[
    {
        "id": 1,
        "chunk": "# Superhydrophilic and Superamphiphilic Coatings  \n\nSandro Olveira, Ana Stojanovic, and Stefan Seeger Department of Chemistry, University of Zurich, Zurich, Switzerland",
        "category": " Abstract"
    },
    {
        "id": 2,
        "chunk": "# 3.1 INTRODUCTION  \n\nThe concept of superhydrophilicity was introduced following intensive research on superhydrophobic surfaces and in response to a request for coatings and surfaces that display strong affinity to water [1]. When water droplets are in contact with a superhydrophilic surface, they completely spread over the surface. When such a surface additionally enables total spreading of oily droplets (superoleophilic property), it is defined as superamphiphilic. Over the past decade, surfaces exhibiting superhydrophilicity or superamphiphilicity have been the subjects of immense interest because of their potential applications in various fields, including the development of ­microfluidic devices, liquid–liquid separation membranes, antifogging, antireflective, self‐cleaning, and antifouling coatings [2–7].  \n\nIn 2000, the term superhydrophilicity was used for the first time in papers ­published by three different research groups from Japan [1, 8, 9]. Earlier, in 1997, Wang et al. published a seminal paper on a superamphiphilic coating that consisted of a $\\mathrm{TiO}_{2}$ polycrystalline film [10].  \n\nThis chapter reviews the current state of research on superhydrophilic and superamphiphilic coatings, and it is organized as follows: Section 3.2 summarizes important fundamentals and definitions that apply to artificial superhydrophilic and superamphiphilic surfaces. The next section (Section 3.3) presents (i) examples of naturally occurring superhydrophilic and/or superamphiphilic surfaces and (ii) the most prominent examples of artificial superwetting coatings are illustrated in Section 3.4. Since high‐quality coatings exhibiting superhydrophilicity or superamphiphilicity cannot be produced without manipulation and control of surface ­chemistry and surface structure, Section  3.5 contains an overview of the most common techniques used for manufacturing such coatings. Then, the next section (Section 3.6) describes the most explored applications of superhydrophilic and superamphiphilic coatings, which include antifogging films, antireflective ­coatings, enhanced boiling heat transfer, separation membranes, and smart surfaces with reversible switching abilities. Section 3.7 provides an overview of commercially available superhydrophilic and superamphiphilic coatings. In the last Section 3.8 conclusions about the current state of research and commercial applications of these superwetting coatings are drawn.",
        "category": " Introduction"
    },
    {
        "id": 3,
        "chunk": "# 3.2 BASIC CONCEPTS OF SUPERHYDROPHILICITY  \n\nSurface wettability is generally characterized by the value of the contact angle. A surface is called superhydrophilic (or alternatively, superwetting) if the apparent contact angle of water on the surface is less than $5^{\\circ}$ . In addition to either superhydrophilicity or superhydrophobicity, a surface can also exhibit either superoleophilicity or superoleophobicity. On a superoleophilic surface, the contact angle of a polar droplet exhibits a value lower than $10^{\\circ}$ . A surface that is both superhydrophilic and superoleophilic is called superamphiphilic.  \n\nThe wettability of a solid surface is controlled by the surface free energy and the geometric structure of the surface. Commonly, the presence of polar groups on the surface decreases the contact angle of liquids. Water can completely spread over a few smooth surfaces, including those of glass [11], quartz [12], amorphous silica [13], gold [14], selected oxides (carrying OH groups on the surface) [15], and selected self‐assembled monolayers with OH‐based functionalities (OH, COOH, POOH). Such strong affinity for water is typically short term, and in the presence of any contamination, the contact angle increases to a few tens of degrees [16, 17]. According to Young’s equation, the contact angle of a liquid drop on a solid surface results from equilibrium between cohesive forces in the liquid and adhesive forces between the solid and the liquid [18]. For a certain liquid, the predominant contribution to the contact angle originates from the interfacial character of the solid material, which is related to its surface structure [19]. Wettability is determined by the surface free energy of a solid surface, which is commonly expressed by Young’s equation,  \n\n$$\n\\mathrm{cos}\\theta={\\frac{\\gamma_{\\mathrm{{sv}}}-\\gamma_{\\mathrm{{sl}}}}{\\gamma_{\\mathrm{{lv}}}}}\n$$  \n\nHere, $\\theta$ is the contact angle in Young’s mode, and $\\gamma_{_\\mathrm{{lv}}},\\gamma_{_\\mathrm{{sv}}},$ and $\\gamma_{_\\mathrm{lv}}$ are the three ­different types of surface tension (liquid/vapor, solid/vapor, and solid/liquid) involved in the system.  \n\nIn recent studies, Drelich et al. suggested that surfaces are truly superhydrophilic only if the surface is textured and/or structured [20, 21]. Rough or porous surfaces ­possess a roughness factor $r$ (where $r$ is the ratio of the real surface area to the projected  \n\nsurface area) that is defined by the Wenzel equation [22]. The Wenzel theory predicts that, for any rough surface, the actual surface area will be greater than the geometric surface area. This surface ratio is called the roughness factor and is defined by  \n\n$$\nr=\\frac{\\mathrm{cos}\\theta}{\\mathrm{cos}\\theta^{*}}\n$$  \n\nwhere $\\theta$ and $\\boldsymbol{\\theta}^{*}$ are the actual and geometric contact angles, respectively. In other words, an increase in the surface area (due to the presence of texture) amplifies the natural hydrophilicity of the material.  \n\nHence, according to the definition by Drelich et al., truly superhydrophilic ­surfaces possess roughness factors greater than one, and water spreads completely over them. Figure  3.1 shows the relationship between the contact angle on a smooth surface (Young’s contact angle, $\\theta$ ) and the minimum value of the roughness factor $(r)$ that is required for the same surface to promote complete spreading of the liquid. The figure shows that with moderate roughening of the surface in which $r$ is between 1.2 and 2, superhydrophilicity should be conceivable on any material having an intrinsic contact angle less than $60^{\\circ}$ . For materials with $\\theta{>}65{-}70^{\\circ}$ , roughening might not be practical, since extremely high values for $r$ are needed. Theoretically, on any rough material, a liquid should spread to zero (or nearly zero) apparent contact angle; however, in practice, liquid penetration into the rough structure of a substrate might be difficult. Such a system is essentially a three‐phase system trapped in a metastable state, and the surface should be treated more like a porous or solid–air composite material [23, 24].  \n\n![](images/58099ab2c5899654780cbdd53546070d6929688f7d4863670ef7bbc2a0bf6de9.jpg)  \nFig. 3.1  Minimum values of roughness factor necessary to promote the complete spreading of liquid on a surface with varying Young’s (intrinsic) contact angle. Used with permission from Ref. 21. $\\mathbb{O}$ Royal Society of Chemistry.  \n\nBesides being rough, porous surfaces also can exhibit superhydrophilic properties via wicking. Wicking or spontaneous imbibition is the suction of a liquid into a porous medium due to negative capillary pressure created at the liquid–air interface [25]. Wetting on a three‐dimensional (3D) porous surface that exhibits a 3D capillary effect was investigated theoretically by Quéré et al. [24, 26, 27] According to the authors, a “hemi‐wicking” behavior is likely on superhydrophilic 3D porous media; this behavior is between droplet spreading and penetration. The critical contact angle, $\\theta_{\\mathrm{c}}$ , below which the penetration of the porous surface by a liquid will take place, is given by  \n\n$$\n\\cos\\theta_{\\mathrm{c}}=\\frac{1-\\varphi_{\\mathrm{s}}}{r-\\varphi_{\\mathrm{s}}}\n$$  \n\nwhere $\\varphi_{s}$ is the solid fraction remaining dry during the wicking process and $r\\left(\\geq1\\right)$ is the surface roughness. For a porous surface, $r$ goes to infinity, and (3.3) predicts that the microstructure will be fully invaded by any liquid having a contact angle (as measured on a flat surface) of less than $90^{\\circ}$ . For rough surfaces ( ${\\bf\\zeta}_{r>1}$ but not infinity), the critical angle can vary between 0 and $90^{\\circ}$ . In the case of 3D porous materials, it is possible to switch from a superhydrophobic to a superhydrophilic state by slight changes in the surface chemistry [28]. This extreme transition in wetting behavior is enabled by the fact that only filled or empty pores are energetically favorable. Thus, compared to roughness‐induced superhydrophilicity, porosity‐induced superhydrophilicity offers some unusual possibilities for designing functional surfaces. In this regard, nanoporous thin films are particularly attractive since, in contrast to ­microporous films, they do not scatter light due to their small pore size [29].  \n\nBesides 3D capillary effects, a surface can be completely wetted by a 2D capillary effect. The 2D capillary effect has been observed on metal oxide semiconductors [30]. In general, ultraviolet (UV) light induces the formation of hydrophilic and oleophilic nanodomains on the surface. These different domains lead to nanostructured and microstructured flow channels for both aqueous and oily liquids. Channels for water flow are formed by oleophilic walls, while those for oily liquids are formed by hydrophilic walls; these structures may exhibit behavior that resembles the 2D capillary effect [31]. The hydrophilic areas are higher in position than oleophilic areas; this increases the formation of flow channels [30]. When a liquid droplet comes in contact with such a surface, it will flow along these nano‐ and microchannels and form a very thin film on the surface. This effect usually leads to superamphiphilicity; it was first observed in $\\mathrm{TiO}_{_2}$ coatings. The existence of surface superamphiphilicity has been explained as follows: after UV illumination, oxygen vacancies are created in the surface, and these vacancies induce the translation of the corresponding $\\mathrm{Ti^{4+}}$ sites to $\\mathrm{Ti}^{3+}$ . The as‐formed $\\mathrm{Ti}^{3+}$ sites are favorable to dissociate water molecules and further monolayers or multilayers of water form by molecular adsorption [32]. This results in the formation of surface hydrophilic domains, while leaving the rest of the surface oleophilic. In many cases, 2D and 3D capillary effects can coexist and work cooperatively on the same surface [30].",
