# Durability of Superamphiphobic Polyester Fabrics in Simulated Aerodynamic Icing Conditions
## 3 Results and Discussion
### Flat Plate Configuration
#### 3.2.2 Superamphiphobic Fabrics  
The oil wetting properties of superamphiphobic fabrics exposed to a water droplet cloud at airspeeds of 90-115 m/s, at freezing and non-freezing temperatures, and on a flat plate at two constant angles of attack are plotted in Figure 6c,d. Figure 6c represents fabrics tested in a warm cloud at 12.5 \({}^{\circ}\)C. It shows that the oil contact angle remained between 140\({}^{\circ}\) and 160\({}^{\circ}\) and an oil roll-off angle below 50\({}^{\circ}\). The higher wear at an angle of attack of 45\({}^{\circ}\) can be rationalized by a higher direct exposure to droplets and air than at 20\({}^{\circ}\). It was expected that the superhydrophobic fabrics would show the same trend, however that was not the case. Oil has a lower surface tension than water and the differences between the two wettabilities could lie in the sensitivity of the testing liquid to different degrees of damage. In general, the optical nature of the fabric (macroscopically visible roughness and lack of reflectivity), made it difficult to obtain an accurate mathematical fit with the oil droplets, resulting in a higher standard deviation than on the superhydrophobic fabrics. Figure 6d represents fabrics tested in a supercooled droplet cloud at \(-5\) \({}^{\circ}\)C. The oil contact angle was constant at 140\({}^{\circ}\) for both angles of attack across all speeds presented. Similar to superhydrophobic fabrics, the superamphiphobic fabrics only showed pinning after exposure to the lowest airspeed and angle of attack, and not at higher airspeeds or angle of attack. At the shallow angle of 20\({}^{\circ}\), most supercooled droplets impacted the leading edge of the substrate holder rather than the fabric. That means that ice grew faster on the leading edge of the sample holder than on the fabric. An ice accretion at the corner of the sample holder has a two-fold effect: protection of the upper surface of the sample holder (and therefore the fabric) from impacting supercooled water droplets, and separation of the aerodynamic boundary layer from the top surface of the sample holder. The contribution to wear of the fabric from the first effect is fairly straightforward. At an angle of attack of 45\({}^{\circ}\), impacting supercooled droplets quickly formed a relatively flat ice layer covering the entire surface of the fabric (illustrated in Figure A2h). The dense ice layer protected the fabric from further impacts. The result of this type of icing encounter was negligible on the wetting properties of the fabrics. At an angle of attack of 20\({}^{\circ}\), the duration of direct droplet impact was shortened by the sample holder-born ice accretion. The micro- and nanostructure of the superamphiphobic fabrics tested at an angle of attack of 20\({}^{\circ}\) in icing conditions are shown by SEM images in Figure 7. Four rows of images corresponded to the four airspeeds tested in that configuration. The three columns were arranged in order of increasing magnification. The inset in each row shows the respective oil droplet at the highest tilt angle before shedding during roll-off angle measurements. The 90 m/s images show a PET fiber with filamentous nanostructures on its side, which decreased in size and length leading to a smooth area on the top of the fiber. The smooth area did not cover the entire length of the fiber. The inset for that speed shows an oil droplet with high contact angle hysteresis. The higher speeds show a similar gradient in length but they did not reduce to a completely smooth surface, only to a slightly less structured one. The insets for these speeds show oil droplets with low contact angle hysteresis. In dry airflow, at a 20\({}^{\circ}\)-angle of attack, the upper surface of a rectangular block is near to flow separation [26]. With ice forming on the leading corner, it is fairly certain that the flow was separated over at least a portion of the fabric. Separated flow can reattach to the wall after a certain chord-length.