As aircrafts face hazardous icing issues (refer to Chapter 1), some of the most exciting and ground-breaking technological solutions are inspired by nature, including the devel-opment of aircrafts themselves. In 1903, two American inventors, Wilbur and Orville Wright, achieved the first flight with a powered, sustained and controlled airplane [46]
shown in Figure 13, surpassing years of problems confronting aeronautical engineers, pioneers and enthusiasts around the world. The two brothers received patent in America and Europe for their work, despite criticism from the press and aviation community [47].
Figure 13. The first flight of December 17, 1903 near Kitty Hawk, NC [48].
Although the Wright brothers made a breakthrough in aviation, their vision on how a machine could fly was not at all new. Since ancient times, people observed how birds fly and studied problems of previous flyers (examples of nature-inspired previous fliers are shown in Figure 14). Likewise, the Wright brothers traveled to picnic areas on their bikes to observe how many birds fly around [49]. They modified early kite and glider experi-ments that did not meet their performance goals, and they built their own models-testing techniques to make a breakthrough flight that changed the future.
Figure 14. Nature-inspired flying designs: (left) Leonardo da Vinci’s 1488 sketch, (right) Jean-Marie Le Bris 1868 flying machine [50].
Decades later, the same process that the Wright brothers used to build the first success-ful airplane is still being used by modern-day tech giants like NASA and Airbus to tackle some of the most challenging problems. Today, NASA engineers are making improve-ments on the original works of Wright Brothers by using computer simulators of their test models [51] to make nature-inspired space exploration. Airbus, a multinational aero-space corporation and the world’s largest airline manufacturer, uses nature and concepts of biomimicry to improve performance of modern-day commercial aircrafts [52],[53],[54].
Figure 15 shows future aircraft designs resembling birds, sharks and eagles.
Figure 15. Airbus studies birds, eagles and sharks to design drag-reducing surfaces, improves aircraft efficiency and reduce emission [52],[54],[55].
Later problems with aviation and air travel were being resolved with cutting edge, nature-inspired thinking. For example, bionic bones resembling bird bones are being integrated in future airplane design, replacing standard machine structure and windows [56]. Figure 16 compares a bird’s bone structure to Airbuse’s 2050 airplane design. In a presentation for future projects, Airbus chief engineer explained that bird bones are light and strong and have porous interior structure that carries tension where necessary and leaves space elsewhere [56]. Today’s successful aviation leaders have the same vision.
Figure 16. Airbus’s 2050 aircraft structure (“bionic bone design) in comparison to a bird’s porous bone structure [56],[57].
In fact, natural systems are studied to solve many challenging modern-day problems, from producing rechargeable batteries and supercapacitors [58] to developing computer
software [59]. The complexity of a problem requires flexible and adaptable solutions;
biological systems, animals and plans have been evolving for millions of years to adapt to many of the same challenges nature poses to us. As such, nature inspired systems are promising to tackle problems with producing icephobic surfaces and structures.
3.1 Slippery Liquid Infused Porous Surface – SLIPS
SLIPS are nature-inspired surfaces designed to repel particles due to their highly slip-pery, non-stick properties. They show water repellency, self-cleaning and anti-fouling properties [60], like many surfaces found in nature such as the lotus leaf, shark skin and the butterfly wings, shown in Figure 17. The SLIPS design consists of a fabricate surface composed of a structured solid, which function is to hold a liquid layer in place [61].
Ideally, particles that touch these surfaces only meet the lubricating layer (i.e. the oils).
And because oils are highly slippery, have no defects and can self-heal, in theory parti-cles can effortlessly and naturally “slip,” slide, or roll off these surfaces.
Figure 17. Microscopic images of shark skin (left), butterfly wings (middle) and the lotus leaf (right) [62],[63],[64].
3.1.1 The Lotus Effect
The lotus leaf is considered the most notable icon for perfect superhydrophobicity and stable self-cleaning properties; It has led to the invention of a new concept: The Lotus Effect [65]. The lotus leaf has a micro/nanoscale double structure composed of many microscale waxy mastoid processes covered with nanoscale particles (the so-called “hi-erarchical structure”) [60]. This structure design acts to entrap air. And since air is highly hydrophobic, water will naturally roll off the surface, carrying any particles along. The nanoscale particles attach to foreign particles which helps them roll-off effortlessly.
3.1.2 Practical implications
In comparison between the lotus leaf (Figure 18) and other structures, the lotus leaf’s papillae (surface asperities) and waxy structure makes for an optimized performance: a perfection of durability and water repellency [65]. Liquid-infused structure rely on
com-plicated process preparation and design for liquid lockability, both considered to be limi-tations for designing SLIPS [60]. Although the production of a durable and porous struc-ture can be challenging, the strucstruc-ture should be oil-infusible and have good oil-lockabil-ity. More recent efforts are incorporating waxes instead of liquids to design structures closer on the lotus effect [66]. Yet, liquid-infused structures are desired in research.
Figure 18. (left) Water droplets on a lotus leaf [67] and (right) a computer graphic showing surface topography of a lotus leaf [68].
3.1.3 Current SLIPS technology
Current technology dealing with SLIPS aim to overcome challenges with oil lockability and complicated structure preparations. Some examples of recent SLIPS designs are shown in Figure 19. In one example, surface pores are created via laser ablation to avoid sacrificial templates and multi-step preparation processes [69]. The polymer-made, laser ablated SLIPS design by Xi’an Jiaotong University of China is claimed to successfully repel water, hexadecane, milk, Coca-Cola, ink, coffee, fruit juice, glycerol, and egg white [69]. In another example, ferrofluids (i.e. magnetic fluid) are used with magnetic fields to lock the lubricant in place to improve liquid lockability [70].
Figure 19. Example of current SLIPS: (left) Laser ablation of coated structures [69];
and (right) Magnetic SLIPS design [70].