        "category": " Results and discussion"
    },
    {
        "id": 4,
        "chunk": "# 3.3  NATURALLY OCCURRING SUPERHYDROPHILIC AND SUPERAMPHIPHILIC SURFACES  \n\nDuring evolution, plants and animals developed complex strategies to handle liquids, particularly for water management. With their sophisticated surface structures, living organisms can react to the environmental changes and deal with the extreme living conditions. There is enormous diversity in biological, multifunctional, and protective surfaces formed in different environments [33]. One of the most prominent examples is the surface of the lotus leaf, which exhibits extraordinary purity because of its extreme water‐repellent properties. The reason for this superhydrophobic behavior, discovered by Barthlott in the 1990s, [34] is a surface structure consisting of an epicuticular wax layer and a microscopic papillae structure. The wax causes low surface energy and the papillae, which consist of further branch‐like nanostructures, provide high surface roughness. These two features, the chemical composition and the topographic micro‐ and nanostructure of the surface, are most important for determining the wettability of a surface [35, 36]. In contrast to the lotus leaf, some plants show superhydrophilic properties. If a drop of water comes in contact with the surface of such plants, it will spread within seconds or even milliseconds. With regard to wetting behavior and environmental conditions, superhydrophilic plants can be divided into three groups: those that are permanently wet, those on which water is absorbed at surfaces, and those on which water spreads over surfaces. [37] The leaves of submerged water plants, for example, Anubias barteri, have permanently wet surfaces that consist of smooth cell surfaces without any waxes, papilla, or hairs. Plants that absorb water have porous surfaces or pores or multicellular hairs. For example, Sphagnum mosses consist of pores $10{-}20\\upmu\\mathrm{m}$ in diameter) that form a sponge‐like surface structure; this enables the plant to absorb amount of water that is up to 20 times the weight of the plant itself. This strategy of absorbing water is particularly important as a way for rootless plants to take up nutrients and as a way for plants to retain water in dry areas where dew is almost the only water source [38]. Ruellia devosiana belongs to the group of plants on which water spreads on surfaces. The leaves of this plant contain a complex surface with different cell types, including hair papilla, papilla cells, and glands; such a complex surface can spread a $5\\mathrm{-}\\upmu\\updownarrow$ drop of water within 0.2 s [38]. This is the fastest spreading behavior known for a plant species. Such a superhydrophilic surface is very important for plants in areas having high densities of precipitation, such as rainforests. Fast spreading creates a larger water–air interface, leading to an increase in evaporation, compared to hydrophilic, hydrophobic, or superhydrophobic surfaces. Through fast evaporation, a smooth gas exchange at the surface is ensured, and the growth of microorganisms at the surface is hindered. [37] The leaves of $R$ . devosiana show another interesting property. If a $10\\mathrm{-}\\upmu\\mathrm{l}$ droplet of oil is placed on its leaf, it also spreads to a flat film within 0.6 s. Surprisingly, water on a vertically oriented leaf stripe can move against gravity. Within 31 s, water on a leaf can move a vertical distance of $5\\mathrm{cm}$ . This phenomenon of water transport without any pressure against gravity might be exploited in applications of liquid‐transport devices [39].",
        "category": " Results and discussion"
    },
    {
        "id": 5,
        "chunk": "# 3.4  ARTIFICIAL SUPERHYDROPHILIC COATINGS",
        "category": " Materials and methods"
    },
    {
        "id": 6,
        "chunk": "# 3.4.1 $\\bar{\\mathsf{T i O}}_{2}$ Coatings  \n\nThe first superamphiphilic coating that consisted of a thin $\\mathrm{TiO}_{2}$ polycrystalline film from anatase sol on a glass substrate was reported by Wang et al. [10]. When the thin $\\mathrm{TiO}_{_2}$ film is exposed to UV radiation, the surface turns from slightly hydrophilic to superhydrophilic. The film exhibits a water contact angle of $72^{\\circ}\\pm1^{\\circ}$ before UV irradiation (Fig. 3.2a). After irradiation, water droplets spread on the film, resulting in a contact angle of $(0\\pm1)^{\\circ}$ (Fig. 3.2b). This change in wettability is more obvious when a $\\mathrm{TiO}_{2}$ ‐coated glass is exposed to water vapor. Without UV irradiation, the glass fogs (Fig. 3.2c), but upon irradiation, the glass becomes transparent (Fig. 3.2d), displaying an outstanding antifogging effect.  \n\nAll nonpolar liquids (e.g., glycerol trioleate and hexadecane) spread across the surface upon UV irradiation, exhibiting contact angles of $(0\\pm1)$ [10]. The same wettability change is observed on both anatase and rutile $\\mathrm{TiO}_{_2}$ surfaces, independent of their photocatalytic activities. Even after storage in dark conditions for a few days, the high amphiphilicity of the $\\mathrm{TiO}_{2}$ surface was maintained. A longer storage period induced a gradual increase in the water contact angle, revealing the surface wettability trend toward hydrophobicity. However, high amphiphilicity was repeatedly regenerated by  \n\n![](images/ea5a66eb4976e966b520bdcb2c949898a2a227968b494bbf07083a78e5fd13cc.jpg)  \nFig. 3.2  (a) A hydrophobic surface before ultraviolet irradiation. (b) A highly hydrophilic surface after ultraviolet irradiation. (c) Exposure of a hydrophobic $\\mathrm{TiO}_{2}$ ‐coated glass to water vapor. The formation of fog (small water droplets) hinders the view of the text on paper placed behind the glass. (d) Result of ultraviolet irradiation, creating an antifogging surface. The high hydrophilicity prevents the formation of water droplets, making the text clearly visible. Used with permission from Ref. 10. $\\mathbb{O}$ Nature Publishing Group.  \n\n![](images/90f7acfeaf337aea9f2861c4d3ff1baec24459e720913fa07bc025b6502a3826.jpg)  \nFig. 3.3  Mechanism of hydrophilicity on surfaces coated by $\\mathrm{TiO}_{2}$ . Under UV radiation, the valence of $\\mathrm{Ti^{4+}}$ changes to $\\mathrm{Ti}^{3+}$ , accompanied by the release of $\\mathbf{O}_{2}$ . This creates oxygen vacancies on the surface that can be occupied by water; hence, the surface becomes more hydrophilic. Used with permission from Ref. 42. $\\mathbb{O}$ Elsevier.  \n\nUV irradiation. The formation of a microstructured composite between hydrophilic and oleophilic phases, which is a result of the photogenerated $\\mathrm{Ti}^{3+}$ defects at definite sites, is considered to account for this unique behavior [40]. As a consequence, water can spread rapidly on a UV‐illuminated $\\mathrm{TiO}_{2}$ surface, which imparts superhydrophilic properties. ZnO possesses the same photoinduced hydrophilicity mechanism [41]. Increasing the concentration of nano‐ $\\mathrm{\\cdotTiO}_{_2}$ on a surface increases the number of accessible oxygen vacancies, thereby increasing the capacity for water absorption. Figure 3.3 illustrates the hydrophilicity mechanism for a $\\mathrm{TiO}_{2}$ ‐coated surface.  \n\nWhen placed in a dark environment, $\\mathrm{TiO}_{_2}$ ‐based surface coatings typically lose their superhydrophilic properties within minutes to hours; this limits their practical applications. In order to improve that Machida et al. found that by adding $30\\mathrm{mol}\\%$ $\\mathrm{SiO}_{_2}$ to a $\\mathrm{TiO}_{_2}$ coating, the contact angle of water is low immediately after the production, and hydrophilicity is preserved in a dark place [43]. At this point, extensive research has focused on chemical modifications to solid surfaces, such as ion doping, [44] metal deposition, [45] semiconductor coupling, [11, 20, 46], and further ­compositing with $\\mathrm{SiO}_{2}$ . [47] Moreover, superhydrophilicity might also be strengthened by increasing surface acidity or the number of hydroxyl groups at the surface.  \n\nToday, there are numerous studies dealing with superhydrophilic self‐cleaning surfaces containing titanium, but the majority of these publications still use glass as a standard substrate. On the other hand, one of excellent potential target for photoinduced cleaning, UV protection, and antimicrobial effects is the textile industry [48]. Special clothes, especially those that are endangered by staining with heavy ­contaminants such as soot, oils, or lubricants, are good candidates for applying superhydrophilic self‐cleaning surfaces. This is particularly important in the case of textile products that are either utilized outdoors or cannot be washed (because of their size or water sensitivity). Recently, a $\\mathrm{TiO}_{_2}$ coating was reported to be deposited on textile material using radiofrequency plasma‐enhanced chemical vapor deposition (RF PECVD) technique [49]. In this procedure, titanium (IV) chloride was used as the titanium source, oxygen was supplied as $\\mathbf{O}_{2}$ gas, and a cotton fabric served as the substrate. The stability of the coating remains unchanged after washing the fabric in a detergent solution, even after subsequent storage for 18 months. The number of nanoparticles absorbed on surfaces of fabrics and subsequent superhydrophilicity are affected by the efficiency of pretreatments [50]. For instance, studies have been performed on the effectiveness of some pretreatment methods, such as activating textile surfaces using plasma and vacuum UV irradiation [50, 51]. In these methods, fabric surfaces were modified by introducing negatively charged groups, thereby increasing the hydrophilicity of the fabrics. Alternatively, cross‐linking agents can be used to immobilize $\\mathrm{TiO}_{_2}$ nanoparticles on surfaces of the wool [52].  \n\nBesides glass and textile substrates, Ti‐containing mesoporous silica thin films (Ti‐MSTFs) have been prepared on Al and Al–Mg alloy substrates via a sol–gel/ spin‐coating method [53]. This coating method is applicable to various materials that have low thermal resistance. The resulting Al and Al–Mg alloy substrates coated with Ti‐MSTFs had highly hydrophilic properties, even under dark conditions, and showed photo‐induced superhydrophilicity under UV irradiation.",
