There are two significant factors in fabricating a superhydrophobic surface: the appropriate hierarchical structure with durable micro/nanoparticles and a low energy surface. For textiles with a micro-scale fiber structure, a common strategy is to coat nanoscale particles onto the fiber surface to achieve the micro/nanoscale structure and subsequently post-fluorinate the hierarchical structure for the low energy. The most common methods for preparing robust superhydrophobic textile surfaces include physical and chemical approaches, such as dip-coating, wet chemical deposition, electro-assisted chemical deposition, spray coating, sol-gel, chemical etching, chemical vapor deposition, plasma processing, and polymer grafting (Wang et al., 2013a; Wang et al., 2013b; Yu et al., 2013; Deng et al., 2014; Lin et al., 2015; Liu et al., 2015; Wu et al., 2016; Li et al., . 2017; Nechyporchuk et al., 2017; Tursi et al., 2019). These available fabrication technologies will be separately discussed in the following categories. The most common approaches to fabricate superhydrophobic coatings on cellulose-based materials include various dip-coating methods, spray-coating, in situ nanorod/ particle growth, CVD and plasma processing techniques.
Polymerization techniques use closely controlled chemical reactions to render a cellulose sample hydrophobic. Polymerization techniques are typically limited by the slow efficiency (Teisala et al., 2014). Cellulose (C6H10O5)n is a long-chain polymeric polysaccharide of glucopyranose repeating units linked together by β-1, 4 glyosidic bond (Fig. 2). It forms the primary structural component of green plants. The primary cell wall of green plants is made of cellulose while the secondary wall contains cellulose with variable amounts of lignin.
The hydroxyl groups of cellulose can be partially or fully reacted with various species to provide cellulose derivatives with useful properties. This provides an opportunity to modify the surface of cellulose/lignocellulosic by chemical reactions. Through derivatization reactions, polymers with special structures and functions can be introduced to modify the surface and attain the desired properties. This has been the subject of intense studies and well documented (Cunha and Gandini, 2010). Due to the abundance of —OH groups in cellulose, chemicals with low surface energy, such as fluorine-containing substituents (Cunha and Gandini, 2010), silicone and hydrocarbon polymers (Pasquini et al., 2006) can be easily introduced by esterification with acid or anhydride (Fig. 3) (Song and Rojas, 2013).
Figure 3. Esterification reactions of cellulose. R is a long fluorine-containing or hydrocarbon chain (Song and Rojas, 2013)
The introduction of fluorinated polymer chains endowed cotton with a water contact angle of 155°. Since the fluorinated polymer chains were covalently attached on the surface, the super-hydrophobic cotton fabric possessed high stability and chemical durability. Lin et al. (2017) fabricated cellulose microspheres via sol-gel transition using NaOH/urea/H2O as the solvent system. Due to the hydrophilic nature of cellulose, contact angle of cellulose microsphere (CM) was lower than 10°, upon modification with Fe3O4 and poly (DOPAm-co-PFOEA) (derived from N-(3, 4-dihydroxyphenethyl) acrylamide (DOPAm) and 2-perfluorooctyl)ethyl acrylate (PFOEA))), poly(DOPAm-co-PFOEA)/Fe3O4/cellulose microspheres (PMCM), water contact angle of the PMCM increased to 154.7°. These results indicated PFOEA moieties containing fluorinated units imparted cellulose-based microspheres a low surface energy, leading to superhydrophobicity development.
The pre-roughening and post-fluorinating technology are the most common methods available to date in the preparation of cellulose-based superhydrophobic textile surfaces. Functionalization with nanoparticles or nanofilaments or a layer of film can usually achieve the required roughness. Nanoparticles (such as SiO2, TiO2, ZnO (Zhou et al., 2013; Lin et al., 2015; Wu et al., 2016)) are often used to decorate textiles surfaces to generate a roughness and durable superhydrophobic surface. In addition, some inorganic or organic chemical materials in the form of nanofilaments, nanofibers and even film layers were also reported in the literature to fabricate superhydrophobic cellulose-based surfaces (Wang et al., 2013c). In this section, we will discuss the fabrication technologies in detail based on the formation type (particles, monofilaments, nano-fiber, and film) on the textile surfaces.
