# Durability of Superamphiphobic Polyester Fabrics in Simulated Aerodynamic Icing Conditions
## 1 Introduction  
Fabric aircraft skins have been historically important in aviation. A fabric taught over rigid wing spars provides a smooth aerodynamic surface with minimal weight, ideal for small and ultralight aircraft. Due to its low cost and light weight, this concept is still being used to this day on some small aircraft. Although more robust light aircraft are built using carbon fiber composites, many recent home-built or experimental aircraft make use of fabric-covered wings [1]. These small aircraft typically fly at airspeeds not exceeding 80 m/s or 300 km/h [2; 3]. At comparatively low speeds and altitudes, contamination of aircraft surfaces is at best an inconvenience and at worst a serious safety concern. A common in-flight external surface contamination is due to insect impact. Another more severe but less frequent source of damage comes from atmosphericicing experienced, e.g., during flight. It is also known as impact icing. This term refers to the impact of supercooled water droplets, which freeze when they collide with the surface of an aircraft. Aircraft icing does not occur during a typical cruise phase of flight, but is common enough after take-off and before landing to be of great concern for pilots and flight planners. If ignored, ice accretion on aerodynamic surfaces can lead to a loss of aircraft control. Fabric covered aircraft typically do not have the energy resources for ice contamination prevention typically used on metal-skin airframes with larger on-board power plants. For example, for the prevention of icing, most metal-skin airframes use some form of heating on critical surfaces to evaporate impacting supercooled droplets. Others use a pneumatic boot, which once inflated, stresses accreted ice to the point of breaking, relieving the surface of ice contamination [4]. Insect contamination and frost are typically washed off when the aircraft is on-ground. The use of excessive pressure during washing can be problematic for fabric-skin aircraft due to a risk of piercing the skin. An easy-to-clean, superamphiphobic fabric would reduce the pressure needed to remove contamination from a fabric-skin aircraft. It could potentially reduce the risk posed by icing contamination by reducing the adhesion strength to ice, or slowing the accretion of ice entirely. On the ground, at high altitudes such as in mountainous regions, and at sea [5], icing similar to that seen in-flight can degrade the performance of signal transmission equipment [6], and greatly increase safety hazards. Ice accumulated on such equipment is often removed by hand, meaning physically removing the ice using a hand-tool. One solution that has been proposed to ease the removal of ice is to loosely cover the equipment with some fabric before ice begins to form [7]. The flexible nature of the fabric means it is easy to bend and therefore break ice apart. It logically follows that a fabric with an icephobic or even hydrophobic coating would further ease that process. A staggering number of hydrophobic technologies and processes has been published in recent years [8; 9]. Hydrophobicity is dependent on the microstructure, nanostructure, and chemistry of a surface [10]. On a superhydrophobic or superamphiphobic surface, the droplet partially rests on an air cushion. The reduced contact area helps to support and repel droplets [11]. To design superhydrophobic or superamphiphobic surfaces, the coatings need to have a low surface free energy. The surface energy is a measure for the amount of energy needed for a fluid (air or liquid) to form an interface with the surface [12]. A microstructure can decrease the contact area that a liquid has with a surface if air cushions are formed between asperities. The droplet is in the so termed \"Cassie state\". A deposited droplet rolls off if the surface is tilted by a few degrees. However, the large spacing between asperities cannot prevent wetting of the surface at high impact velocities.