        "category": " Results and discussion"
    },
    {
        "id": 7,
        "chunk": "# 3.4.2 $\\mathsf{S i O}_{2}$ Coatings  \n\nPrevious examples illustrate that addition of $\\mathrm{SiO}_{2}$ to titanium enhances the ­durability of superhydrophilic coatings. However, very recently, superhydrophilic coatings have been reported to be produced from mesoporous $\\mathrm{SiO}_{_2}$ without the addition of titanium [54]. The early theory established by Quéré et al. suggests that it is possible to enhance the wetting property of a surface by introducing roughness at the right scale [23, 55]. As a result, a mesoporous $\\mathrm{SiO}_{_2}$ superhydrophilic thin film was ­produced by the sol–gel method. When one coating layer of the film was applied, the contact angle for water decreased below $5^{\\circ}$ in $4\\mathrm{s}$ ; on films coated 6–12 times, the contact angle decreased below $5^{\\circ}$ in less than 1 s. Thus, superhydrophilicity increases as the number of coatings increases. The mechanism for such behavior can be understood from the simple relation derived by Wenzel et al. (see Section 3.2).  \n\nIn addition to mesoporous substances, nanostructured materials having structural elements between 1 and $100\\mathrm{nm}$ have the potential to improve surface functionalities of thin films. Materials with well‐ordered pores have been intensely investigated in many fields, including catalytic chemistry, adsorption chemistry, electrochemistry, and materials science. The most important families of silica‐based porous materials are zeolites, which have microporous structures with pore sizes less than $2\\mathrm{nm}$ , and mesoporous silicas, which have mesoporous structures with pore sizes between 2 and $50\\mathrm{nm}$ . Mesoporous structures have been formed via evaporation‐induced self‐assembly methods. The significant hydrophilic behavior can be achieved when transparent mesoporous silica thin films containing single‐site photocatalysts are used. The single‐site photocatalysts include moieties of Ti‐, V‐, Cr‐, Mo‐, and W‐oxide; [56] these photocatalysts show exclusive and remarkable catalytic properties that are not demonstrated by bulk catalysts [57, 58]. Because of electron localization, substitution sites for heteroatoms would attract water molecules; therefore, these materials become hydrophilic. After coating, the materials exhibit significant hydrophilic properties under dark conditions and photoinduced superhydrophilicity under UV irradiation. Among them, the W‐containing mesoporous silica thin film shows the best hydrophilic properties. This coating can be applied to various materials, including Al, Al–Mg alloys, and polycarbonate.",
        "category": " Results and discussion"
    },
    {
        "id": 8,
        "chunk": "# 3.5 METHODS FOR FABRICATING SUPERHYDROPHILIC AND SUPERAMPHIPHILIC SURFACES  \n\nMost solids are naturally rough, but their roughness is usually insufficient to reinforce a superhydrophilic state on a material surface. Inspired by naturally occurring examples of plants with superwetting properties for both water and oily liquids, such as R. devosiana, scientists have started creating artificial surfaces that exhibit similar properties [37]. Within the past decade, many different methods and materials have been implemented for the production of superhydrophilic and superamphiphilic surfaces on various substrates. Here, we review the most common techniques used for producing these coatings.",
        "category": " Results and discussion"
    },
    {
        "id": 9,
        "chunk": "# 3.5.1 Sol–Gel Method  \n\nIn general, the sol–gel method is used to synthesize porous network structures. It is a low‐temperature technique that is simple, affordable, and easy to control. By adjusting the composition of the precursor solution along with the hydrolysis and polycondensation processes, the resulting films can exhibit different morphologies and can contain different chemical components at the surface. The sol–gel method is widely used and is a convenient process for coating surfaces to obtain superhydrophilic and/ or superamphiphilic properties.  \n\n$\\mathrm{TiO}_{2}$ –polydimethylsiloxane $(\\mathrm{TiO}_{2}\\mathrm{-PDMS})$ composite films can be prepared by the sol–gel method from a $\\mathrm{Ti(OBu)_{4}}$ ‐benzoylacetone solution containing PDMS [59]. Contact angles measured for the $\\mathrm{TiO}_{2}{\\mathrm{-PDMS}}$ thin films show a wettability transition from hydrophobic to superhydrophilic states after treatment with oxygen plasma for $\\mathrm{{1s.Ma}}$ et al. [60] used this conventional technique to produce transparent mesoporous silica coatings that showed permanent superamphiphilicity with very fast spreading rates of within a few microseconds. To form the sol, they mixed tetraethyl orthosilicate (TEOS) and a poloxamer (Pluronic F‐127) with nitric acid; the solution was stirred at room temperature for $2\\mathrm{h}$ . Next, glass substrates were spin‐ coated with the sol, and further drying steps were performed. The approach aimed to create a superamphiphilic surface by increasing the roughness of an amphiphilic ­surface. Therefore, they introduced mesopores on the surface and obtained a ­superamphiphilic coating without using UV illumination. The ability of water and oily liquids, such as hexadecane, to spread with a contact angle of $0^{\\circ}$ on this surface is caused by high surface energy and high surface roughness. The high surface energy results from Si–O–Si and Si–OH groups present on the surface. The high capillary pressure inside the mesopores leads to the fast spreading mentioned above. Furthermore, the coating is transparent, because the pore diameter is in the range of $10\\mathrm{nm}$ , and therefore much less than the wavelength of visible light. This is an important feature with regard to possible industrial applications.  \n\nKako and Ye used a similar sol–gel method to produce a complex oxide $\\mathbf{(InNbO_{4})}$ coating with UV‐induced superamphiphilic properties [61]. A powder of $\\mathrm{In}(\\mathrm{NO}_{3})_{3}$ and niobium ethoxide was mixed in ethanol and spin‐coated on a quartz substrate. After 30 s of UV exposure, the contact angle of a water droplet changes from $55^{\\circ}$ to less than $5^{\\circ}$ , indicating superhydrophilicity. Moreover, the contact angles of $\\mathrm{CH}_{2}\\mathrm{I}_{2}$ and dodecane decrease to $15^{\\circ}$ and $0^{\\circ}$ , respectively, on such a surface. $\\mathrm{InNbO}_{4}$ is the first complex oxide to show superamphiphilicity after UV illumination.",
        "category": " Materials and methods"
    },
    {
        "id": 10,
        "chunk": "# 3.5.2 Layer‐By‐Layer Assembly  \n\nIn 1966, Iler published a method for creating multilayers of inorganic “colloidal particles” without using any organic molecules; the method is now known as layer‐by‐ layer (LbL) deposition [62]. He reported that multilayers of oppositely charged nanoparticles can be assembled by the sequential adsorption of oppositely charged nanoparticles onto substrates from aqueous suspensions. Although Iler’s work did not attract attention at the time, 25 years later, Decher et al. developed the LbL process to fabricate multilayer thin films from oppositely charged polyelectrolytes. [63] Over the past 15 years, the LbL technique has received enormous attention. To enhance the wettability of the films, micro‐ or nanoparticles are commonly integrated to increase surface roughness. The LbL assembly has the advantage of precisely controlling the film thickness; thus, it is a desirable technique for fabricating ­transparent coatings. Liu and He [64] reported an LbL method for obtaining superhydrophilic coatings, in which raspberry‐like silica nanospheres were prepared by the electrostatic self‐assembly of polyelectrolytes and monodispersed silica nanoparticles of two different sizes.  \n\nAlternatively, the production of superhydrophilic coatings composed completely of nanoparticles has been proposed [65]. In addition, thin films consisting of $\\mathrm{TiO}_{_2}$ and $\\mathrm{SiO}_{2}$ nanoparticles have been prepared via LbL deposition. The presence of nanopores in $\\mathrm{TiO}_{2}/\\mathrm{SiO}_{2}$ nanoparticle coatings leads to useful ­functionalities, including antireflective and antifogging properties. The last step of this method includes calcination at high temperatures. This well‐known calcination process burns out the polymer component of the film and fuses the silica nanoparticles together via the formation of stable siloxane bridges [66]. Unfortunately, the high temperatures employed during this curing process also limit the substrate materials that can be coated; plastics with low melting points are not suitable for this type of coating.",
        "category": " Results and discussion"
    },
    {
        "id": 11,
