different wall friction stresses. Direct numerical flow simulations (DNSs) of a NACA 0012 profile at an angle of attack of 5\({}^{\circ}\) and Reynolds number, \(Re=5\times 10^{4}\), showed that the coefficient of pressure along the airfoil suction side is higher in magnitude near the leading edge than that on the pressure side [28] (Reynolds number (Re) is the ratio between inertial forces and viscous forces of a fluid in motion). Wall friction shear forces are dependent on the flow velocity, and since the flow was faster over the suction side, the wall shear stresses were higher. The more intense of the stresses was evidently on the suction side, where flow separation is likely to occur (Figure 11c,e). We expect that the pressure difference between the bottom and the top of the airfoil was due to a small angle of attack tolerance during installation of the profile in the test section, and due to manufacturing tolerances, which led to a slightly shorter bottom side. These small geometric deviations were amplified by the high Reynolds number at 120 m/s (\(Re=1.6\times 10^{6}\)), which forced the airfoil to pitch upwards slightly. The only resistance to this pitching was contact friction with the test section walls, governed by the hand-tightness of the screws used to fasten the airfoil along a single axis. A small angle of attack at such high Reynolds numbers can lead to wall shear stress differences over the top and bottom surfaces (8 Pa difference, calculations shown in Appendix B), and flow instabilities [24], especially in a straight, fixed-size test section. According to [28],  
Figure 5: Wetting properties of the superamphiphobic fabric exposed to aerodynamic wear conditions.