Dip coating is the most common method used to induce a hydrophobic surface on cellulose, which exhibits good mechanical durability. It usually includes three process steps. In the first step, the surface was dipped in the coating slurry, then the surface was dried and finally cured. The coating slurry typically has multiple components which often consist of an organic solvent, surface roughening agent and polymers to increase binding. In some cases, the surface will require additional steps to induce superhydrophobicity.
Wang et al. (Wang et al., 2013c) fabricated a superhydrophobic fabric via a two-step dip-coating chemical route. As shown in Fig. 4a, after coating, the fabric surface showed considerable liquid repellence to liquids with various surface tension and the contact angle remained above 150° when the tested liquid has a surface tension higher than 22.1 mN/m (Fig. 4b). The dependency of the sliding angle under constant liquid volume (35 mL) and constant liquid weight (0.35 g) over different surface tension was also investigated, and the results indicated that the sliding angle decreased with the increase of liquid surface tension (Fig. 4c). Moreover, the excellent wettability was confirmed by observing the contact angle changes on an extended period of time (Fig. 4d). Moreover, Nguyen-Tri et al. (2019) prepared robust superhydrophobic cotton fibers by simple dip-coating approach using chemical and plasma-etching pretreatments (Fig. 5). As is shown in Table 1, with different input variables and etching techniques, superhydrophobic cotton fabrics with high chemical and mechanical durability were successfully prepared, with contact angles up to 173°.
Figure 4. Chemical structures of coating materials and procedure for coating treatment (a). Relationship of contact angle (b) and sliding angle (c) with surface tension and CA changes of water, hexadecane and ethanol with time (d). Redrawn from (Wang et al., 2013c)
Figure 5. Schematic illustration of preparation of superhydrophobic cotton fabric by alkali or plasma pretreatments
Sample Pretreatment Solution A step1 Solution B step2 Contact angle (°) a Water/ethanol SiO2 (8%) +water (300 mL) + acetic acid (2 mL) TEOS (10%) 91±1 b NaOH (0.5 mol/L) SiO2 (8%) TEOS (10%) 147±1 c NaOH (0.5 mol/L) SiO2 (10%) TEOS (10%) 152±1 d NaOH (0.5 mol/L) SiO2 (12%) TEOS (10%) 160±2 e NaOH (0.5 mol/L) SiO2 (12%) TEOS (10%) 173±2 f NaOH (0.5 mol/L) SiO2 (12%) TEOS (10%) 173±2 g NaOH (0.5 mol/L) SiO2 (12%) 2wt% of acrylic resin TEOS (10%) 167±2 Note: Exceptionally, solution B is prepared in benzene instead of toluene (Nguyen-Tri et al., 2019). TEOS, tetraethylorthosilicate.
Table 1. Treatment conditions for cotton febrics by one-step (a) and two-step (b–f) procedures
Chemical bath deposition (CBD) technique is a low-cost fabrication process, and includes some of the important parameters such as pH, bath temperature, composition and deposition time which play a decisive role in uniformly depositing thin films by using the CBD (Enrı́quez, 2003; Cortes et al., 2004; Waldiya et al., 2019).
Stanssens et al. (2011) synthesized a series of organic nanoparticles by imidization of poly (styrene-maleic anhydride) copolymers under pure conditions or in presence of palm oil, which can be applied as a top-coating onto cellulosic substrates. Results showed that surface treatment of paper with organic nanoparticles provides super-hydrophobic surfaces, which are characterized by a static contact angle of 148°.
Wang et al. (2015) prepared a one-way transport oil droplet functional fabric surface by a two-step coating process to apply flower-like ZnO nanorods, fluorinated decyl polyhedral oligomeric silsesquioxane (FD-POSS), and hydrolyzed fluorinated alkylsilane. After the superhydrophobic fabric was exposed to UV light on one side, the treated fabric showed interesting one-way oil transport ability. In addition, the selective oil-transportation depends on the specific surface tension of the liquid, and by changing the UV irradiation time, different types of oils can be selected. Such a one-way oil fluid transport material has potential application in detecting liquid surface tension (Fig. 6a).
