Inspired by the living organisms in nature, taking cues from nature and mimicking biological systems has become one of the most popular and interesting research topics in current nanotechnology.Scientists have recently developed several biomimetic superhydrophobic surfaces, inspired by nature, such as lotus leaves, butterfly wings, exhibiting outstanding superhydrophobicity, attributable to numerous smart and simple routes. Although several examples, such as the lotus effect, explicitly tell us that biomimicry is unlike the mere replication or repetition of biological structures. Highlighting the work on both natural superhydrophobic surfaces and biomimetic superhydrophobic surfaces in this feature article and highlight some of the recent developments in the recent years, including the different smart routes for constructing rough surfaces and many chemical improvements that contribute to superhydrophobicity. To date, scientists and researchers are still updating their roles and implementations.
Functional fabrics such as anti-bacterial, odour-proof, antistatic, waterproof and moisture-permeable, oil-proof and pollution-proof are becoming increasingly popular with consumers with the improvement in people’s living standards. This also means that there is a rise in the consumer demand for practical fabric products. With the growth of the textile industry, practical textile auxiliaries, such as waterproofing agents and finishing agents for quick decontamination, have been commonly used.However there are a few drawbacks with artificial coating materials that can not be overlooked. For instance, paraffin-aluminium soaps have poor washability and rough handling. Instead of aluminium salt, if zirconium salt is used, the effect will be improved, but the cost will be increased. To improve hand feeling, hydroxymethyl melamine derivatives must be combined with resin.
Another finishing technique that is used widely is Chromium Stearate Complexes. The chromium ion in the structure makes the fabric green, making it more difficult to dye, and the chromium ion does not meet environmental protection requirements. When working, pyridine quaternary ammonium salts will produce unpleasant gases that will impact the working environment. The fabrics treated are prone to yellowing and decoloration. Silicone feels poor in the hand and has poor durability. To increase film toughness and strength, it needs to be shared with hydroxy silicone oil emulsion. Fluorocarbon Waterproof and Oil-proof Agent usage is increasingly restricted due to the environmental toxicity of perfluorocarbons.[7,8] The fluorine-free waterproofing agent currently on the market has only a waterproofing effect, no oil effect, and no pollution prevention effect. Imitating nature is a simplified means of generating certain properties. Superhydrophobic and superoleophobic properties found in nature are therefore given a brief review here.
Biospecies exhibit remarkable properties such as superhydrophobicity with high or low adhesion, UV defence and self-cleaning ability in order to adapt to their surrounding environment and better survival in nature. To regulate interfacial interactions, such as superhydrophobic adhesion, nature also uses topographic patterning. Lotus leaves, for example , show a self-cleaning feature where, when the leaves are slightly inclined, pollutants can be quickly pushed away from the surface.
Due to its great advantages in both theoretical science and practical applications, superhydrophobicity has received considerable attention in recent years. Superhydrophobic surfaces with unique liquid-solid adhesion, on the other hand, have recently drawn considerable interest in their possible applications. Anti-corrosion coatings, anti-icing coatings, liquid-repellent textiles, oil / water separation, assembly of nanoparticles, microfluidic devices, printing techniques, optical devices, high-sensitive sensors or batteries, materials with superhydrophobic or superoleophobic properties are in significant demand.In all of these uses, liquid penetration (oil / water separation, anti-fogging), ion penetration (anti-corrosion, water desalination, batteries), heat transfer (anti-icing) can be minimised by the presence of an air layer trapped within the surface roughness, while surface roughness can boost the intrinsic properties of the materials (optical, electronic, catalytic properties). It is also absolutely vital that the superhydrophobic coating is durable, which ensures that, even after high pressure, the materials maintain their properties. Mimicking nature is a simpler way to produce certain properties. Therefore, a brief analysis of superhydrophobic and superoleophobic properties discovered in nature is given here.
The static contact angle, which is defined as the angle a liquid makes with a solid, is the primary parameter that characterises wetting of a surface. The angle of touch depends on many factors, including the energy of the surface, the roughness of the surface and its cleanliness.If the static contact angle value is (a) 0° ≤ θ ≤ 90°, the liquid wets the surface, while the contact angle value is (b) 90° ≥ θ ≥ 180° if the liquid does not wet the surface. Surfaces consisting of high surface energy polar molecules tend to be hydrophilic, whereas those made up of low energy non-polar molecules tend to be hydrophobic. Surfaces with a contact angle below 10 ° are considered to be super hydrophilic, while surfaces with a contact angle between (c) 150 ° and 180 ° are considered to be super hydrophilic. Wettability of a surface mainly depends on two factors: (i) the surface free energy, and (ii) the surface roughness. The self-cleaning property of a surface depends on its smoothness – extremely smooth surfaces show a reduced soiling behaviour, because the particles have only low mechanical hold and can be removed either by air or liquid. A real self-cleaning effect can be achieved when overlapping structures with dimensions of a few micrometres and superimposed structures of 50-100 nm are added to surfaces, and if the surface chemistry is hydrophobic. The surface structure significantly minimises the effective surface contact area of dirt particles and, thus, adhesion is very limited. Dirt particles are separated when a drop of water rolls over such a surface. The adhesion energy of the particle to the solid surface is very low due to the roughness of the surface and the low contact area.[6,8]
Hydrophobic Characteristics in Nature
The wettability of various animal and plant surfaces has been designed for various purposes by Evolution. From hydrophilic to super hydrophobic, the wetting aspect of various natural surfaces varies. Without wetting the surfaces, some of the natural surfaces are so hydrophobic that water droplets will roll over them. The classic example of this type of surface is the surface of the lotus leaf and the ‘lotus leaf effect’ phenomena. Rose petals, duck feathers and butterfly wings are other examples of such textures. Self cleaning properties are created by the super-hydrophobicity of their surfaces, i.e. when the water droplet rolls over the surface, it removes all the dirt on the surface , i.e. it self-cleans.
