Special Issue “Superhydrophobic and Icephobic Coatings as Passive Ice Protection Systems for Aeronautical Applications”

The formation of ice can be very dangerous to flight safety, especially in cold climates, since ice accumulated on the surfaces of the aircraft can alter the aerodynamics, increase the weight, and reduce lift, leading to catastrophic stall situations in some cases. Currently, risks caused by ice accretion are mitigated by using energy-demanding active ice protection systems (IPSs), which work by either preventing ice (anti-icing) or removing ice (de-icing). However, for future sustainable aviation, low-energy-demanding IPS must be designed. Hybrid IPSs, which combine active IPSs with passive superhydrophobic/icephobic coatings, able to prevent, delay, and/or reduce the ice accretion, might represent a valuable solution, reducing the energy consumption and the CO₂ emissions [1,2,3,4,5]. Recently, Morita et al. [6] found that the combination of icephobic coatings with electrothermal heating IPS reduces the energy requirements by more than 70% at LWC of 0.6 g/m³ and a median volume diameter (MVD) of 15 μm at 75 m/s. However, in wet icing conditions, more than a 30% reduction in power is achieved. A reduced power consumption by more than 50% compared with IPS without coatings was also observed by Yu et al. [7], which demonstrated the ability of superhydrophobic coatings to prevent run-back ice.

In this scenario, this Special Issue collects research achievements, ideas, chemical formulations, applications of superhydrophobic and icephobic coatings as passive IPS working alone or in combination with active IPSs, IPS parameter design, and icing wind tunnel (IWT) test campaigns, covering diverse technologies and application domains, such as aeronautical and energetic.

Some studies have focused on the design of the hybrid IPSs, providing numerical tools able to simulate the effects of superhydrophobic and icephobic coatings under icing conditions [8], including the sensitivities of IPSs to icing environmental parameters [7]. These first results demonstrated that it is actually possible to predict the beneficial effect of properly designed coatings for ice mitigation purposes in real-world applications [8]. Physical parameters, coating compositions, structure, roughness, morphology, and durability are properties not to be neglected in the design and development of reliable IPSs in aircraft maintenance [9].

In addition to high superhydrophobicity or icephobicity, a protective coating to be used on aircraft must meet comprehensive property requirements, such as workability, heat and low temperature resistance, weatherability, erosion durability, and reparability. Among these, erosion resistance is one of the most important; for instance, coatings applied to the leading edge of a wing or helicopter rotor blades, where icing tends to occur and cause the most negative impact on aircraft operation, may experience excessive wear due to the high-speed (e.g., 100–300 m/s) impact of sand, airborne dust, rain droplets, and hail [10]. Elastomeric coatings with water contact angles ranging from 100 to 115°, high mechanical strength (19–27 MPa), high elongation at break (640–730%), and low tensile set (20–35%) have been demonstrated to reduce the ice adhesion from 622 kPa to 480–220 kPa, while combining excellent erosion resistance against both high-speed solid particles and water droplets [10].

The question remains as to whether such coatings could provide durability and performance in relevant flight icing conditions. Superhydrophobic coatings were demonstrated to be helpful in reducing the ice accretion by 12% to 100% at temperatures higher than −12 °C and velocities of 50 and 95 m/s during an IWT test campaign carried out on coatings applied on two NACA0015 wing profiles, with no active IPS switched on [11].

The use of passive IPSs becomes essential during takeoff operations since, according to the Federal Aviation Administration regulation, active IPSs cannot be switched on until the aircraft reaches 400 feet above the takeoff surface to avoid engine thrust reduction [12]. IWT tests performed at the takeoff and first climb conditions [13] on a nacelle lip-skin at T = −5 °C and −12 °C, v = 70 and 95 m/s, at an altitude of 3000 m, with a median volumetric diameter (MVD) equal to 20 μm and liquid water content (LWC) equal to 0.3 and 1 g/m³, demonstrated that the application of a superhydrophobic coating with active IPS switched off reduces the ice thickness, up to −49%, and the ice accreted impingement length up to −10%. At a higher LWC, i.e., 1 g/m³, a reduced length and number of ice rivulets have also been observed for coated configurations [14].

A fundamental parameter to know about a coatings’ performance in icing flight conditions is the ice adhesion strength on the aircraft surfaces. The lack of a standard for the ice adhesion measurements offers many opportunities and opens new routes in this area. The effects of ice types, test parameters, and surface properties on the measurement data of ice adhesion centrifuge tests have been studied elsewhere [15]. Surfaces with low ice adhesion strength, achieved through the application of coatings [16] or through laser texturing [17], might also be highly useful in facilitating or assisting the manual de-icing process performed by the crew members [18].

Technological solutions found for the aeronautical applications can be transferred in diverse domains, since ice accretion poses serious problems not only in the aviation industry, but also for dams and locks, express trains, air conditioners, refrigerators, wind turbines, solar panels, power lines, suspension bridges, heat pumps, and offshore oil platforms [19]. For instance, one of the problems involving the use of photovoltaic technology to produce renewable energy is that photovoltaic panels are subject to a significant loss of efficiency due to the accumulation of dust and dirt and, during the winter season, of snow and ice, which reduce or stop the energy production. The application of transparent coatings with superhydrophobic self-cleaning and icephobic properties might be proposed as valuable solution [20]. The results demonstrated that the ice adhesion of photovoltaic panels decreased by 69%, and the freezing delay time increased 17-fold compared with those of the unmodified surface. Jointly, the contact angle hysteresis and roll-off angle of coated surfaces were significantly reduced, and transparency, which is a key requirement for photovoltaic applications, was preserved.

Although their application on vehicles’ surfaces is still challenging, the development of hybrid IPSs offers new perspectives in the field of aviation, paving the way for more sustainable and efficient solutions for flight safety.