        "chunk": "# 3.5.3 Electrochemical Methods  \n\nElectrochemical methods include electrochemical deposition, anodization, galvanic cell reactions, and electrochemical polymerization. These are facile methods for ­constructing rough surfaces, regardless of the size and shape of the substrate.  \n\nShibuichi et al. have shown that the treatment of the surface of an aluminum plate with anode oxidation leads to superamphiphilic properties [67]. The plate was dipped into an acidic solution, and oxidation was performed at a current density of $10\\mathrm{mA}/\\mathrm{cm}^{2}$ . This method increases surface roughness; in fact, the analysis of the aluminum ­surface showed it to be fractal. Such a rough surface showed superwettable properties for both polar and nonpolar solvents.  \n\nNanostructured conducting polymers generally show superhydrophilic properties. Among these, the surface of polyaniline exhibits amphiphilic behavior. Therefore, Zhang et al. used an electrochemical‐template‐free method for the direct deposition of nanostructured polyaniline (PANI) on a substrate such as stainless steel [68]. This substrate was chosen, because it is widely used in industrial equipment. The PANI nanofibers were prepared using sulfuric acid and aniline at $0.85\\mathrm{V}$ on a stainless steel electrode. The structure and size of the nanofibers can be controlled by polymerization time. A water droplet on this modified PANI surface spreads very fast and reaches a contact angle of $5^{\\circ}$ within a few milliseconds. Droplets of organic solvents, such as acetone and hexane, spread even faster than a water droplet. Such a ­superamphiphilic surface is of high interest from both scientific and industrial ­viewpoints, as PANI shows simple nonredox doping/dedoping chemistry, is environmentally stable, and the superwetting property is permanently stable. This method exemplifies a convenient way for producing polymer‐functionalized surfaces that exhibit superamphiphilic properties.",
        "category": " Results and discussion"
    },
    {
        "id": 12,
        "chunk": "# 3.5.4 Electrospinning  \n\nElectrospinning is a technique used to produce fibers with diameters ranging from micrometers to nanometers [69, 70]. In this technique, the sample solution is pumped through a nozzle to which a high electric voltage is applied. Owing to the evaporation of the solvent, the solution jet solidifies and forms fibers that are deposited on a collector. The final morphology strongly depends on the starting solution concentration.  \n\nSuperhydrophilic surfaces can be generated by $\\scriptstyle\\alpha-\\mathrm{Fe}_{2}\\mathrm{O}_{3}$ nanofibers with contact angles of $0^{\\circ}$ for water [71]. The $\\scriptstyle\\mathbf{\\alpha}\\mathbf{-Fe}_{2}\\mathbf{O}_{3}$ nanofibers are produced by electrospun poly(vinyl alcohol)/ferrous acetate composite nanofiber precursors and high‐­temperature calcination in air. The experimental results show that the morphology and crystalline phase of $\\scriptstyle\\alpha-\\mathrm{Fe}_{2}\\mathrm{O}_{3}$ nanofibers are influenced by the content of ferrous acetate in composite nanofibers and the calcination temperature. By controlling the calcination temperature, the magnetic property of $\\scriptstyle\\alpha-\\mathrm{Fe}_{2}\\mathrm{O}_{3}$ nanofibers can be tuned from superparamagnetic to ferromagnetic.  \n\nSuperamphiphilic coating applicable to textiles can be produced by electrospinning. Lim et al. [72] fabricated Janus fabrics with superwetting properties by using polyacrylonitrile (PAN) as the starting material. The method is very useful, because it enables the synthesis of micro‐ and nanofibers with definite diameters that can be coated on different substrates. To produce superamphiphilic Janus fabrics, a polymer solution of PAN, dimethylformamide, and TEOS was electrospun into nanofibers. Next, the fibrous mats were heated at $200^{\\circ}\\mathrm{C}$ , and the polymer solution mentioned earlier was electrospun onto the treated mats. Then, the mats were peeled from the substrate. This simple method provides a new way for producing functionally smart materials, which are of high interest for possible industrial applications.",
        "category": " Results and discussion"
    },
    {
        "id": 13,
        "chunk": "# 3.5.5 Etching  \n\nEtching is often used as an additional step in many procedures to improve the roughness of substrates on different scales. For example, Kim et al. demonstrated a facile chemical etching method for fabricating a superamphiphilic surface on a silicon wafer [73]. On a clean silicon wafer, gold nanoclusters were deposited by thermal evaporation and etched in an $\\mathrm{HF/H}_{2}\\mathrm{O}_{2}/\\mathrm{DI}$ water solution. The gold clusters catalyzed the etching process at room temperature; therefore, the wafer was selectively etched. Next, the surface was coated with a self‐assembled monolayer material. The surface of the silicon wafer showed superamphiphilic behavior, upon exposure of a coated wafer to deep UV light $(\\lambda\\sim254\\mathrm{nm})$ .  \n\nA silicon wafer exhibiting superhydrophilicity can be produced by an electroless (EE) silicon etching method [74]. The EE method is a top–down technique that can be used to modify the morphology of the surface with nanoscale structures over a large area at room temperature.  \n\nIn industry, titanium alloys are often coated with commercially pure titanium (Ti) via physical vapor deposition (PVD), especially if they are used in the clinical sector, such as in the hospital. The coated surfaces have high roughness and show superoleophilicity. A droplet of mineral oil spreads with a critical contact angle of about $0^{\\circ}$ , while the contact angle of water droplet was $145^{\\circ}$ , indicating hydrophobicity. Jennissen and Lüers developed a simple chemical treatment to improve the wetting properties of such commercially available Ti‐PVD surfaces to superamphiphilicity [75]. After cleaning, the samples are etched with chromosulfuric acid at $240^{\\circ}\\mathrm{C}$ for $30\\mathrm{min}$ . A water droplet spread on the treated Ti‐PVD surface to a critical contact angle of $0^{\\circ}$ , indicating additional superhydrophilicity.",
        "category": " Results and discussion"
    },
    {
        "id": 14,
        "chunk": "# 3.5.6  Plasma Treatment  \n\nBecause of its ease in handling and very effective results, plasma treatment is one of the most used techniques for producing superhydrophilic or superamphiphilic ­surfaces. Oxygen plasma treatment modifies the surface properties of polymers, resulting in the hydrophilization of their surfaces [76]. The roughness and ­morphology of a treated polymer surface depend on the time of exposure [77].  \n\nZimmermann et al. used this method to prepare superamphiphilic surfaces based on a silicone nanofilament coating [78]. First, a desired substrate, such as a glass slide, was coated with silicone nanofilaments via a simple CVD coating at room temperature. The resulting surface was completely wetted by a droplet of hexadecane, which indicates superoleophilic and superhydrophobic properties. After exposing the surface to oxygen plasma, droplets of both hexadecane and  water spread completely on the surface, thus showing superamphiphilic properties.  \n\nLiu et al. used a plasma method to introduce superamphiphilic properties to a ­surface consisting of carbon nanotubes (CNT) that were decorated with silver nanoparticles (Ag) [79]. First, a silicon‐wafer substrate was coated with a paste of $\\mathbf{Ag}@\\mathbf{CNT}.$ By treating the modified substrate with Ar plasma, the contact angle of a water droplet decreased from $85^{\\circ}$ to almost $0^{\\circ}$ within $5\\mathrm{{min}}$ . The treated surface also showed superoleophilic properties with contact angles of $0^{\\circ}$ for diiodomethane and ethylene glycol. The plasma treatment increases the number of hydrophilic functional groups, such as hydroxyl and carboxyl groups, on the surface. In addition, proton donor components are increased, which leads to an increase in oleophilicity due to the capillary effect. This approach can be used, for instance, for the production of superamphiphilic Ag‐CNT electrodes.  \n\nPolymeric superamphiphilic surfaces were produced by Ellinas et al. using a plasma etching method [80]. Polystyrene colloid particles were deposited on a poly(methylmethacrylate) (PMMA) surface via spin coating. This colloidal lithography was followed by oxygen plasma etching, leading to a surface with highly ordered arrays of micropillars of PMMA. The height and diameters of the pillars can be adjusted very accurately by controlling the etching time and voltage. This method led to high surface roughness due to the introduced micro/nanoscale topography. The superwetting properties of such a nanotextured surface are stable over a long time.",
        "category": " Results and discussion"
    },
    {
        "id": 15,
        "chunk": "# 3.5.7  Hydrothermal Method  \n\nThe hydrothermal method is a wet chemical procedure in which single crystals are synthesized in hot water under high pressure. This method allows precise control over crystal morphology and composition.  \n\nA microwave‐assisted hydrothermal method for the fabrication of superamphiphilic titanate network (STN) films was developed by Li et al. [81]. The SNT films consisted of twisted multiwalled titanate nanotubes grown on a Ti foil. Compared to other hydrothermal methods, this method is rapid $(10\\mathrm{min})$ and simple. Droplets of both polar and nonpolar solvents spread immediately with contact angles below $1^{\\circ}$ . On untreated Ti samples, the contact angles for water and $\\mathrm{CCl}_{4}$ were $73^{\\circ}$ and $15^{\\circ}$ , respectively. This facile production method also allows the integration of other atoms, such as PbS or CdS. This is of high interest for possible indoor applications, because Ti‐containing coatings demand UV light. By modifying the surface with materials sensitive to visible light, such as $\\mathrm{Pb}\\mathrm{S}$ or CdS, the capability for light absorption can be shifted to longer wavelengths. Films of PbS‐STN and CdS‐STN were produced via the same microwave‐assisted hydrothermal method and showed superamphiphilicity similar to the STN films. Compared to $\\mathrm{TiO}_{2}$ surfaces, the superamphiphilicity remained permanently stable, even after 6 months, without the need for light illumination.",