The surface of the lotus leaf was examined under the electron microscope to investigate the reason for the lotus leaf effect. While the lotus leaf is clean and smooth to the naked eye, it is not so on the nanoscale. On the opposite, due to papillose epidermal cells that shape the papillae or micro-asperities, it is rough. The surface of the papillas is also rough, in addition to the microscale roughness. Three-dimensional epicuticular waxes, which are hydrophobic long-chain hydrocarbons, shape the nanoscale roughness. So the surface of the lotus leaf essentially consists of three-dimensional nipple-like structures systematically arranged made of nanosized wax crystal shapes that are not greater than a few nanometers in size, but are water-repellent. Two important factors depend on this superhydrophobicity of a surface. First, the solid surface has low surface energy and chemical composition and, second, a high degree of surface roughness.Two important factors depend on this superhydrophobicity of a surface. First, the solid surface has low surface energy and chemical composition and, second, a high degree of surface roughness. A decreased contact area with water is caused by this rough structure on the surface of the lotus leaves.The water penetration is prevented by the presence of wax crystals of hydrophobic nano-size. The water forms droplets and falls over the surface as a result.
Inspired by the non-wetting phenomena of duck feathers, the water repellent property of duck feathers was studied at the nanoscale. The microstructures of the duck feather were investigated by a scanning electron microscope (SEM) imaging method through a step-by-step magnifying procedure. The SEM results show that duck feathers have a multi-scale structure and that this multi-scale structure as well as the preening oil are responsible for their super hydrophobic behavior. The microstructures of the duck feather were simulated on textile substrates using the biopolymer chitosan as building blocks through a novel surface solution precipitation (SSP) method, and then the textile substrates were further modified with a silicone compound to achieve low surface energy. The resultant textiles exhibit super water repellent properties, thus providing a simple bionic way to create super hydrophobic surfaces on soft substrates using flexible material as building blocks. Compared with lotus leaves, duck feathers are soft and flexible, and the water repellent characteristic of duck feathers is durable, thus it provides an ideal model for the fabrication of super hydrophobic surfaces on soft substrates. In this paper, we report the fabrication of super hydrophobic surfaces on textile substrates by mimicking the surface structures of duck feathers and explore their application in self-cleaning textiles.
Diverse surface structures and wettability are often exhibited by land plants. The petal of Rosa montana, for instance, consists of conical cells with a nano folding cuticulum. The petals of red roses have also been stated to have superhydrophobic properties with high adhesion. The scale of the conical cells was 16 mm in diameter and 7 mm in height, also known as micropapillae. It is also important to note that the size of the red petals is greater than that of the lotus leaves (1 11 mm) for both the microstructures and nanostructures. Indeed, the water could join, but not inside the manifolds, the large spaces between the large micropapillae. For peanut leaves, similar properties were also recorded. The wild pansy (Viola tricolour), however, which has much closer surface structures than the rose petals, showed self-cleaning characteristics. This is possibly due to changes in the structure’s dimensions. In the literature, superhydrophobic leaves containing hair are also mentioned. Although Lady’s Mantle was found to have vertical hairs, on ragwort and poplar leaves, horizontal hairs were detected. The white colour of the plants contributes to high reflectance properties in the above cases. It is also important to cite the examples of Strelitzia reginae and Oryza sativa (rice) leaves to finish with superhydrophobic plants.The superhydrophobic properties of these leaves have been shown to be anisotropic. Tauthors found on their surface the existence of parallel microgrooves. After the leaf was inclined in the direction perpendicular to the microgrooves, a water droplet deposited on the leaf stayed fixed on it, but the droplet could be displaced in the direction parallel to the microgrooves. In addition , the presence of microgrooves, if in the same direction, may also reduce the water flux drag.