A robust flower-like hierarchically structured cotton fabric surface was prepared in situ via a one-pot hydrothermal technology, in which the cotton piece was immersed into a solution mixture and treated at various temperatures ranging from 120 ℃ to 200 ℃ (Fig. 6b). Subsequently the remarkable superhydrophobic cotton surface was developed by the chemical modification with fluoroalkylsilane. The flower-like hierarchical TiO2 micro-nanoparticles were evenly coated on the cellulose fiber surface. Such special wetting superhydrophobic coating showed excellent water-repellent ability with a static contact angle larger than 160° and a dynamic sliding angle less than 5°. The facile strategy for fabricating hierarchical structure superhydrophobic TiO2@Cotton is expected to be helpful for designing and developing self-cleaning materials in air and underwater superhydrophobic/super-oleophilic materials.
Layer-by-layer (LbL) assembly is considered as a simple and efficient approach, which could incorporate functional films on substrates and have been widely applied in many fields (Zhang and Xiao, 2017). Specially, owning to the facile and effective features, the electrostatic LbL self-assembly is rather appealing. It is only based on alternating adsorption of oppositely charged polyelectrolyte nano materials onto the surface of substrates without any additional advanced equipment and complex procedures (Tian et al., 2016; Xiao et al., 2016; Zeng et al., 2018; Fu et al., 2019; ).
Zhao et al. (2010) successfully prepared a superhydrophobic fabric surface via a versatile electrostatic LbL self-assembly method and a post-fluorinating strategy to construct polyelectrolyte/silica nanoparticle multilayers. Before the electrostatic self-assembly, the pristine cotton fabric was treated with specific solution to form a charged polymer film. Moreover, the surface morphology and hydrophobicity of cotton fabric were tuned by the number of silica-nanoparticle multilayers. The fabric exhibited a weave structure and the pure cotton fiber was very smooth (Figs. 7a and 7b). When the surface was assembled with 1 or 3 layers of (poly(allylaminehydrochloride)-silica (PAH-SiO2), the silica-nanoparticles were dispersed onto the fiber surface randomly (Figs. 7c and 7d) and such a surface showed sticky property with high contact angle hysteresis. An increasing number of assemble layers could result in the homogeneous coverage and aggregation of silica nano-particles coated on cotton fiber surface (Figs. 7e–7f). The surfaces exhibited excellent super-hydrophobicity with high static contact angles and low contact angle hysteresis. The superhydrophobic surface can withstand at least 30 cycles of machine washing due to the excellent affinity between the PAH-SiO2 and cotton fiber.
Figure 7. The SEM images of untreated cotton fabric (a and b) and cotton fabrics assembled with (PAH-SiO2)n multilayers: (c) n= 1, (d) n=3, (e) n=5, and (f) n=7. Reprinted from Zhao et al., 2010 with permission from Elsevier
In-situ nanorod/particle growth can occur in the gas or liquid phase and is used to assemble nano and micro structures in a controlled manner on the sample in question. This can be used to induce superhydrophobicity, however it is typically a very time-consuming process (Teisala et al., 2014).
Wang et al. (2013a) reported a general methodology for in situ growth of transition metals and the oxide nanoparticles on fabric and sponge to realize stable surface roughness, which can readily coordinate with thiol, resulting in a special wetting property (Fig. 8). It was also demonstrated that multi-scale surface roughness and the special wettability can be controlled via efficient control of the growth of nucleation of Group VIII and IB nanocrystals. The Group VIII and IB metal oxides, as well as simple metallic substances, such as Fe, Co, Ni, Cu, and Ag could enable the fabric/sponge with different colors. The as-selected transition-metal element can not only strongly bond with thiols, but also possess some special properties that can be utilized to realize multifunctional integration. The multifunctional applications were mainly derived from magnetic recycling, semiconducting, and antibacterial material properties. Moreover, the as-prepared fabric and sponge via the in-situ growth method followed by thiol modification possess anti-wettability towards water and can selectively absorb and filtrate oils from water with high efficiency (Li et al., 2017).