        "category": " Results and discussion"
    },
    {
        "id": 16,
        "chunk": "# 3.5.8 Dip Coating  \n\nDip coating is a very simple and popular method for creating thin films. Uniform films can be applied to flat or cylindrical substrates. A related technique often used in industrial applications is spin coating.  \n\nChen et al. developed a superhydrophilic, superamphiphilic, scratch‐resistant coating via a dip‐coating process [82]. The coating consisted of aggregated zeolite nanoparticles, which resulted in the useful properties mentioned earlier. The ­prepared surfaces had higher roughness and strength compared to amorphous $\\mathrm{SiO}_{2}$ surfaces. In  addition, the coatings showed a high antireflective and superamphiphilic properties that are of high interest in, for instance, the production of solar panels.  \n\nIn addition, surfaces with tunable wettability for guiding water droplets can be produced by dip coating [83]. For example, silicon nanowires can be dip‐coated in dodecyltrichlorosilane to attain a superhydrophobic state, and then the wettability can be converted to hydrophilic via UV‐enhanced photodecomposition.",
        "category": " Results and discussion"
    },
    {
        "id": 17,
        "chunk": "# 3.5.9  Phase Separation  \n\nThe phase‐separation method is commonly used for the fabrication of porous polymer coatings. In this method, the starting material is typically a polymer solution or a polymer blend, and phase separation is induced by changing temperature or pressure or both. For example, porous polymers with switchable wettability can be produced by the condensation of organo‐triethoxysilane in a mixture of an organic solvent and water [28].  \n\nIn another approach, Zhang et al. reported a one‐step production method for a superhydrophilic polymer surface. A nylon 6,6 plate was swelled by formic acid and then immersed in a coagulate bath to induce precipitation. Microparticles with ­nano‐ protrusions were generated and linked together, covering the surface. After drying, the as‐formed surface showed superhydrophilic abilities due to the hydrophilic nature of nylon and the network of micro/nano flower‐like particles [84]. Poly(l‐lactic acid) substrates can be prepared using a phase‐separation‐based method. Using argon‐ plasma posttreatment, the wettability of the surfaces can be controlled in the range from superhydrophobic to superhydrophilic [85].",
        "category": " Results and discussion"
    },
    {
        "id": 18,
        "chunk": "# 3.5.10  Templating Method  \n\nTemplating is an effective method for constructing surfaces with highly controlled morphology. The inverse of a template can be formed and replicated again for producing positive replicas and offering the possibility to template natural biosurfaces.  \n\nNanoporous anodic aluminum oxide (AAO) has been commonly used for the pressure‐driven imprint process. By choosing AAO replications with different pore diameters and channel lengths, the diameter and height of surface‐projecting ­nanostructures can be controlled [86–88].  \n\nAnother widely used template material is polystyrene. Li et al. used a polystyrene colloidal monolayer as a template to produce a hierarchically ordered $\\mathrm{TiO}_{_2}$ ­hemispherical array with hexagonal not‐close‐packed tops [89]. The obtained coating exhibited excellent superhydrophilicity with a contact angle of $0^{\\circ}$ without UV radiation. A very novel approach for making superhydrophobic/superhydrophilic patterns is based on printing an “ink” (an ethanol solution of a phospholipid) on a porous superhydrophobic surface [90].",
        "category": " Results and discussion"
    },
    {
        "id": 19,
        "chunk": "# 3.6 APPLICATIONS",
        "category": " Results and discussion"
    },
    {
        "id": 20,
        "chunk": "# 3.6.1 Self‐Cleaning  \n\nOne of the most attractive applications, which is already being commercialized, is the production of self‐cleaning coatings. Self‐cleaning coatings are broadly classified into two major categories: superhydrophilic (water contact angles close to $0^{\\circ}$ ) and superhydrophobic (water contact angles ${>}150^{\\circ}$ ). Both clean themselves by the action of water. On a superhydrophilic surface, water spreads, and pollutants can be removed by a stream of water [91]. Water spreads because adhesive forces between the liquid and the substrate play a more important role than internal cohesive forces within the liquid [92]. In addition, these strong adhesive forces between water and surface prevent interactions between impurities and the surface, enabling their easy removal [93]. Water penetrates between impurities and the surface so that the impurities can be washed away [37]. On superhydrophilic surfaces, traces of water will evaporate much faster than on superhydrophobic surfaces, thereby contributing to a cleaner surface. On superhydrophilic coatings consisting of $\\mathrm{TiO}_{_2}$ or other semiconductor materials, two self‐cleaning mechanisms can occur [94]. One is the photocatalytic effect that is induced by sunlight to chemically breakdown organic pollutants. This effect is a consequence of its semiconductor nature [95]. Photocatalytic properties of $\\mathrm{TiO}_{_2}$ are beyond the scope of this chapter and can be found elsewhere [96]. The decomposed impurities can be easily washed away by water that immediately spreads to a film on superhydrophilic surfaces [97]. The other self‐cleaning mechanism involved in $\\mathrm{TiO}_{_2}$ coatings is the superoleophilic effect. Superamphiphilic surfaces enhance self‐ cleaning, because oily liquids spread completely, thereby increasing the contact area with the $\\mathrm{TiO}_{_2}$ coating and promoting the faster decomposition of contaminants.  \n\nThe self‐cleaning features of superhydrophilic and/or superamphiphilic surfaces are useful for a broad range of possible applications. For example, a soiled kitchen exhaust fan consisting of plastic cannot be cleaned only with water. However, if the exhaust fan is coated with a superhydrophilic and/or superamphiphilic surface, oily contaminants can easily be removed by a stream of water [97]. Traffic signs soiled by exhaust gases from automobiles could be cleaned in the same way or even by rainfall [94]. Self‐cleaning is of particular interest for building walls and windows where cleaning is very difficult. A building wall (such as in $\\operatorname{Fig}3.4)$ can be cleaned just by rainwater, saving cleaning costs and time [97].  \n\n![](images/738e07800e5961b6c141cc72b9059c992528b5f4ae5aafa29f5d3e77b08e1d00.jpg)  \nFig. 3.4  Comparison of a conventional tent material (left) and a self‐cleaning material coated with $\\mathrm{TiO}_{_2}$ (right). Used with permission from Ref. 98. $\\circleddash$ The Japan Society of Applied Physics.",
        "category": " Results and discussion"
    },
    {
        "id": 21,
        "chunk": "# 3.6.2  Antifogging and Antireflective Coatings  \n\nAntifogging surfaces are needed to maintain visibility through transparent surfaces in high‐humidity environments. Since relative humidity is a strong function of temperature, a vapor can easily reach its saturation limit in response to changes in ­temperature. In addition, condensation often occurs when a cold surface rapidly comes into contact with warm moist air. The resulting condensation appears as tiny droplets. These droplets randomly scatter light, causing the surfaces to be translucent or foggy. A superhydrophilic coating can prevent fogging, because water spreads on the rough hydrophilic surface to form a thin film instead of droplets. In addition to antifogging, superhydrophilic coatings commonly exhibit antireflective properties.  \n\nGenerally speaking, surface roughness and transparency are competitive ­properties. Hydrophilicity increases as surface roughness increases, while the transparency of rough surfaces often decreases owing to the Mie scattering effect [99]. Antireflective properties reduce glare and maximize the amount of light passing through, an effect that shows promise for improving materials used in greenhouses and solar‐cell panels. As a consequence, it is of great importance to develop a simple method for fabricating transparent and superhydrophilic coatings with specific functional properties. To facilitate commercialization, such coatings also need to be mechanically stable and cost‐effective.  \n\nAntireflective properties are very important for the production of solar modules, because solar conversion efficiency depends on optical transmission [82]. Son et al. showed that the efficiency of solar panels coated with a superhydrophilic surface is reduced by only $1.4\\%$ after 12 weeks in an outdoor environment. In comparison, the efficiency of uncoated solar panels is reduced by $7.8\\%$ [3].  \n\nRubner’s Research Group at the Massachusetts Institute of Technology (MIT) fabricated a superhydrophilic polyelectrolyte multilayer film with exceptional ­antifogging and antireflective properties [29]. This film was created from LbL assembled $\\mathrm{SiO}_{2}$ nanoparticles and a polycation. With suitable control over the processing conditions $\\mathrm{\\Phi_{\\mathrm{pH}}}$ , concentration, etc.) and proper choice of nanoparticle size, ­multifunctional nanoporous thin‐film coatings were created. The resulting film was superhydrophilic, antifogging, and significantly suppressed the reflection of light (antireflective). To improve mechanical durability, the final deposited film was heated to about $500^{\\circ}\\mathrm{C}$ for $^{4\\mathrm{h}}$ . After this process, the resulting thin‐film coating could withstand aggressive rubbing and easily passed a standard scotch‐tape peel test. A simple experiment was carried out to demonstrate the antifogging properties of the obtained superhydrophilic coating. Two glass slides, one coated and the other uncoated, were placed in a refrigerator at $-18^{\\circ}\\mathrm{C}$ for some time. After removal from the refrigerator, the coated slide remained transparent and not fogged, while water condensate readily formed on the uncoated slide. On a coated surface, water remains on a wet surface as a continuous sheet instead of dewetting to form droplets. This property of coated films provides the opportunity for maintaining good visibility even when the nanopores in the film are fully saturated with water. Since these coatings are nanoporous structures composed of $\\mathrm{SiO}_{2}$ and air‐filled pores, their refractive index lies between those of silica $(n\\approx1.45)$ and air $(n=1)$ ); this makes them good candidates for antireflective applications.  \n\nAs one of the first and most prominent superhydrophilic coatings, $\\mathrm{TiO}_{_2}$ (discussed earlier) also possesses impressive antifogging properties. However, the need for UV radiation to produce desirable surface properties remains a major drawback to using this coating material [10]. To eliminate this drawback, porous $\\mathrm{TiO}_{2}/\\mathrm{SiO}_{2}$ (TS) composite thin‐film coatings with superhydrophilic performance have been produced by a sol–gel process under template‐free conditions [47]. When the $\\mathrm{SiO}_{_2}$ content was set to $20\\%$ , the TS composite coatings exhibited a water contact angle of $2.5^{\\circ}$ without UV radiation, and the time needed for a water droplet to completely spread over the surface was less than $1\\mathrm{s}$ . The resulting coatings showed excellent antifogging properties, which are attributed to the instantaneous spreading of droplets absorbed on the coated glass surface to form sheets similar to a water membrane. The water therefore evaporates immediately, providing quicker drying of the surface and keeping it clear for a long time (Fig. 3.5).  \n\nThere are approaches other than $\\mathrm{TiO}_{_2}$ technology for obtaining antifogging and antireflective coatings [100, 101]. You et al. produced a thin‐film coating of $\\mathrm{La(OH)}_{3}$ nanorods on a glass substrate by simple self‐stacking methods [102]. These single layer coatings significantly reduced reflective losses for visible light. To improve the roughness of these coatings, silica nanoparticles were deposited. The resulting $\\mathrm{La(OH)}_{3}/\\mathrm{SiO}_{2}$ film showed nanoporosity‐driven superhydrophilicity and the antifogging property with no significant loss in the antireflective property.  \n\n![](images/7e8aef52f636ab3c39a32114aa4ec0cfc504461e96e4e790e350ce07e50faf72.jpg)  \nFig. 3.5  Comparison of antifogging behavior of a bare glass slide (right) with a slide partially coated (left) with superhydrophilic porous TS film. Used with permission from Ref. 47. $\\mathbb{O}$ Elsevier.  \n\nRecently, a novel antifogging coating has been developed for plastic substrates; the coating consists of a hydrophilic/hydrophobic bilayer structure [103]. The bottom layer is hydrophobic colloidal silica, which acts as a mechanical support and a hydrophobic barrier against water penetration. Atop this layer, an antifogging coating was applied; this top layer incorporates a superhydrophilic species synthesized from Tween‐20 (surfactant), isophorone diisocyanate (coupling agent), and 2‐hydroxyethyl methacrylate (monomer). The resulting coating was transparent, wearable, and could be soaked in water for 7 days at $25^{\\circ}\\mathrm{C}$ without downgrading its antifogging capability.  \n\nBesides superhydrophilic and highly hydrophilic surfaces [104–107], the so‐ called zwitter‐wettable surfaces also exhibit excellent antifogging and antifrost properties [108]. Zwitter‐wettable coatings have the ability to rapidly absorb molecular water from the environment while simultaneously appearing hydrophobic when probed with water droplets (Fig. 3.6). They are prepared by using hydrogen‐bonding assisted LbL assembly of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA). In an additional step, functionalizing the nanoblended PVA/PAA multilayer with poly(ethylene glycol methyl ether) (PEG) segments produced significantly enhanced antifogging and frost‐resistant properties.  \n\nAntifogging coatings undoubtedly impact diverse applications such as sports and sanitary equipment, lenses for optical devices, automobile windshields, windows, eyeglasses, camera lenses, or any other transparent glass or plastic surface. For example, when a food item is packaged and displayed in a refrigerated cabinet, the relative humidity inside the package increases because of the decrease in temperature. Consequently, water tends to condense on the inner surfaces of packages, which, if treated to be antifogging, can enhance the visual display of the packaged items. Commercially available antifogging and antireflective coatings will be discussed in the following sections.  \n\n![](images/5fb70577a11ea5208f36f871501b6bacd16bec26e0b181f085b8064f3371faa9.jpg)  \nFig. 3.6  (a) Temporal evolution of water‐drop profiles on a PEG‐functionalized PVA/PAA multilayer film at $37^{\\circ}\\mathrm{C}$ and $80\\%$ RH. (b) Changes in water contact angle over time for a PVA/PAA multilayer film at $37^{\\circ}\\mathrm{C}$ and $80\\%$ RH. (c) Changes in water contact angle over time for a PEG‐functionalized PVA/PAA multilayer film at $22\\pm1^{\\circ}\\mathrm{C}$ and $40\\pm10\\%$ RH and at $37^{\\circ}\\mathrm{C}$ and $80\\%$ RH. (d) Photograph of a water drop placed on a PEG‐functionalized PVA/ PAA multilayer film after being transferred from $-20^{\\circ}\\mathrm{C}$ to $22\\pm1^{\\circ}\\mathrm{C}$ , $40\\pm10\\%$ RH. Inset photograph shows the magnified image of the water drop with a contact angle above $90^{\\circ}$ . Only the glass coated with PEG‐functionalized PVA/PAA multilayer resists formation of frost. (e) Schematic of zwitter‐wettability. MIT logo in the figure is used with permission from the Massachusetts Institute of Technology. Used with permission from Ref. 108. $\\mathbb{O}$ American Chemical Society.",
        "category": " Results and discussion"
    },
    {
        "id": 22,
        "chunk": "# 3.6.3  Antifouling Properties  \n\nFouling is the deposition of an unwanted material on solid surfaces to the detriment of function. In marine engineering, fouling refers to the growth of microorganisms, algae, plants, etc., on a surface immersed in seawater.  \n\nIn membrane technologies, fouling is the deposition of retained particles, macromolecules, and salts at the membrane surface or inside the pores. This fouling is caused by interactions between the membrane surface and the foulants in many ­different forms. The foulants not only physically interact with the membrane surface but also chemically degrade the membrane material [109]. It is generally assumed that fouling decreases with an increase in the hydrophilicity of the polymeric material. This assumption seems reasonable, since with an increase in membrane surface hydrophobicity, hydrophobic organic molecules are driven toward the surface, enhancing surface contamination. Water separation membranes should be designed to maximize their surface affinity with water so as to increase their resistance to fouling [110]. Antifouling properties arise because of the strong hydration layer of the hydrophilic surface, which opposes the adsorption of molecules and particles on the membrane surface [111]. Elimelech et al. experimentally confirmed these hypotheses by producing superhydrophilic thin‐film composite membranes on which increased resistance to fouling was observed [112, 113].  \n\nBiomedical devices can be subjected to fouling via the deposition of surplus cells, proteins, and biomolecules. Patel et al. recently examined two types of superhydrophilic surfaces as potential surfaces in microfluidic devices: [6] polyester films treated by oxygen plasma and indium–tin‐oxide‐coated glasses treated by an electrochemical method. Fluorescence microscopy studies confirmed the significantly reduced adhesion of fluorescein and fluorescent proteins after the surfaces were treated to be superhydrophilic, thereby indicating their potential for antifouling applications.  \n\nMany types of hydrophilic polymer surfaces with suitable wettability and antifouling properties, especially bio‐antifouling, have been proposed and prepared by surface‐initiated controlled radical polymerization of vinyl monomers with specially designed hydrophilic functional groups; this process leads to densely grafted polymers on the solid surface, which are the so‐called polymer brushes [114, 115]. Kobayashi et al. investigated the behavior of different polyelectrolyte polymer brushes [116]. They observed that polyelectrolyte brushes repel both air bubbles and hexadecane in water. Even when silicone oil was spread on the polyelectrolyte brush surfaces in air, once they were immersed in water, the oil quickly rolls up and detaches from the brush surface. Figure 3.7 shows the contact angle of silicone oil on the surface of a poly[2-(methacryloyloxy)ethyl phosphorylcholin] (PMCP) brush and on an unmodified silicon wafer in air and in water. The oil detachment observed on the superhydrophilic polyelectrolyte brush in water was caused by low adhesive forces between the brush and the oil; this could contribute to its excellent antifouling and self‐cleaning properties.",
        "category": " Results and discussion"
    },
    {
        "id": 23,
