Recent development in the fabrication of self-healing superhydrophobic surfaces

Chemical Engineering Journal 373 (2019) 531–546

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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Review

Recent development in the fabrication of self-healing superhydrophobic surfaces

T

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Ebenezer Kobina Sama, Daniel Kobina Sama, Xiaomeng Lva, , Botao Liua, Xinxin Xiaoa, ⁎ Shanhe Gonga, Weiting Yub, Jie Chenc, Jun Liuc, a

School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China College of Environment, Zhejiang University of Technology, Hangzhou, Zhejiang 310014, PR China c School of the Environment and Safety Engineering, Jiangsu University, Zhenjiang 212013, PR China b

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

introduction about super• Brief hydrophobic surfaces and self-healing superhydrophobic surfaces.

Recent and different fabrication tech• niques and methods are reviewed. of current challenges and • Discussion future developments.

A R T I C LE I N FO

A B S T R A C T

Keywords: Self-healing Superhydrophobic Coatings Durability

Some plants and insects exhibit superhydrophobicity in nature. Inspired by this, many researchers have designed superhydrophobic surfaces by studying the structures of those plants and animals. Hence, man-made superhydrophobic surfaces have gained tremendous interest because they can be used in diverse fields. Superhydrophobicity occurs by combining micro-/nanoscale rough structures with low surface energy materials to produce a water-repelling surface. However, superhydrophobic surfaces have not been able to be used in real life applications because of their poor durability and short life span. Self-healing is a good strategy to increase the resilience and life span of a superhydrophobic surface. It has been suggested that embedding a superhydrophobic surface with a self-healing ability will extend the lifespan of the surface for practical applications. A lot of reviews talk about superhydrophobic surfaces but very few discuss self-healing superhydrophobic surfaces. In this review, recent progress in the fabrication of self-healing superhydrophobic surfaces, characterization, applications of superhydrophobic surfaces and different methods for fabrication are discussed. Also, some ideas for the way forward on future researches are discussed.

⁎

Corresponding authors. E-mail addresses: [email protected] (X. Lv), [email protected] (J. Liu).

https://doi.org/10.1016/j.cej.2019.05.077 Received 25 March 2019; Received in revised form 4 May 2019; Accepted 13 May 2019 Available online 15 May 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

Chemical Engineering Journal 373 (2019) 531–546

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Fig. 1. (a) SEM image of PMC/TiO2 nanocomposite coated wood. (b) Water contact angles and sliding angles after mechanical abrasion. (c) SEM image of the coated wood surface after sandpaper abrasion. (d) Changes in water contact angle with ultraviolet irradiation time for the treated wood with and without a PDMS base coat. (e) Changes in water contact angles in the self-healing process for the superhydrophobic surface. (f) Changes in sliding angles in the self-healing process for the superhydrophobic surface. Reproduced with permission from Ref. [69].

1. Introduction

droplet slide away from its surface reluctantly (it is said to have a low contact angle hysteresis) [17]. A surface’s hydrophobicity is measured on the basis of its water contact angle which is the angle between the liquid and solid surface [18,19]. The water contact angle of a surface ranges from 0° (a surface that is completely wetted by water) to 180° (a surface that is nonwetting by water) [20]. Also, a superhydrophobic surface should have a small contact angle hysteresis and a low sliding angle [21,22]. The rolloff behavior of a superhydrophobic surface is attributed to its small sliding angle and small contact angle hysteresis. Sliding angle is described as the smallest angle a surface has to be inclined to cause a

1.1. Superhydrophobic surfaces Superhydrophobic surfaces have in recent times gained much attention because they can be used in diverse areas such as self-cleaning [1], anti-corrosive [2–4], drag reduction [5–8], oil-water separation [9–11] and so on [12–15]. Due to this, a lot of research work began in the field of superhydrophobic surfaces in the 1990s because of their various applications [16]. A superhydrophobic surface is described as a surface that has a water contact angle to be greater than 150° and water