Figure 8. Schematic illustration of preparation procedure of superhydrophobic fabric from in situ growth of transition-metal/metal oxide nanocrystals with thiol modification. Redrawn from (Wang et al., 2013a)
Chemical vapor deposition (CVD) is a chemical reaction which transforms gaseous molecules, called precursor, into a solid material, in the form of thin film or powder, on the surface of substrate (Alf et al., 2009, Asatekin et al., 2010). The CVD approach is regarded as a simple and effective method to deposit super-hydrophobic coatings onto substrates. For vapor deposition, the reaction temperature in the chamber is required to be higher than the boiling point of the precursor. For example, silane precursors of trichloromethylsilane (TCMS) and dimethyl-dichlorosilane (DMDCS) require a temperature higher or close to the boiling point of the precursor, 66 ℃, and 68 –70 ℃, respectively (Li et al., 2008; Oh et al., 2011). Cotton fabrics or filter papers were placed in a sealed chamber for a set time, into which a vapor of precursor was introduced. The precursor adsorbed onto the cotton fibers' surface and penetrated into the fibers; and the reaction between halide and hydroxyl groups then took place. An efficient reaction enhanced the content of covalently bound silicone to the fibers' surfaces. After the deposition, condensation of silane polymers was required. This step was conducted in an aqueous solution of pyridine (1 mol/L) at room temperature to hydrolyze the remaining Si–Cl bonds (Li et al., 2008). Finally, subsequent polymerization of Si–OH was accomplished in an oven at 150 ℃ for 10 min, which resulted in a nano-scaled silicone coating tightly attached to the surface. Therefore, this method can be regarded as a special sol-gel process in which the precursor attaches to the surface in gaseous phase rather than in solution. One disadvantage of this process is that the wet tensile strength can be reduced, which is caused by the generation of hydrogen chloride during reaction which degrades the acid-sensitive cellulose fibers (Oh et al., 2011).
Wang et al. (2013b) also reported a multifunctional fabric with electrical conductivity and super-hydrophobicity by chemical vapor phase polymerization of 3, 4-ethylenedioxythiophene (PEDOT) in the presence of FeCl3•6H2O, fluorinated decyl polyhedral oligomeric silsesquioxanes (FD-POSS) and a fluorinated alkyl silane (FAS). The chemical structure and the chemical vapor phase polymerization are illustrated in Figs. 9a and 9b. The addition of FD-POSS and FAS endowed the conductive coating with stable and durable super-hydrophobicity with a contact angle of 169° and 156° to water and hexadecane, respectively; however, the fabric surface with only PEDOT coating showed unstable hydrophobic properties and the water droplet will spread into the fabric matrix (Fig. 9c) (Wang et al., 2013b). In addition, the surface exhibited an excellent super-hydrophobicity to liquids with surface tension higher than 27 mN/m (Fig. 9d). The FD-POSS and FAS were found to play an important role in enhancing the washing and abrasion durability and self-healing function of the coating; meanwhile it showed little influence on the conductivity of coating. This facile and novel strategy is expected to move forward the textile industry to durable and smart applications (Wang et al., 2013b; Zhou et al., 2013).
Figure 9. (a) Chemical structures of FD-POSS, FAS and EDOT, (b) illustration of vapor-phase polymerization to form PEDOT/FD-POSS/FAS coating on fabrics, (c) contact angle of the coated fabric changing over time from initial fluid-fabric contact, and (d) dependency of contact angle on surface tension of fluids. Redrawn from (Wang et al., 2013b; Zhou, et al., 2013)
Chemical etching increases the surface roughness of fibers substrates. In case of hydrophobized microscale cellulose membranes, a simple chemical etching can yield the surface super-antiwetting behavior due to the wetting state transition (Liu et al., 2016). Zhou et al. (2015) reported a robust, chemical stable superhydrophobic fabric prepared via a one-pot wetting coating method using a coating solution containing poly (vinylidene fluoride cohexfluoropropylene) (PVDF-HFP), fluoroalkylsilane (FAS) and a volatile solvent (such as acetone) (Figs. 10a and 10b). The particle-free coating made the coated fabric super-repellent to liquids with a surface tension greater than 21.5 mN/m in acetone solution (Fig. 10c). Such a fabric surface showed good stability under continued liquid dropping with the static contact angle unchanged. Wu et al. (2014) reported an extremely simple solution soaking coating in fluoropolymers (FPs) process for preparing extremely durable superhydrophobic textiles. The textiles coated under the optimal conditions show excellent superhydrophobicity and chemical stabilities.