        "chunk": "# 3.6.4  Enhanced Boiling Heat Transfer  \n\nOver the past 80 years heat transfer under boiling conditions has been investigated by numerous scientists worldwide [117, 118]. One of the most important parameters in the performance of pool boiling is the critical heat flux (CHF). The CHF is the maximum heat flux at which boiling heat transfer sustains its high cooling efficiency. When a surface reaches CHF, it is coated with a vapor film, which then interferes with contact between the surface and the liquid and decreases the heat‐transfer efficiency. Subsequently, the system temperature increases, and if it exceeds the limits of its containment materials, system breakdown occurs. Because of this, every  \n\nThe right figures are schematic images of the oil on brush in the left photographs.  \n\n![](images/e8c93088012c2c75308480157da59d04009220e8d32f247d573032a41554532d.jpg)  \nFig. 3.7  Wettability‐reversion phenomena of a silicone oil droplet $(5.0\\upmu\\mathrm{l}$ , Shin‐Etsu Chemical Co. KF‐96‐100CS) on a PMPC brush (a, b) in air and (c,d) in water. Photograph (c) (side view) displays an oil droplet on a PMPC brush substrate in water, showing superoleophobicity, with a contact angle of $173^{\\circ}$ . Used with permission from Ref. 116. $\\mathbb{O}$ American Chemical Society.  \n\nsystem integrates a safety margin by operating at a heat flux much lower than CHF;   \nof course, this safety measure limits the efficiency of the system.  \n\nIn 1993, Wang and Dhir showed that CHF can be increased by enhancing surface wettability [119]. Since then, a number of studies have reported that surface wettability is an important factor affecting boiling heat transfer [120, 121]. In 1995, Choi introduced the concept of nanofluids [122]. Nanofluids are a new class of nanotechnology‐based heat‐transfer fluids, engineered by dispersing and stably suspending nanoparticles (with dimensions on the order of $1{-}50\\mathrm{nm},$ ) in traditional heat‐transfer fluids. The base fluids include water, ethylene, oil, biofluids, and polymer solutions. Various materials are commonly used as nanoparticles, including chemically stable metals (e.g., copper, gold, silver), metal oxides (e.g., alumina, bismuth oxide, silica, titania, zirconia), ­several allotropes of carbon (e.g., diamond, CNTs, fullerenes), and functionalized nanoparticles [123]. Vertically aligned nanoforests of hydrophilic/ superhydrophilic nanorods [124], nanowires [125], and water‐based alumina nanofluids [126] have shown the potential for considerably improving boiling heat transfer. The increase in CHF is attributed to roughness, high surface‐tension forces of superhydrophilic ­nanostructures for pumping in fresh liquid, and capillary wicking phenomena [127].  \n\nPhan et al. investigated the influence of surface wettability on nucleate boiling heat transfer by varying the water contact angle [120]. It was found that increasing surface wettability increases the vapor‐bubble departure radius and reduces the bubble emission frequency. Hsu et al. coated a plain copper surface with silica nanoparticles and found that the superhydrophilic surface with a contact angle less than $10^{\\circ}$ has a larger CHF than the hydrophilic one with a contact angle of $16^{\\circ}$ [121]. The superhydrophilic surface exhibits an increase in CHF of approximately $100\\%$ compared to a plain copper surface. For superhydrophilic surfaces, small bubbles form on the surface when the wall temperature is $100^{\\circ}\\mathrm{C}$ (illustration in Fig.  3.8). These small growth bubbles move and merge with other bubbles and then depart the surface. However, the size and number of growth bubbles on the heating surface both increase when the surface is more hydrophobic, as shown in Fig. 3.8b and c. Fig. 3.8d shows the effects of growth bubbles during boiling on a superhydrophobic surface; bubbles spread over the surface and coalesce with bubbles formed at other sites, causing a large area of the surface to be covered with a vapor film [13]. This vapor film interferes with contact between the surface and the surrounding liquid and decreases heat‐transfer efficiency.  \n\n![](images/a97b98487fb68e6d375d4359d39d2cbd32bc15639c8d2bba193cdbfe47cc6f73.jpg)  \nFig. 3.8  Effects of surface wettability on the growth of bubbles during boiling on (a) superhydrophilic surface, (b) hydrophilic surface, (c) hydrophobic surface, and (d) superhydrophobic surface. In (d), the bubbles coalesce into a thin film that impedes heat transfer from the surface. Used with permission from Ref. 120. $\\mathbb{C}$ Elsevier.  \n\nSince CHF is the upper limit for nucleate boiling, the enhancement of CHF offers the potential for major improvements in the performance of many practical applications. For example, the use of superhydrophilic coatings with higher CHFs could enable the effective thermal management of even smaller and more powerful electronic devices, improve power‐up rates in commercial nuclear plants, allow the design of more compact heat exchangers for the chemical industry, among others.  \n\n![](images/e692db6669748958a96d7a9ade29a69edaf3dfc237822d31e2beb07714a86132.jpg)  \nFig. 3.9  Energy‐saving system in which exterior surfaces of buildings are covered with a $\\mathrm{TiO}_{2}$ coating. The coating is made superhydrophilic by exposure to UV radiation in sunlight and then by pumping of stored rainwater over those surfaces. Evaporation of water from the surfaces helps cool the building and reduces load on air conditioning equipment. Used with permission from Ref. 98. $\\circledcirc$ The Japan Society of Applied Physics.",
        "category": " Results and discussion"
    },
    {
        "id": 24,
        "chunk": "# 3.6.5 Efficient Water Evaporation  \n\nA current problem in big cities around the world is the so‐called “heat island phenomenon.” Increasing amounts of exhaust gases from traffic and decreasing areas of lakes and green land significantly increase temperatures in cities. Hashimoto et al. suggested the use of superhydrophilic surfaces to prevent the heat island phenomenon. The facades of a building equipped with a superhydrophilic surface can be completely covered by a thin water film. Through quick and efficient water evaporation, the building can be cooled by the flux in latent heat (Fig. 3.9). Therefore, if small amounts of collected rainwater are continuously sprinkled onto the superhydrophilic surface, the temperature increase in cities could be reduced. Thinner water layers (in the range of $0.1\\mathrm{mm}$ ) improve the efficiency of cooling buildings and the surrounding air. The decrease in temperature brings another positive impact: less air conditioning is needed, leading to a total decrease in energy consumption of more than $10\\%$ .",
        "category": " Results and discussion"
    },
    {
        "id": 25,
        "chunk": "# 3.6.6 Switchable and Patterned Wettability Coatings  \n\nCombining the two extremes, superhydrophilicity and superhydrophobicity on the same surface in precise 2D patterns opens the possibilities for exciting new functionalitiesinawidevarietyofapplications.Generally,superhydrophilic–­superhydrophobic patterned surfaces are used to control bioadhesive and nonbioadhesive regions. The “switch” between the two regimes can be triggered by heat, light, or a solvent.  \n\nSwitching between superhydrophobicity and superhydrophilicity in porous materials was predicted theoretically and demonstrated experimentally using a thermally induced change in contact angle [28]. The porous materials used in that study were produced by a phase‐separation method. Reaction occurs through the hydrolysis of the ethoxy groups and the polymerization of the silanol groups thus formed. Polymerization causes a decrease in the dipole moment, leading to hydrophobic phase separation. The dried material had organic groups on its surface, causing the foam to be superhydrophobic, but after thermal treatment to $400^{\\circ}\\mathrm{C}$ , the surface was superhydrophilic. [48] This sudden hydrophilic–hydrophobic transition is due to the cross‐linking of the silica backbone, which causes the redistribution of organic groups from the surface into the bulk of the material. In porous materials, the transition is very sharp, and because the pores can only be empty or filled, partial states are not energetically favored. Beside porous surfaces, rough surfaces also can exhibit wetting transitions [74].  \n\nHan et al. showed that superhydrophilic channels photo‐patterned in a superhydrophobic porous polymer layer can separate peptides of different hydrophobicities and isoelectric points by 2D thin‐layer chromatography [128]. Recently, another facile and versatile method has been presented for creating superhydrophilic patterns in superhydrophobic porous polymer films by UV‐initiated photografting [129]. The extreme difference in wettability between superhydrophilic and superhydrophobic areas permits the use of superhydrophilic patterns as microfluidic channels. The method allows precise control of the size and geometry of photo‐grafted superhydrophilic patterns. For mixed polymer brushes that consist of incompatible hydrophobic and hydrophilic components attached to a substrate, the top morphology and composition of the films can be switched by exposure to different solvents, which in turn results in changes in the surface energy and water contact angle [30, 130, 131].  \n\nThere are many emerging technologies that can benefit from the combination of extreme wettabilities on one substrate. Some of those advantages are patterning of complex geometries with liquids, production of cell microarrays, offset printing, and control of the adhesion of proteins, cells, or bacteria.",
        "category": " Results and discussion"
    },
    {
        "id": 26,
        "chunk": "# 3.6.7 Other Applications  \n\nBesides the main applications of self‐cleaning and antifogging, superhydrophilic surfaces are also very suitable for filtration processes. For example, Sun et al. found that a superhydrophilic polypropylene filter shows remarkable filtration efficiency and may serve as an ultrafilter. Opposite, uncoated polypropylenes filter presents poor filtration properties because of its hydrophobic properties [132]. The superhydrophilic property of substrates, particularly of metals, can also be used for the attachment of biocoats as BMP‐2. These coatings exhibit bioactive properties in bone and are therefore suited for implants [133].  \n\nThe examples of applications mentioned above illustrate the immense range of potential applications of superhydrophilic surfaces. There is continuing development of novel and improved superhydrophilic surfaces for new applications. However, the remaining question is how many of possible applications are already being implemented in real products. This question is addressed in the following section.",
