# Durability of Superamphiphobic Polyester Fabrics in Simulated Aerodynamic Icing Conditions
## 3 Results and Discussion
### Flat Plate Configuration
#### 3.2.2 Superamphiphobic Fabrics
The more ice accreted, the further the reattachment point would be (see Figure 8). The fabrics in icing conditions at 20\({}^{\circ}\)-angle of attack and in 90 m/s airflow were subjected to one continuous icing cycle for 10 min, and were therefore subjected to the most intense back-flow for the longest time. For fabrics in the same conditions at 100 and 110 m/s, the ice accretion was removed after 3 min of icing, and they went through 3 icing cycles. For 115 m/s, the ice accretion was removed after 5 min of icing and 2 cycles were completed. The short cycle duration, despite higher speeds, meant that the effect of flow separation was smaller than at 90 m/s, and resulted in less severe damage to the SNF. The mechanism of SNF damage is debatable, but the authors believe it is due to two major contributors, as illustrated in Figure 9. The first is that the aerodynamic wall shear stress caused a detachment of isolated feathery ice grains by cantilever loading, which pulled on SNF (Figure 9a). The second was the impact of these detached ice grains on the fabric surface caused by aerodynamic backflow (Figure 9b). Figure 8: Sketch of air pathlines over a rectangular body after 3 and 10 min of icing. Bluish area: Sketch of accreted ice. Figure 7: SEM images of superamphiphobic fabrics tested in a flat plate configuration at an angle of attack of 20\({}^{\circ}\) and in icing conditions. Each row consists of increasing magnification of a single fabric tested at the airspeed indicated on the left. The inset images are 10 \(\upmu\)L oil droplets on the respective fabric sample following durability testing and tilted at the angle specified beside the image. Isolated feathery ice grains generally grew along the airflow freestream velocity directions. The average momentum of supercooled droplets is usually enough to penetrate the aerodynamic boundary layer [30]. A laminar aerodynamic boundary layer flows parallel to a wall, and the direction of a turbulent boundary layer, which is likely the case here, flows more chaotically but on average also in a direction parallel to the wall. Thus, the boundary layer airflow causes a drag force in the order of 25-150 \(\upmu\)N, on isolated feathery ice grains with a component that is perpendicular to their direction of growth; assuming a small cylindrical feature at a small angle from parallel to the airflow (details of calculations included in Appendix B). With this force system in mind, the ice grains could be sheared or bent off the surface of the fabrics as observed during the experiments. The drag forces calculated would result in a pulling stress at the base of the ice grain between 3 and 18 kPa. The separation of isolated feathery ice grains from the fabric could have caused more harm than good. The advantageous part is that an icephobic surface typically has low adhesion strength to ice, and if ice grains fly off, then large ice accretions are less likely to grow. The disadvantage is that the ice grains cause damage downstream, or remain suspended in recirculated flow. Ice grains act as solid particles and could cause erosive damage to exposed portions of SNF on superficial polyester fibers when they were recirculated in the separated backflow above the fabric. SNF were clearly shortened after durability tests (Figure 7). However, a base layer of the coating remained on the fibers of the fabric. This base layer of silicone provided a hydrophobic surface chemistry on top of the microstructure given by the polyester fiber weave. The surface chemistry and microstructure were not changed by the cloud and ice exposure, resulting in only very subtle changes in contact angle measurements. The evidence suggests that low roll-off angles were mainly attributable to the nanoscopic form factor and aspect ratio of the network of long SNF.