Figure 10. (a) Chemical structures of coating materials and procedure for coating treatment, (b) contact angle change with time when liquid drop evaporated) and (c) dependency of CA and SA on surface tension of liquids. Redrawn from (Zhou et al., 2015)
In addition to the above methods, there are some other methods. Daoud et al. (2006) utilized pulsed laser deposition to fabricate thin polytetrafluoroethylene (PTFE) films on cotton fabrics (Fig. 11). The film deposition was carried out in vacuum at room temperature, and the deposition time was about 3 min. The PTFE coated fabrics showed superhydrophobic properties with water contact angle of 151°. The SEM image revealed that the PTFE film on cotton had a granular surface structure where the grain size was about 50–70 nm in diameter. This was apparently the first straightforward one-step approach to fabricate a superhydrophobic coating on cotton. That is, any type of chemical modification, drying, or curing steps were not needed after application of the coating (Daoud et al., 2006). Moridi et al. (2018) prepared super-hydrophobic on the fabric by exposing to air corona discharge treatment for 300 W-10 min without any extra chemical modification, indicating a static contact angle of 167°. Tursi et al. (2019) grafted cellulose fiber extracted from Spanish Broom by means of low plasma process at a very low input power, which could be simply scaled for industrial purposes. Results showed that fluorine grafted cellulose has a water contact angle higher than 160° and the adsorption capacity is higher than 270 mg/g, making it effective adsorbent material for removing hydrocarbons from water. Li et al. (2011) grafted cellulose microfibrils with poly (butyl acrylate) (PBA) by atom transfer radical polymerization (ATRP) of butyl acrylate (BA) on the surface of 2-bromoisobutyryl-functionalized cellulose microfibril (CMF) generating highly hydrophobic microfibrils (CMF-PBA).
Figure 11. Schematic of a Pulsed Laser Deposition system (a) polytetrafluoroethylene (PTFE) deposited on a cotton fiber by pulsed laser deposition (PLD) (Song and Rojas, 2013) and (b) high magnification SEM micrograph showing granular morphology and nanostructure of PTFE film (c). Reprinted from Daoud et al., 2006 with permission from Elsevier
Zhang et al. (2011) presented a solvothermal synthesis of nano-porous polydivinylbenzene (PDVB) powder and demonstrated an innovative approach to apply the superhydrophobic nano-powder coating on various substrates. The as-prepared PDVB powder took a form of a solid monolith, by which the substrates were simply wiped to paint the superhydrophobic and transparent nano-porous polymer coating. Attachment of the polymer powder on the rough paper surface occurred by the electrostatic interaction, and average thickness of the coating was estimated to be approximately 10 μm. Water contact angle on the polymer coated paper was 157° and droplets could roll off the surface at sliding angle of 6°. The superhydrophobic properties of the coating remained stable and the contact angle decreased only 3° in 24 h. In addition, the superhydrophobic properties of the coating were reported to remain stable also in humid conditions. Each category of methods discussed could be carried out using a wide array of chemicals and physical techniques. The following discussion focused on the specific applications of superhydrophobic-modified cellulose.
2.1. Polymer grafting
2.2. Dip-coating method
2.3. Chemical bath deposition
2.4. Electrostatic layer by layer self-assembly
2.5. In-situ growth processing
2.6. Chemical vapor deposition
2.7. Wetting chemical etching
2.8. Other methods
Superhydrophobic textiles fabricated by constructing appropriate surface roughness and suitable chemistry have been successfully demonstrated for a range of applications, such as oil/water separation, self-cleaning, and multifunctional materials containing UV-shielding, flame-retardant, anti-icing, and photocatalytic properties, as well as some smart materials with self-healing, stimuli-responsive, patterning and asymmetric response (Cao et al., 2009; Wang et al., 2013a; Arslan et al., 2016; Peng et al., 2016; Shibraen et al., 2016; Jiang et al., 2017; Chen et al., 2018; Gao et al., 2018; Mi et al., 2018; Schlaich et al., 2018). In this section, we will mainly focus on the functional applications of textiles (mainly on cellulose based materials) with special wettability surfaces.