        "category": " Results and discussion"
    },
    {
        "id": 27,
        "chunk": "# 3.7 COMMERCIAL COATINGS  \n\nSome of the aforementioned technologies are already commercialized products available in the market. This section gives a sampling of companies that offer superhydrophilic products in different industrial fields. On the one hand, there are finished products for end users, and on the other hand, there are chemical solutions that can be easily sprayed onto a surface to obtain superhydrophilic properties.  \n\nIn particular, coatings containing $\\mathrm{TiO}_{_2}$ nanoparticles are widely offered for different uses. One of the first products in the market with a self‐cleaning property was a glass cover for lights in tunnels [98]. These covers are mainly used in tunnels in Japan; they remain clean due to a photocatalytic process induced by sodium lamps that emit UV light. In contrast, conventional glass covers are covered by exhaust compounds (Fig. 3.10), leading to dark tunnels.  \n\nThe German company “GXC Coatings GmbH” offers a group of GXC NuGlass PK coatings that modify different surface substrates, including glass, polycarbonate, polymethylmethacrylate, and various metals. The coating includes $\\mathrm{TiO}_{_2}$ nanoparticles and exhibits a self‐cleaning effect. It is mainly used in cover panels for exterior lighting systems in the automotive industry, for covers of measuring instruments, and in architectural elements [134].  \n\nSelf‐cleaning windows constitute a major part of the market of superhydrophilic products. The first glass with self‐cleaning properties, “Pilkington Active,” was introduced by the British company Pilkington in 2001 [135]. The technology is based on $\\mathrm{TiO}_{_2}$ particles; the surface exhibits superhydrophilicity and can decompose dirt with the help of sunlight [136, 137]. Pilkington ActiveTM windows are easily cleaned by rain and are therefore used in many public and private buildings, including the Town Hall in France, the Britomart Transport Centre in New Zealand, and the Hilton Hotel in Finland. Besides the construction industry, the technology is also used in the automotive industry for the production of side mirrors for cars [98, 138].  \n\n![](images/1605b7c2f52dae9604eb960a5ec26a136e6e76dc65a1168ee184239df3fc1f9a.jpg)  \nFig. 3.10  Glass covers on lighting fixtures in a highway tunnel. Without a $\\mathrm{TiO}_{_2}$ coating, the covers are darkened by automobile exhaust (left). With a $\\mathrm{TiO}_{_2}$ coating, the covers remain clean (right). Used with permission from Ref. 98. $\\mathbb{O}$ The Japan Society of Applied Physics.  \n\nIn 2001, Saint Gobain, a competitor of Pilkington, presented a product called SGG Aquaclean at the international building exhibition Batimat [139, 140]. In contrast to most other self‐cleaning surfaces, the self‐cleaning effect is based only on the superhydrophilic property of the surface without any catalytic effect; it can be applied to all building elements that are exposed to rain. Only 1 year later, PPG Industries introduced a self‐cleaning glass called Sun Clean [141]. Similar to almost all ­self‐ cleaning windows, the Sun Clean coating is based on $\\mathrm{TiO}_{_2}$ technology and is mainly used in commercial building applications [142]. There are many other competitors in the market offering self‐cleaning windows, including, for example, Neat Glass from Cardinal Glass Industries [143]. This company claims that Neat Glass shows the same self‐cleaning effects as Pilkington Active and Sun Clean, but it also transmits more visible light and reflects less. This coating consists of a mixture of titanium dioxide and silicon dioxide [144].  \n\nBesides self‐cleaning windows, there are other superhydrophilic materials developed for use in the construction industry. In 2002, the Japanese company Toto Ltd. introduced a superhydrophilic and photocatalytic paint for walls called Hydrotect [145, 146]. The system consists of three‐layer painting that can be easily applied on outside walls to produce a self‐cleaning surface. The technology is based on $\\mathrm{TiO}_{_2}$ nanoparticles; besides self‐cleaning, it also leads to surfaces that can purify the atmosphere by a catalytic process. Toto Ltd. claims that up to now, more than 1000 public and private buildings are equipped with the Hydrotect technology. Compared to uncoated buildings, the walls stay much cleaner (see Fig. 3.11). For example, the facade of a Toyota factory in Aichi is coated with Hydrotect to make a contribution toward sustainable production [148]. One of the leading manufacturers of ceramic tiles, Casalgrande Padana, developed, in cooperation with Toto Ltd Bios Self Cleaning Ceramics®, a product line of porcelain tiles with self‐cleaning properties. The products are based on the Hydrotect technology from Toto and can be used in both interior and exterior architectural applications [149]. The tiles also exhibit antibacterial and smell‐reducing properties, making them very suited for indoor applications [150, 151]. Alcoa Architectural Products used the Hydrotect technology to offer a self‐cleaning aluminum panel suited for facades; the product is called Reynobond with EcoClean [152]. The panels also clean the surrounding air by a photocatalytic effect [153].  \n\n![](images/592069fe82d20bd92347cf5da99fc1289b60fe2307eef760fbfa9111f65ecedc.jpg)  \nFig. 3.11  Effects of outdoor weathering (left) on a superhydrophilic building façade and (right) on a façade not treated with a superhydrophilic coating. Used with permission from Ref. 147. $\\mathbb{O}$ Alcoa Inc.  \n\nSelf‐cleaning is not the only important property of superhydrophilic surfaces that has been commercialized. There are already products in the market with antifogging properties. The company Akzente Oberflächen‐ und Vertriebs GmbH in Germany developed an antifogging coating based on plastics [154]. Acryl groups are a main component used during the synthesis of the plastics. The antifogging coating is mainly used in the production of motorcycle helmets and goggles.  \n\nThe products mentioned above are mainly suited for specific applications. In addition, some superhydrophilic products in the market can be used in our daily lives. Superhydrophilic coating solutions that can be easily sprayed on various substrates to get self‐cleaning glasses or antifogging contact lenses are offered by several companies, including Laiyang Zixilai Environmental Protection Technology Co., Ltd. and Tomorrow Nano Science and Technology Co. in China and iCoat Company in America [155–157]. For example, the product IC No‐Fog from iCoat can be applied to various lens materials, both plastic and glass.  \n\nThis selection of superhydrophilic products shows that such novel technologies will improve living standards and make contributions to solve important everyday problems.",
        "category": " Results and discussion"
    },
    {
        "id": 28,
        "chunk": "# 3.8 CONCLUSIONS AND OUTLOOK  \n\nThe production of superhydrophilic and superamphiphilic coatings is a new and interesting field in research and industry. Since 2000, research on superwetting coatings has expanded, resulting in a significant increase in the number of publications.  \n\nGenerally, artificial superhydrophilicity and/or superamphiphilicity can be achieved by two ways: a texture‐induced method or a photoinduced strategy. Texture‐ induced superhydrophilicity is attained by creating rough surface structures on materials having high surface energies. Alternatively, photoinduced superhydrophilicity using $\\mathrm{TiO}_{_2}$ and $z_{\\mathrm{{nO}}}$ has attracted significant attention as an intriguing phenomenon that can provide antifogging and self‐cleaning properties. Regardless of which mechanism is used, it is clear that the incorporation of superhydrophilicity into commercial products can have significant benefits. Superhydrophilic coatings are already being used to impart features such as self‐cleaning, antifogging, and antireflecting; these coatings can also improve heat transfer from solid surfaces, which enables the fast cooling of surfaces.  \n\nSignificant studies have focused on the production of superhydrophilic surfaces with additional features, including wettability switching, patterning, and gradient wetting. These surfaces are expected to have exciting applications in microfluidics, microarrays, offset printing, etc.  \n\nHowever, there are still many challenges in the production of artificial superhydrophilic coatings. For instance, the production of photoinduced coatings having superhydrophobicity faces serious obstacles in terms of cost and materials.When texture‐induced superhydrophilicity is used as a production method, another challenge is poor mechanical stability, which is currently addressed through self‐­curing and self‐healing mechanisms. The investigation of these mechanisms should provide further future opportunities. In particular, coatings containing $\\mathrm{TiO}_{_2}$ have to be improved, because they are easily wiped off or damaged due to the lack of hardness. Up to now, most photoinduced coatings can only be used for outdoor applications, because superhydrophilicity is induced by UV light. Therefore, there is a need to develop superhydrophilic coatings with high hardness and optical transparency but without any need for external stimuli. Some research groups have already started to develop such coatings, but they are in the minority [158–160]. Although some superhydrophilic coatings are in the market, there is a demand for new technologies that provide superhydrophilic coatings with stable properties and that can be scaled‐up for commercial production.",
        "category": " Conclusions"
    },
    {
        "id": 29,
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        "category": " References"
    }
]