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Fig. 2. (a) Water contact angle (left) and sliding angle (right) of AP/TiO2/OTS coated substrates showing glass (1), ceramic (2), fabric (3), nickel foam (4), and stainless steel mesh (5). Insets show pictures of a water droplet rolling down on the coated glass. (b) Contact angle variation of AP/TiO2/OTS coated substrates with sandpaper abrasion cycles. Insets are how the sandpaper abrasion test is done. (c) Contact angle variation of AP/TiO2/OTS coated substrates with sand impact cycles. Insets are how the sand impact test is done. (d) Schematic illustration of a water droplet on the surface of AP/TiO2/OTS coated fabric during multiple self-healing processes of O2 plasma etching and high-temperature repairing [70].

several methods have been used for imparting surface roughness to substrates. These methods include sol-gel method [35], lithography [36], template methods [37], colloidal self-assembly [38] and electrospinning [39] to name a few. These methods have successfully been used to achieve superhydrophobicity by combining surface roughness and low surface energy hydrophobic material [40]. Generally, having low surface energy, high coating conformity and high roughness result in a surface having a large water droplet contact angle, low contact angle hysteresis and great superhydrophobicity [41,42]. Chandler et al. [43] confirmed in their work the importance of a nanostructure in repelling water. According to their work, the typical length of hydrophobic interaction between water molecules was around 100 nm thereby demonstrating the significance of having a nanostructure to prevent the entry of water. Barthlott and Neinhuis in 1997 realized the water-repellent feature on lotus leaves was due to it having microscaled papillae and hydrophobic epicuticular wax [44]. Koch et al. proved the importance two-tier microstructures have on superhydrophobicity. In their work, microstructures, nanostructures and hierarchical structures were fabricated based on the structure of lotus leaf. They found out the microstructures and nanostructures lead to superhydrophobicity but the hierarchical structures lead to greater superhydrophobicity and had a small contact angle hysteresis even much better compared to that of natural lotus leaves. They also realized water droplets could not roll over on the microstructured surfaces but could do so easily on the micro-/nanoscale two-tier surfaces [45]. Therefore, having a micro-/nanoscale two-tier structure instead of a

water droplet to roll [23]. Contact angle hysteresis can be defined as the difference between the advancing contact angle (contact angle measured with increasing droplet volume before wetting line starts to advance) and receding contact angle (contact angle measured with decreasing droplet volume before wetting line is receding). The contact angle hysteresis of a superhydrophobic surface should be less than 10° [24]. However, it should be noted that two different surfaces may have similar water contact angles but can have very different water contact angle hysteresis [25,26]. This shows that the water contact angle alone cannot be used to classify a surface’s superhydrophobicity because a surface can have a water contact angle of 150° but water droplets do not slide readily on the surface. Mechanical abrasion, for example, is known to increase contact angle hysteresis but rarely affects the water contact angle. Two important features are required for a surface to achieve superhydrophobicity. A micro or nanostructured surface topography and nonpolar surface chemistry [27,28]. Combining a micro or nanoscale hierarchical structure with materials with low surface energy has resulted in the fabrication of a lot of superhydrophobic surfaces [29,30]. There are some superhydrophobic surfaces that exist naturally. The surfaces of lotus leaves, insect legs, wings and animal feathers have high water repellency and self-cleaning properties. This is due to the surfaces having micro/nanoscale rough composition [31–33]. By learning from nature, a lot of man-made superhydrophobic surfaces have been fabricated by combining materials with micro/nanoscale roughness and low surface energy materials [34]. Based on this idea,

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Fig. 3. (a) SEM images of the coating with 40 wt% of MS nanoparticles. (b) Mechanical stability of the superhydrophobic coating. (c) The contact angles of water droplets with different pH value on the superhydrophobic coating. (d) The process of O2 plasma etching and high-temperature repairing. Reproduced with permission from Ref. [71].

