# Durability of Superamphiphobic Polyester Fabrics in Simulated Aerodynamic Icing Conditions
## 3 Results and Discussion
### Airfoil Configuration
Pressure side “P2” (hollow square), pressure side “P1” (filled square), stagnation line “LE” (concentric triangles), suction side “S1” (filled circle), suction side “S2” (hollow circle) positions, equilibrium contact angle averaged over all positions (flat bar), and roll-off angle averaged over all positions (cross). (**a**) After exposure to a warm droplet cloud at speeds from 20 to 120 m/s, for a cumulative duration of 50 min. (**b**) After exposure to a warm droplet cloud at 90 m/s for up to 50 min. (**c**) SEM images of polyester fibers on fabrics exposed to 120 m/s (top) and 50 m/s for 3 icing cycles (bottom). (**d**) After exposure to 3 icing cycles at 50 m/s. Lines are shown as a guide for the eye. the skin friction coefficient along the airfoil peaks near the leading edge and decreases rapidly toward the trailing edge. To isolate the duration of exposure from airspeed, separate tests were done at a constant speed but increasing exposure time (Figure 5b). An airspeed of 95 m/s was used since it was suspected that the same damage seen at 120 m/s could be done at a slightly lower speed, but with longer exposure time. Contact angle measurements exceeded 160\({}^{\circ}\) (165\({}^{\circ}\)\(\pm\) 3\({}^{\circ}\)) for the full 50 min showing no measurable change with increasing exposure time. Roll-off angle measurements slightly increased after the first 10 min of exposure, and then remained constant at 22\({}^{\circ}\)\(\pm\) 5\({}^{\circ}\). This indicates that the full damage occurred within the first 10 min of testing, and did not increase with increasing exposure time. This result suggests that SNF were removed or broken when wall shear stresses (roughly 6 kPa for an airflow at 120 m/s) exceeded their tensile strength. The absence of time-dependent damage suggests that they did not show signs of creep for the durations tested. Cloud exposures were in 10-min intervals, and the SNF had time to resettle between exposure sessions. The main knowledge gained here is that maximum damage at a certain airspeed occurred within the first 10 min of exposure and that the coating was removed only beyond a critical shear stress threshold--independently of exposure time. The state of the superamphiphobic coating following exposure to harsh aerodynamic conditions while on a NACA 0012 profile is shown by SEM images in Figure 5c. The fabric whose wetting properties measurement showed full pinning in Figure 5a is shown in Figure 5c. The image shows two smooth PET fiber surfaces with well-defined boundaries. An image of a fiber on the fabric tested in icing conditions was captured in Figure 5c. It appears to have a porous surface with filamentous features sizing an order of magnitude less than the scale bar of 1 \(\upmu\)m. The SEM image of the fabric after exposure to 120 m/s in Figure 5c is validation that the coating was removed from the top and side faces of the polyester fibers, creating pinning sites during contact angle and roll-off angle measurements. Wetting properties of a superamphiphobic fabric on an airfoil exposed to icing conditions at \(-\)3 \({}^{\circ}\)C and an airspeed of 50 m/s are plotted in Figure 5d. The water contact angle exceeded 160\({}^{\circ}\) for the full 40 min of exposure. The water roll-off angle of the pressure and suction side increased steadily to 40\({}^{\circ}\) after 20 min of exposure after which it remained constant, whereas the leading edge roll-off angle increased steadily to 15\({}^{\circ}\) after which it remained constant. Therefore, ice accretion caused damage to areas subjected to aerodynamic shear, but not on the stagnation line. The ice grains that formed on the stagnation line were densely packed, forming a solid block of ice that protected a small area of the fabric from airflow and direct impact of droplets (pathline illustration in Figure A2g and experiment image in Figure A3). and A few millimeters downstream along the chord-length, ice grains were more dispersed. Boundary layer air could flow between ice grains, pulling them away from their anchor-point on the fabric.