The superhydrophobicity of insect wings is an advantage in reducing the pollution of dust / particles and enhancing their flight ability. The Barthlott group researched the surface structures and wettability of 97 wings of insects. As recorded by Watson and coworkers for termite wings, they found different families with extremely hydrophobic wings, including mayflies, dragonflies, stoneflies, lacewings, scorpionflies, alderflies, caddisflies, butterflies, moths and flies. Various morphologies, such as cloth-like microstructures, hair or scales, have been published. The Goodwyn group investigated the structures of various butterflies with hydrophobic or superhydrophobic features and different colours. Although no simple pattern was seen in the scales of the transparent butterfly wings of the genus Parnassius glacialis, the white translucent regions of Parantica sita were highly ordered and arranged in lines forming periodic and parallel porous microstructures.[13,11]
In addition, the sizes of butterflies, such as Morpho aega, overlap in only one direction at the microscale. As a result, the wings of these insects, also called anisotropicity, are superhydrophobic but with lateral adhesion. Only if the wing is inclined in one direction will it roll off the surface when a water droplet is deposited on the wing. Many insects’ feet are superhydrophobic as well.
Application in textile
Understanding comprehensively the roles of surface energy and roughness for natural dewetting surfaces has led to the development of a number of biomimetic superhydrophobic surfaces. Many efforts have been made to obtain artificial surfaces with biomimetic micro/nano binary textures by a variety of methods, such as the sol-gel method, photolithography, laser/plasma/chemical etching, microcontact printing, local anodic oxidation, electrochemical deposition and chemical vapor deposition. However, due to either the complicated preparation procedures or high cost, practical applications of most of these strategies are still very tough. It is highly desirable to achieve surfaces with complex biomimetic micro/nano textures under even milder conditions, such as room temperature.
As far as application in textile materials is concerned, the natural lotus effect phenomenon is useful. In textile materials, if this can be imitated, then a whole range of products such as umbrella, rainwear, carpets, upholstery, protective clothing, sportswear, automotive interior fabrics, etc., and even self-cleaning clothing, can be made. The first patent on hydrophobic textiles was filed in this respect in 1945; alkyl silane was used in hydrophobic textile materials. By using non-polar hydrophobic agents such as paraffin wax, silicone, silane and fluorinated polymers, the hydrophobic properties of a surface can be achieved. Paraffin wax, silicones and silanes, however, only waterproof the textile surface, which is painful in the case of clothing.Due to their high water and oil resistance, organic solvent resistance and lubricity, a variety of fluorine-based polymers are common for this purpose. In addition, the treated fabric helps water and moisture vapours to pass through it. Because of these benefits, fluoropolymers have been used since the 1960s in the textile industry. In 1964, Dettre and Johnson first researched the lotus effect phenomenon using paraffin or PTFE telomere-coated glass beads. Recent approaches to this form of finishing include the achievement of nano-microscale surface topography by fibre-attached nanoparticles that enhance surface roughness. In commercial terms, silicate and fluorocarbon nanoparticles are used for this purpose. It has been stated that cotton fabric can achieve super hydrophobicity using a homogeneous silica-copper hybrid nano-composite fabric.
Using a simple solution precipitation technique, artificial duck feather structures were developed on textile substrates, inspired by the novel water repellent phenomenon of duck feathers. The microstructures of duck feathers have been studied using the SEM imaging technique. The SEM images demonstrate, in their texture, that duck feathers have multi-scaled structures. The multi-scaled structures on duck feathers provide ample surface roughness for water repellent features. The duck feather exhibits super water repellent properties. The multi-scaled duck feather structures were designed to manufacture hydrophobic textiles on textile substrates. The soft biopolymer chitosan was used as building blocks to create nanosized roughness on cotton and polyester fibres and then the textiles were further treated with polysiloxane emulsion to achieve low surface energy The resulting textiles show super water repellent properties with a handy handle. Several publications have already reported on the production of super repelling surfaces using re-entrant geometry, but fluorinated compounds have been used in all these publications because they have high hydrophobic properties while having relatively significant oleophobic properties compared to hydrocarbons.
Although superhydrophobicity is only a recently developed concept, it has already become important to a lot of research and will be potentially important to people’s life. As stated in previous sections, a great amount of effort has been put into the research to understand the mechanisms that are related to superhydrophobicity on solid surfaces. This nature-inspired theory is an interdisciplinary subject which involves physics, chemistry, material science, and even biology. In the cutting edge research, nanotechnologies are effectively used to explore the intrinsic essence of the subject. Hence, with more developed equipment and further investigation, we are able to see the “real” structures of the superhydrophobic models that are found in nature and fabricate manmade superhydrophobic surfaces which have significant water-repellency and durability. The achievement of superhydrophobicity-related study is phenomenal and this tendency will maintain and continue. As discussed previously, creating a superhydrophobic surface may involve several procedures, during which a number of techniques are used together. But in some cases, it is also possible to create superhydrophobic surfaces using relatively simple and prompt methods. No matter how the superhydrophobic surfaces are made, it is always important to keep the process under control and obtain the required effect precisely. Even greater progress is in prospect due to increasing attention and developing technique. The mostly expected breakthroughs in the future research will be concentrated on precisely controlling surface roughness and structure in both small and large scales, understanding more details of the wetting process, especially the wetting transitions, and turning up the application in an even broader range of materials.
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