layer after they get destructed [61,62]. Self-healing superhydrophobic surfaces have in recent times been developed to tackle the issue of durability and also prolong the lifespan of the surfaces. Two main techniques are used in the fabrication of self-healing superhydrophobic surfaces. The first one deals with storing hydrophobic components inside the rough nanomaterials. In the case of damage to the surface, the hydrophobic components move to the damaged area and hence complete the self-healing process [63–65]. The second approach has to do with the regeneration of micro-/nanoscale structures which causes the damaged surfaces to regain their superhydrophobicity [48,58,66]. However, embedding self-healing properties onto a superhydrophobic surface is normally realized by supplying the surface with hydrophobic components stored inside the nanomaterials. Self-healing superhydrophobic surfaces are not normally fabricated using regeneration of the topographic surface. Our research group has also made progress in the field of superhydrophobic surfaces. The main use of superhydrophobic surfaces in our group is for oil/water separation. Lü et al. [67] fabricated a superhydrophobic surface by dipping commercial polyurethane sponge into a solution of silica nanoparticles/graphene oxide. The coated sponge could absorb organic solvents/oils up to 180 times its own weight. It also showed excellent recyclability. Lv et al. [68] also fabricated polysiloxane/polyurethane sponge by sol-gel and dip-coating process. The sponge could also absorb oil and organic solvents up to 150 times its own weight. The sponge also showed good thermal stability and excellent recyclability. Recently, our group has also started working in coating superhydrophobic surfaces on metals to prevent corrosion but a report is yet to be published. In this review, we discuss the recent progress in the fabrication of

single micro-scaled structure could lead to steady superhydrophobic surfaces [46]. 1.2. Self-healing superhydrophobic surfaces Artificial superhydrophobic surfaces have gained much awareness because of their usage in a lot of fields but the practical application of superhydrophobic surfaces are hindered because of their poor durability [47]. Self-healing gives a productive approach to further improve the long term durability of a superhydrophobic surface. Due to the degradation of the fragile micro-/nanoscale structure of superhydrophobic surfaces, the surfaces lose their superhydrophobicity after mechanical or chemical damage which reduces the lifespan of superhydrophobic surfaces [48]. Therefore, it is important to enhance the durability of superhydrophobic surfaces so they can be used for applications in real life world [30,49,50]. However, a lot of challenges remain in embedding superhydrophobic surfaces with durable and selfhealing abilities. Self-healing can help to improve the durability of superhydrophobic surfaces because of the regenerating property against chemical or physical destructions. Therefore, embedding a superhydrophobic surface with self-healing ability is the best approach to achieve long term durability when exposed to the outside environment [51–54]. Superhydrophobic surfaces with high mechanical stability were initially fabricated to tackle the issue of durability [55,56]. However, new studies have shown that self-healing superhydrophobic surfaces are better to enhance the durability and increase the lifespan of the surfaces [57–59]. The idea of self-healing comes from biology [60]. Some plants, for example, have a self-healing phenomenon which helps to keep their superhydrophobicity by restoring the epicuticular wax 534

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Fig. 4. (a) SEM image of the super-hydrophobic EP + PDMS@SiO2 coated on Mg alloys substrate. High and low-temperature stability tests for super-hydrophobic EP + PDMS@SiO2 coatings. Contact angles and sliding angles as a function of immersion time in (b) boiling water (c) liquid nitrogen. (d) Reversible superhydrophobic and super-hydrophilic self-healing cycles process of the EP + PDMS@ SiO2 coating. Reproduced with permission from Ref. [72].

contact angle was 152.2° with a sliding angle of 5°. The mechanical durability of the coated wood was tested using sandpaper abrasion. The surface still maintained its superhydrophobicity with a water contact angle greater than 150° and a sliding angle below 10° after 500 cm of abrasion distance (Fig. 1b). The SEM image showed no damage to the hierarchical structures even though scratches were seen (Fig. 1c). The surface still had a low sliding angle in spite of the scratches. The superhydrophobic coating was then subjected to UV irradiation. Solid wood without precoated PDMS was used as a control for this procedure. The water contact angle of the solid wood without PDMS coating dropped rapidly and became superhydrophilic after 10 h of UV irradiation. Meanwhile, the water contact angle of the solid with PDMS coating was above 150° after 10 h of irradiation but water droplets got stuck to its surface signifying a rise in its contact angle hysteresis (Fig. 1d). In order to check the self-healing property, the coated substrate was subjected to heat treatment which caused the surface to recover. The UV irradiation destroyed the PMC which exposed the TiO2 nanoparticles and caused the surface to become sticky. The heat treatment caused the hydrophobic PDMS molecules to move to the surface to reduce the surface energy and restore the superhydrophobicity. After 20 cycles of ultraviolet irradiation and heat treatment, the water contact angle was still above 150° (Fig. 1e). The sliding angle increased to above 25° after ultraviolet irradiation but dropped to below 10° after every treatment. Surprisingly after the first 10 irradiation and heating cycles, the sliding angle was still below 10° for the remaining cycles of UV irradiation without any heat treatment (Fig. 1f). Guo et al. [70] also prepared a robust, self-healing superhydrophobic surface by spraying a solution containing aluminum

