Abstract
Beautiful, powerful, dangerous, cold Ice has a magic, can't be controlled Stronger than one, stronger than ten Stronger than a hundred men - The lyrics of “Frozen Heart” from the movie Frozen
Anyone who has slipped on an icy sidewalk or had to scrape ice off their vehicle understands the dangers of ice buildup. Uncontrolled ice formation and buildup on surfaces pose significant challenges across various industrial sectors, from transportation and clean energy to civil engineering. While ice can be hazardous on the ground, it becomes disastrous in the air. This was tragically demonstrated by the crashes of Air Florida Flight 90 (1982), Continental Flight 1713 (1988), and USAir Flight 405 (1992), all of which resulted in numerous fatalities due to improper or inadequate deicing operations. Icing events also affect other transportation industries, such as the railway sector, where iced overhead catenaries can cause equipment malfunctions and train delays, and the maritime sector, where ice formation compromises safety and operation. Similarly, in the energy sector, ice accretion reduces the efficiency of wind turbines, disrupts power line networks, and hinders solar cell performance by scattering and reflecting sunlight.
Deicing is a reactive approach implemented after ice has formed; however, a more effective strategy is to prevent ice formation in the first place. Icephobic surfaces are engineered to repel ice or hinder the development of ice films. This can be accomplished through morphological modifications to the surface or by applying specialized ice-phobic coatings.
The earliest known antifreeze coating dates to 1918, with a U.S. patent suggesting a coating made of a solution of calcium chloride, caramel, glucose, and borax in water for internal combustion engine radiators to prevent freezing in cold climates. 1 Later, de-icing lacquers for aviation included nitrocellulose, ethyl-cellulose, cellulose acetate, and polybutylene in alkyds and vinyls with plasticizers. 2
The development of coatings for ice phobic surfaces requires understanding the mechanism of ice formation. The coatings can prevent ice nucleation, icing propagation and/or ice adhesion. Icephobic coatings are different from water-repellant coatings. While water repellence aids in icephobicity, the materials needed for delaying ice formation and reducing ice adhesion differ from those for water repellence. For instance, microscale textures create hydrophobic surfaces by trapping air pockets. However, they may also promote ice nucleation and enhance ice adhesion, making them less effective for icephobicity despite their water-repelling properties. 3 Other strategies such as lubricant-infused surfaces (LIS), and photothermal surfaces are not fail-proof; LIS can fail due to lubricant depletion, 4 and photothermal surfaces require continuous solar exposure. Despite such challenges, considerable materials development efforts are ongoing for the development of icephobic coatings.
Despite the ongoing debate about the effectiveness of hydrophobic and superhydrophobic coatings for ice repellency, 5 superhydrophobic coatings remain the most frequently studied. Superhydrophobic surfaces have been positioned to act in two ways – preventing/delaying ice nucleation and reducing the adhesion of ice to the substrate.
Superhydrophobic coatings lower the freezing temperature of water and thereby reduce the likelihood of ice formation or delay ice formation [Figure 1]. This delay allows more time to remove water droplets before freezing occurs. Textured superhydrophobic coatings resist liquid and freezing solutions, extending the freezing delay of water droplets for hours at −18 °C. Sessile droplets remain metastably supercooled on these surfaces due to the mechanical stability of the textures.6,7
Lowering of freezing temperature with an increase in hydrophobicity. Image reproduced without modification from. 7
Superhydrophobic coatings can also exhibit icephobic properties because water droplets impact, retract, and bounce off these surfaces without leaving any residue, even at temperatures as low as −25 °C [Figure 2].8,9
Droplets of water fully retract and shed only in superhydrophobic surfaces to prevent ice formation, at temperatures close to −25 °C. Image reproduced without modification from. 9
The reduction of the adhesion of ice on the surface is known as interfacial cavitation. Untreated surfaces can have ice adhesion as high as 1600 kPa. Icephobic coatings are designed to exhibit much lower ice-adhesion forces, typically 100 kPa or less. Interfacial cavitation involves the interaction between a soft surface, like a polymeric elastomer, and a hard surface, such as ice. When the elastomer attaches to the ice, it deforms while the ice remains rigid. This deformation creates a stress concentration at the interface, which makes it easier for the ice to break free. Thus, the ice adhesion strength is closely linked to the coating's Young's modulus. 10
Soft coatings have been widely researched for their interfacial cavitation properties Among the various materials studied, silicone-based elastomeric coatings have been particularly prominent. These coatings, especially those with a spongy structure, exhibit good icephobic characteristics. The degree of hydrogen bonding in these coatings enhances their mechanical strength and improves adhesion to the substrate without compromising their icephobic properties. 11
Ice adhesion can also be reduced using composite coatings. For instance, a self-formed superhydrophobic composite coating was achieved using a combination of polydimethylsiloxane (PDMS), polyvinylidene (PVDF), and SiO2 powder. 7 This composite reduced ice adhesion and had mechanical properties due to the multilevel nanostructure created by the interactions between the components [Figure 3].