Superhydrophobic and self-cleaning surfaces are based on the surface micro/nano morphologies (Lu et al., 2015). The self-cleaning property of superhydrophobic cellulose is summarized as three types: physical self-cleaning, chemical self-cleaning and biological self-cleaning. Physical self-cleaning is mainly mimicking the lotus leaf surface and characterized by measuring the water contact angle and the sliding angle. Chemical self-cleaning refers to the degradation of the color stain or pollution solution using the photocatalytic effect. The biological self-cleaning corresponds to antibacterial activity of functional fabric against a Gram-positive bacterium (e.g., Staphyloccocus aureus) and a Gram-negative bacterium (e.g., Escherichia coli).
Typically, superhydrophobic surfaces with the water contact angle above 150° and an ultralow sliding angle are endowed with physical self-cleaning properties. Lin et al. (2015) reported a self-cleaning cotton fabric surface coated with a superhydrophobic and super-oleophobic thin composite polymer film consisting of modified SiO2 nanoparticles and a fluoropolymer. A water droplet and a sunflower oil rolled off from the as-prepared cotton fabric surface and brought away dirt along with it. A clear track was left behind by a spherical water droplet or oil droplet. Similarly, the cotton fabric surface coated with micro/nanoparticles subsequently modified with fluoroalkylsilane exhibited a super-antiwetting property with self-cleaning and oil/ water separation ability (Li et al., 2015b). Yu et al. (2013) used a new strategy to covalently immobilize TiO2 nanoparticles onto the fabric surface by grafting polymerization of 2-hydroxyethyl acrylate (HEA) under X-ray irradiation. The resulting functional fabric showed photocatalytic self-cleaning performance when using oleic acid dyed with oil red as the organic stain. As a comparison, the colors of pristine cotton were always red no matter how long it was exposed to the UV irradiation. Reversely, the red colors of various cotton-g-TiO2 gradually disappeared under ultraviolet irradiation because of the photocatalytic effect of TiO2 nanoparticles on the surface.
Based on the principle of separating two different surface tension solution mixtures, the special wettability textile can be divided into two types: as a filtration membrane and as an absorption material. The filtration membrane allows only oil or water to permeate through and repels the other phase, resulting in a selective separation. The absorption material can selectively absorb water or oil on the surface, and thus prevent the other phase from permeating into the absorbent. A facile and inexpensively one pot sonochemistry irradiation method was developed for constructing two-side superhydrophobic fabric incorporated with SiO2 nanoparticles (Li et al., 2015a). The resulting fabric exhibited both super-hydrophobicity and superoleophilicity with high water contact angle of 159° and oil contact angle of nearly 0°, which is suitable for oil/water separation. The fabric can be used for capturing and separating various oils both on the water surface and underwater, including toluene, chloroform, kerosene, etc. Furthermore, a superhydrophobic contact angle above 150° and excellent separation efficiency beyond 94.6% were observed after 40 separation cycles. In addition, the obtained fabric showed stable and robust super-hydrophobicity against hot water, strong acid, alkaline, salt solution and mechanical abrasion. This stable and durable superhydrophobic fabric surface has great potential for practical applications.
Deng et al. (2014) reported a simple and practical route to develop a superhydrophobic SiO2-TiO2@PDMS hybrid film via a sol-gel method, which endows the coated polyester-cotton fabric surface with super-hydrophobicity and photocatalytic effect. The SiO2-TiO2@PDMS hybrid film can be produced on a large scale and the film has high thermal stability at temperatures up to 400 ℃. The as-prepared large-scale superhydrophobic cloth was also used as a filter to separate the oil/water mixture. Besides, the "filter cloth" exhibited a considerable separation efficiency with a water contact angle above 150° and a sliding angle about 8° after ten times of the separation experiment.
Gao et al. (2017) prepared a hybrid polyvinylidene fluoride (PVDF)/SiO2 microspheres based on electro-spraying and casting for super-hydrophobic coating. It was found that gravity driven oil-water separation was achieved by using the filter paper coated with the super-hydrophobic hybrid microspheres. More importantly, the coated filter paper could not only separate the oil with the pure water droplets but also the corrosive droplets including the salt, acid and alkali solution. Compared with the filter paper coated hybrid microspheres, the free-standing membrane composed of hybrid microspheres and ultrathin threads displayed a higher oil-water separation efficiency. In addition, the flexible membrane could be used as the adsorbent for different kinds of oil.
