# Durability of Superamphiphobic Polyester Fabrics in Simulated Aerodynamic Icing Conditions
## 2 Materials and Methods
### Icing Wind Tunnel Testing
Fabrics were placed mid-span of the airfoil and covered up to the 45-mm chord-wise position. Three identical superamphiphobic fabrics were tested in this configuration, illustrated in Figure 3.  
Polyester fabrics were coated as square sheets with a side length of 100 mm. To fit the flat plate substrate holder, the corners of the sheets were cut as shown in Figure 10a. An aluminum plate (50 mm \(\times\) 50 mm \(\times\) 2 mm) was used as the core support for the fabric. The effective area of the fabric was placed on one face of the aluminum plates and the side flaps were folded over to the back (Figure 10b), where they were held in place using adhesive tape. One superamphiphobic and one superhydrophobic fabric were tested simultaneously side-by-side in the icing wind tunnel as illustrated in Figure 4. Figure 4b is a side-view diagram of the flat-plate configuration showing direction of airflow and the angle of attack (AoA). A total of 14 superhydrophobic and 14 superamphiphobic fabrics were tested in this configuration, each one exposed to a single combination of airspeed, temperature, and angle of attack. The environmental conditions that the fabrics were exposed to are listed in Table 1. For the two used configurations, above freezing and below freezing air temperatures were tested. Airspeeds ranged from 20 to 120 m/s for the airfoil, and from 90 to 115 m/s for the flat plate. The airfoil was tested at a single 0\({}^{\circ}\)-angle of attack. The airfoil shape was such that the fabric's effective incident angle with air ranged from 0\({}^{\circ}\) at the maximum airfoil thickness (S2 and P2 in Figure 3), to 90\({}^{\circ}\) at the stagnation line (LE in Figure 3). The flat plate was placed in either one of two angles of attack: 20\({}^{\circ}\) or 45\({}^{\circ}\). The sample holder was in-fact a rectangular prism rather than a thin plate (leading edge flat face was 15 mm thick) so a minimum angle of attack of 20\({}^{\circ}\) was needed to prevent flow separation on the top surface [26] and to ensure interaction of the fabric with impacting water droplets. A 45\({}^{\circ}\)-angle of attack provided more direct exposure to the droplets and raised the stagnation line above the bottom edge of the fabric. The total duration of exposure was kept constant at 10 consecutive minutes for the flat plate configuration to allow for a higher number of test conditions, whereas on the airfoil configuration, cumulative exposure time ranged until 50 min. Instead of using fixed liquid water content (conventional metric for the concentration of water droplets in a cloud), a fixed water volume flow rate of 10 cm\({}^{3}\)/min was introduced into the air through the air atomizing nozzles. The difference in droplet impact rate between a constant liquid water content and constant feed rate is subtle for the range of conditions tested (liquid water content was between 0.25 and 0.36 g/m\({}^{3}\)). The leading edge portion of the airfoil was subjected to an impact rate of 4 and 10 droplet impacts/s for airspeeds of 50 and 120 m/s respectively. For an exposure of 10 min that accumulated to 2500 and 6200 impacts, respectively. For the total of 50 min of exposure at progressively increasing airspeeds, the number of droplet impacts over the leading edge of the airfoil is estimated at 18,000.