# Durability of Superamphiphobic Polyester Fabrics in Simulated Aerodynamic Icing Conditions
## 1 Introduction
To increase the energy barrier against wetting of the surface by impacting droplets, a nanostructured surface is favorable. The so termed \"Cassie-to-Wenzel transition\" is prevented or delayed [13]. Additionally, droplets are less strongly pinned to nanostructures than to microstructures, thus greatly increasing their mobility on a surface [14]. However, microstructures show superior mechanical stability. Rigid nanostructures can be easily eroded. Typically, nanostructures have poor mechanical durability due to their small cross-sections, which induce higher stress for a given force. Surface chemistry modifications (e.g., hydrophobization or fluorination) conceptually do not fail mechanically, but rather from exposure to corrosive chemicals, extreme temperatures, or certain wavelengths of radiation such as UV [15; 16]. Coatings composed of silicone nanofilaments (SNFs) have shown superhydrophobicity [17], and durable superamphiphobicity [18]. They are chemically inert so they do not decompose in harsh chemical environments, nor do they degrade from exposure to UV [19]. Conceptually, they show higher toughness (can be deformed more before breaking) than hard nanostructures due to their flexibility. However, on flat glass plates, low mechanical stability of SNF coatings has been reported [20]. Enhanced durability has been achieved using fabric substrates instead of flat glass [21]. Fibers and weaves provide a microstructure to the SNF, which protects the sides not facing the shear stress from abrasion, conserving superamphiphobicity. The sponginess of fabrics effectively reduces contact pressure with SNF, reducing the likelihood of them being abraded. The durability of superamphiphobic fabrics has been evaluated by prolonged chemical exposure, and by exposure to an urban external environment on the outside of a car. The car was driven at speeds up to 120 km/h (33 m/s) and traveled a distance of more than 5000 km. The most severely damaged fabric suffered a water roll-off-angle increase of 20\({}^{\circ}\) (from 5\({}^{\circ}\) to 25\({}^{\circ}\)) and hexadecane roll-off-angle increase of 40\({}^{\circ}\) (from 10\({}^{\circ}\) to 50\({}^{\circ}\)). Despite its degraded performance, SNF remained attached to the polyester fibers after the 257 days of exposure [18]. The goal of the study was to determine whether superamphiphobic coatings based on SNF are mechanically durable enough for typical flight conditions, including atmospheric icing conditions, experienced by a low-speed aircraft. We investigate whether the bond between SNF coatings and a polyester fabric substrate can withstand aerodynamic shear forces for airspeeds up to 120 m/s (430 km/h). Aerodynamic friction can be considerable, especially at high Reynolds numbers (\(5.8\times 10^{5}\) during cruise conditions of one exemplary fabric-covered wing aircraft [3]). On textured surfaces, such as fabrics, boundary layers become turbulent rather early on. The wear due to aerodynamic friction depends also on parameters such as an angle of attack and air temperature. In an icing wind tunnel, we introduce a cloud of micrometric water droplets to simulate flying into a cloud. This impacting spray is analogous to fog and to sea spray but with non-salinized water. Above freezing temperatures, the impact of droplets can resemble liquid-particle erosion. At sub-zero temperatures, these water droplets are supercooled and freeze on impact, simulating in-flight impact icing. The median volumetric diameter of the particles was 20 \(\upmu\)m, smaller than the mean spacing between fiber bundles. That means that droplets can penetrate into the fabric, affecting non-superficial polyester fibers. We demonstrate that the coated fabrics can withstand impact of supercooled water droplets up to 120 m/s for several minutes.