Cai et al. (2015) reported that highly porous TiO2 microspheres had been prepared via a template-assisted sol-gel process with cellulose nanofibril aerogel as microsphere template. The modified porous titanium dioxide microspheres showed a typical super-hydrophobic property. The method reported in this study may be applied to fabricate other inorganic materials with desired porous structure (Cai et al., 2015). Xu et al. (2018) synthesized a porous three-dimensional (3D) carbon aerogel by an environmentally friendly freeze-drying process and then carbonized of cellulose nanofibers (CNFs), poly (vinyl alcohol) (PVA) and graphene oxide (GO) to yield CNF/PVA/GO carbon aerogels, which had a water contact angle of 156° and high oil absorption capacity (97 times of its own weight).
He et al. (2018) prepared bacterial cellulose aerogels/silica aerogels (BCAs/SAs) using three-dimensional self-assembled BC skeleton as reinforcement and methyltriethoxysilane derived silica aerogels as filler through vacuum infiltration and freeze drying, which exhibited super-hydrophobicity with a contact angle of 152° and super-oleophilicity resulting from the methyl groups on the surface of silica aerogel filler. This endows the BCAs/SAs outstanding oil absorbing capability with the quality factor Q (The quality factor (Q) was calculated with the weight (wt) of BCAs/SAs before and after absorption as following equation: Q = (wtafter – wtbefore)/wtbefore) from 8 to 14 for organic solvents and oils.
Sobhana et al. (2017) reported that cellulose was hydrophobized by ecofriendly stearic acid through inorganic linker/interface/sandwich material namely layered double hydroxides (LDH) since the layers have affinity to both cellulose and stearic acid at molecular level. The novel idea of utilizing the charged centers on the LDH has been effectively materialized to make conjugation with hydrophobic stearic acid and hydrophilic cellulose simultaneously, which offers not only hydrophobicity but super-hydrophobicity to cellulose network. The oil absorption and tensile strength studies show that the stearic acid -LDH-CE (stearic acid fabricated LDH- cellulose hereafter called as SA-LDH-CEL hybrid fibres) is a mandatory requirement for water-proof packaging, thin films, paper, sorbent and sanitary materials.
Oil-water separation is one of the most widely used superhydrophobic modified fibers. After oil-water separation, we can further adsorb heavy metals from the separated water, so as to further expand its application. Patrick et al. (2019) reviewed on the improving heavy metal ion removal in wastewater.
The self-healing superhydrophobic surface can restore its water repellent property after being destroyed by acid, base, chemical reagents, and mechanical and laundering abrasion. Generally, the destroyed surface is repaired to original superhydrophobicity by heating treatment, humid environmental conditioning, and ironing treatment and so on.
Zhou et al. (2013) reported a two-step wet-chemistry coating method for durable self-healing superhydrophobic surfaces. The coated fabric exhibited excellent durability to acid, UV light, mechanical and washing abrasion. After being damaged, the fabric can restore its super liquid-repellent performance by a short-time heating or room temperature ageing. Wang et al. (2013b) described a simple one pot mist copolymerization technology to construct healable superhydrophobic fabric. Moreover, the modified cotton fabric surface can recover its superhydrophobicity by ironing treatment after 60 cycles of laundering or 2000 times of Martindale abrasion.
A flame-retardant and self-healing superhydrophobic coating was successfully obtained onto the cotton fabric surface via the solution-dipping method (Chen et al., 2015). After etching with O2 plasma, the resulting cotton surface was superhydrophilic. The coating can repetitively and spontaneously restore the superhydrophobicity by placing the damaged cotton fabric in a humid environment with a relative humidity of 35% for about one hour. Similarly, the F-POSS/AgNPs/PEI (fluorinated decyl polyhedral oligomeric silsesquioxane/silver nanoparticles/poly(ethylenimine) coated cotton fabric surface was fully superhydrophilic when exposed to O2 plasma; however, it can recover its original superhydrophobic state under an ambient environment at 25 ℃ and a relative humidity (RH) of 55%. The etching/healing process can be repeated for at least 16 cycles without apparent changes, showing a strong healing ability of damaged superhydrophobic fabric (Wu et al., 2016).