Composite icephobic coating and ice adhesion strength. Image adapted from. 7
An interesting study at ETH Zurich demonstrated how low-pressure-induced levitating water droplets over superhydrophobic surfaces could potentially be used to design icephobic coatings. These coatings, made of a grid of microscopic silicon pillars coated with a Teflon-like material, exhibit extreme water repellence. When the surrounding pressure is reduced, the water droplets resting on these surfaces levitate due to the air trapped in the channels beneath them [Figure 4]. This vacuum effect induces boiling at the droplet's edges, causing water to evaporate and generating pressure below the droplet. Once this pressure surpasses a critical threshold, the droplet spontaneously bounces off the surface. Importantly, during evaporation, the droplet may become supercooled, dropping below the normal freezing point. When it eventually freezes, its temperature rises to 0°C, triggering a rapid increase in vaporization. This enhanced vaporization further promotes levitation, allowing ice droplets to be efficiently removed from the surface. 12
Water droplets solidifying on, and launching from, superhydrophobic fluoropolymer–carbon-nanofibre composite surface at standard temperature with low-pressure and low-humidity conditions. Image reproduced from. 12
Some icephobic coatings have been inspired by nature. Penguins, thriving in the coldest environments, are known to have icephobic feathers that prevent ice formation and maintain insulation. The air-infused microscale and nanoscale hierarchical rough structures of the Humboldt penguin endow their body feathers with remarkable hydrophobicity (water contact angle ≈ 147°) and antiadhesion characteristics (water adhesive force ≈ 23.4 μN), even for supercooled water microdroplets. These feathers trap air, effectively repelling water and exhibiting excellent antifrosting and anti-icing properties. Inspired by this natural design, scientists have developed a polyimide nanofiber coating on an asymmetric electrode through electrospinning, acting as an artificial replica of a penguin's body feather [Figure 5]. 13 The unique microstructure of this coating results in a gradient density of surface chemical substances, essential for altering the water contact angle and adhesive force. As the distance between adjacent fibers increases, the static water contact angles decrease from approximately 154° to 105°, and the water adhesion forces rise from 37 to 102 μN. This membrane effectively pins supercooled water microdroplets, preventing their coalescence, and ultimately achieves icephobicity.
(a) elaborate wrinkles on the barbules and hamuli of penguin feathers (b) oriented nanoscaled grooves on the barbules of penguin feathers (c) SEM of the gradient polyimide nanofiber membrane. Image adapted from. 13
In another biomimetic approach, the antifreeze properties of fish proteins have been mimicked to produce self-healing anti-icing coatings. Anti-icing coatings often sustain mechanical damage from hailstorms, sandstorms, and icing-deicing cycles, creating surface defects that increase water molecule adsorption and heat transfer, which accelerates ice nucleation and propagation. Inspired by natural antifreeze proteins (AFPs) found in polar fish, a self-healing coating has been developed [Figure 6]. This coating effectively inhibits ice nucleation (below −29.4 °C), reduces ice propagation (less than 0.00048 cm²/s), and minimizes ice adhesion (below 38.9 kPa), while autonomously self-healing at −20 °C. 14
Fish antifreeze protein-inspired icephobic coating. Image adapted from. 14
Ice-Skating- inspired Deicing Coating. Schematic reproduced without modification from. 15
Another interesting icephobic coating concept has been inspired by ice skating. Ice skaters glide across the ice using a self-lubricating water layer that forms between the ice and their skate blades. Similarly, researchers have developed a robust anti-icing coating that creates a self-lubricating liquid water layer (SLWL) between the ice and the coated surface [Figure 7]. 15 To achieve this, a micropore-arrayed silicon wafer surface was first prepared by photolithography. Cross-linked hygroscopic polymers, synthesized by free radical polymerization, were then grafted inside these micropores. When the temperature drops, the hygroscopic polymer network inside the micropores absorbs water and swells. If the temperature continues to decrease and enough water is absorbed, the water-swollen polymer network bulges out of the micropores. The swollen polymers merge due to molecular attractions, forming a self-lubricating liquid water layer. This layer reduces the adhesion between the ice and the surface, allowing the ice to slide off easily. (Figure 8).
(a) Blowing away of ice from lubricant-based icephobic coatings (b) ice adhesion strength reduction due to lubricant-based coating on different substrates. Image adapted from. 16
A related anti-icing coating features an aqueous lubricating layer made of polyurethane polymers with hydrophilic di-methylol-propionic acid-based pendant groups. 16 These hydrophilic components absorb water in humid environments, and when in contact with ice or snow, the pendant groups ionize, melting the ice and swelling. This process lowers water activity, significantly reducing ice adhesion strength. The ice formed on this coating can be easily blown off. The coating maintains low ice adhesion even at temperatures as low as −53 °C.
A physical concept called stress-localization has been proposed to develop icephobic surfaces with ice adhesion as low as 1 kPa while maintaining exceptional mechanical, chemical, and environmental durability. 17 In an isotropic elastomer (Phase I), where the interfacial adhesion strength is directly dependent on the shear modulus, introducing local phases with a lower shear modulus (Phase II) at the ice–material interface allows ice to detach from Phase II with minimal force [Figure 9]. This detachment creates a local cavity (crack) between the ice and Phase II. The crack induces an elastic stress field around it, which generates a shear stress field that opens the crack front and propagates the crack along the interface. Thus, the induced stress field from the local phases promotes crack growth and fracture, effectively reducing ice adhesion.
Stress-localized ice-phobic coating. Image adapted from. 17
The research and development of icephobic coatings have shown significant promise in mitigating ice formation and adhesion across various surfaces. The market already offers solutions, such as SilicoTek's Notak® and Sunrise Scientific Inc.'s Syneffex™. NASA's development of silane-based polymer coatings aims to reduce impact ice adhesion strength, while NEI Corporation's NANOMYTE® SuperAiTM technology provides a superior ice adhesion reduction factor for use on aircraft surfaces. With their versatile applications, from enhancing aircraft safety to improving the efficiency of power lines and wind turbines, icephobic coatings will play a crucial role in a wide spectrum of industries.
Footnotes
Declaration of conflicting interests
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The authors received no financial support for the research, authorship, and/or publication of this article.