# Durability of Superamphiphobic Polyester Fabrics in Simulated Aerodynamic Icing Conditions
## 2 Materials and Methods
### Icing Wind Tunnel Testing  
The icing and contamination research facility (iCORE) is a closed-loop wind tunnel equipped with a test section 100 mm wide and 150 mm high. The wind tunnel is fitted with an additional cooling section and spray section, Figure 2. The coolant flowing through the heat exchanger in the cooling section is pressurized by three compressors, one of which with variable power, allowing the air in the wind tunnel to reach temperatures as low as \(-40\)\({}^{\circ}\)C. The spray section, upstream of the test section, includes three horizontally aligned, internal mix, air atomizing nozzles capable of producing water droplet clouds with a median volumetric droplet diameter (MVD) of \(2R=20\)\(\upmu\)m, and a resulting liquid water content (LWC) of up to 2.0 g/m\({}^{3}\) in the test section. The icing wind tunnel is driven by a fan,  
Figure 1: SEM images of polyester fibers before coating, after a single coating process (1-step), and after a double coating process (2-step), each shown at two magnifications. which can force air into the tests section up to a speed of \(U_{\infty}\) = 140 m/s. Assuming laminar flow, a water droplet in the test section would have a Weber number, We \(=\ \varrho U_{\infty}^{2}\frac{R}{\gamma}=2800\), and a dynamic wetting pressure of P \(=\ 0.5\varrho U_{\infty}^{2}=\ 9.8\ \times\ 10^{6}\) Pa. Here, \(\varrho\) is the density of water, \(U_{\infty}\) the impact velocity, \(\gamma\) the interfacial tension, and \(R\) the radius of the droplet. Further details on the icing wind tunnel and its calibration have been presented by [PERSON] et al. (2020). The superamphiphobic fabrics were mounted in the icing wind tunnel test section in two different configurations: on the leading edge of an airfoil (Figure 3), and on the top of an inclined flat plate (Figure 4). The airfoil profile provided a more realistic aerodynamic shape, giving insight into the specific wear conditions that the fabrics would see in-flight. The NACA 0012 shape was chosen because it is a relatively simple shape without camber, making it easy to manufacture and analyze ([PERSON], 2019; [PERSON], 2019; [PERSON], 2019). The flat plate configuration was useful for studying relatively uniform wear conditions and controlling the angle of attack. Figure 4: Configuration of fabric mounted to a flat plate sample holder in the icing wind tunnel test section. (**a**) Side-by-side arrangement of fabrics in the flat plate substrate holder; (**b**) Side view of the configuration in the test section showing the direction of airflow, top and bottom test section walls, and the two tested angles of the sample holder. Figure 3: Diagram of the NACA 0012 support with 180 mm-chord length and labels for positions on the fabrics. LE: leading edge, or stagnation line; S1, S2: suction side; P1, P2: pressure side. Figure 2: icing wind tunnel. A NACA 0012 shape was used for exploring three pressure zones, namely, stagnation, suction, and pressure. The stagnation line, referred to as the leading edge (LE) in Figure 3, gets its name because that is where air hits the object and coms to a halt; it is a theoretical singularity point but in practice is the region of highest pressure. If the airfoil were tiled upward or if the centerline were curved (non-zero camber), then the stagnation line would be slightly below the centerline (also referred to as chord). The consequence would be that air passing over the top side would need to go faster than air passing under the bottom side because it would have to cover a slightly longer distance on the top in the same amount of time. As air velocity increases, static pressure decreases (in an adiabatic system), meaning that the air on the top side has a lower static pressure than on the bottom side, giving it the name, suction side. The airfoil configuration was tested in a 0\({}^{\circ}\)-angle of attack, so the absence of any camber meant the pressure and suction sides were not effective as such; they were just for designation and labeling.