Most reported self-healing surfaces are achieved by migrating low surface energy molecules into the damaged surface to recover the special wettability. Though much literature reported on self-healing coating with extreme wettability, the usage of fluorine-containing agents is harmful for human body and environment. From this perspective, it is essential to create liquid repellent coating with long-term durability, self-healing and non-toxicity, which is believed to be an efficient way to overcome the poor durability caused by physical and chemical damages (Li et al., 2017). Liu et al. (2015) reported a new approach to construct self-healing super-wettability without any fluorine-containing agents. In this work, polydopamine@octadecylamine (PDA@OCA) nanocapsules were used as coating materials and added onto fabric surface using an in-situ polymerization method. When the coated fabric was destroyed and lost its liquid repellency, the OCA molecules could migrate to fabric surface and restore its wettability only on heating treatment (Zhou et al., 2013). The coated fabric was treated with O2 plasma was hydrophilic with contact angles of almost 0° for any liquids. Interestingly, when the fabric treated by plasma was heated at 80 ℃, its liquid repellency was recovered. Moreover, the self-healing could be repeated more than 10 times. The contact angles for various liquids (water, juice, coffee and milk) on coated fabric surface after 10 cycles of plasma and heat curing indicated that its self-healing property could be easily obtained by simple heating.
Recently electromagnetic interference (EMI) shielding fabrics have attracted much attention for their wide use in protecting people from harm by a variety of electromagnetic radiation sources, such as microwave oven, TV set, computer, communication devices and mobile phone.
Zou et al. (2015) reported a superhydrophobic cotton surface with durable electromagnetic interference shielding. The specific process is cotton fabric was deposited on a coating solution containing of Nafion and multiwalled carbon nanotubes (MWCNTs), in which Nafion was used as a linker between the pristine cotton fiber and hydrophobic MWCNTs, meanwhile providing a homogenous dispersion. After six cycles of the deposition process, the cotton fabric coated with Nafion-MWCNTs exhibited a water contact angle of 154.6° (Fig. 12a) and a favorable electromagnetic interference shielding effectiveness of 9 dB (Fig. 12b). Moreover, the resultant fabric also possessed good durability in electromagnetic interference shielding after soaking in water for four days or washing with the American Association of Textile Chemists and Colorists (AATCC) standard (61-2013 test no. 2A) due to its excellent superhydrophobicity and chemical stability.
Figure 12. (a) Water contact angle with various numbers of deposition; (b) Electromagnetic interference shielding effectiveness (EMI SE) of the fabrics with different deposition number at 4.5 GHz. Redrawn from Zou et al., 2015
Another attractive application of superhydrophobic surfaces, in addition to the extraordinary water-repellence, is their excellent capability to reduce accumulation of snow and ice, even completely prevent ice formation in a low humidity environment. Recently, numerous studies of superhydrophobic coating on rigid metal substrates demonstrated lower adhesions to both liquid water droplets and ice than bare metals or metals covered with hydrophobic coating (Farhadi et al., 2011). The anti-icing properties of superhydrophobic surfaces have great potential applications in aircrafts, optical lenses, energy transmission system, power lines, wind turbines, and highways as well as building constructions (Li et al., 2017). However, reports on potential anti-icing application by flexible textile surfaces are very rare. Farhadi et al. (2011) reported the anti-icing property of industrial nonwoven geo-textiles with four different samples (Table 2), which exhibited de-icing and anti-icing properties compared with the controls. It was also found that surface morphologies as well as surface tension of the substrate have a great effect on anti-icing properties (Farhadi et al., 2011).
Sample Description Preparation CA (°) CAH (°) A CeO2-Zonyl 8740 Spin-coating 152.4±2.6 5.7±2.0 B FAS-13 Etching/dip-coating 153.2±2.4 6.1±1.7 C Ag-Zonyl 8740 Spin-coating/anneal/dip-coating 155.1±1.8 5.3±1.9 D TiO2-RTVSR Etching/Spin-coating 154.8±2.1 6.8±1.5 Notes: CA, contact angle; CAH, contact angle hysteresis. Source: Farhadi et al., 2011
Table 2. Preparation and properties of the samples