self-healing superhydrophobic surfaces using different methods. Although a lot of papers have talked about work in this field, a few excellent papers with new approaches are discussed. This review also talks about using cost-effective and environmentally friendly materials for fabrication of the surfaces from the pointview of self-sealing mechanism: (1) Fabrication of self-healing superhydrophobic surfaces by storing hydrophobic components; (2) Fabrication of self-healing superhydrophobic surfaces by regeneration of the topographic surface. It is expected that this review will generate a lot of interest in the fabrication and application of self-healing superhydrophobic surfaces in various fields.

2. Fabrication of self-healing superhydrophobic surfaces 2.1. Fabrication of self-healing superhydrophobic surfaces by storing hydrophobic components Wang et al. [69] also fabricated a self-healing superhydrophobic surface by spray coating perfluoroalkyl methacrylic copolymer (PMC) and titanium dioxide (TiO2) nanocomposites on hydrophilic solid wood which was precoated with polydimethylsiloxane (PDMS). TiO2 nanoparticles provide the rough surface of the material. PMC serves as a binder to hold nanoparticles onto the PDMS coated wood. PDMS acts a reservoir for hydrophobe which moves to surface to bring back the superhydrophobicity if the surface is damaged. The scanning electron microscopy (SEM) image showed a rough surface with micro-/nanoscale structures scattered on the surface (Fig. 1a). It also reveals that the small nanoparticles came together to form bigger particles. The hierarchical structures are essential for superhydrophobicity. The water 535

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Fig. 5. (a) SEM images of F-POSS/APP/bPEI-coated fabric. (b) SEM images of F-POSS/APP/bPEI-coated fabric after vertical flame testing. (c) SEM images of the FPOSS/APP/bPEI-coated cotton fabric after 1000 cycles of abrasion. (d) Abrasion area on the F-POSS/APP/bPEI-coated cotton fabric marked with a rectangle. (e) Fabric recorded at 4 s after ignition. (f) Fabric after vertical flame testing. Reproduced with permission from Ref. [73].

amphiphile but regained its superhydrophobicity after heat treatment. Jiang et al. [71] also fabricated a robust self-healing superhydrophobic surface by spray coating polysiloxane emulsion combining POSS modified SiO2 nanoparticles on different substrates. It was observed that using only 40% of modified SiO2 nanoparticles was enough to achieve superhydrophobicity. Increasing the amount of modified SiO2 nanoparticles caused no significant increase in the contact angle. The SEM showed spherical structures which confirmed the presence of silica nanoparticles and also hierarchical roughness (Fig. 3a). The coating was applied on substrates such as aluminum plate, cotton fabric, glass and wood and the surfaces were also not wetted by liquids such as soy, milk, orange juice, coffee and tea. The coatings remained superhydrophobic after 30 cycles of sand impact tests (Fig. 3b). There was also no significant change in contact angle after the coatings were immersed in acid/alkali solutions ranging from pH 2 to 12 (Fig. 3c). The self-healing ability was tested by O2 plasma treatment. The coatings became superhydrophilic after plasma treatment but could regain its superhydrophobicity after heating at a high temperature (Fig. 3d). The easy movement of fluorinated alkyl chains at room temperature or heating at high temperature caused the coating to regain its superhydrophobicity. The plasma/heating healing process was done for 10 cycles and the coating recovered in each cycle. In this work, they focused on only the contact angle neglecting the sliding angle or contact angle hysteresis which is also requirements for a superhydrophobic surface. Liu et al. [72] fabricated a superhydrophobic surface with selfhealing and anti-corrosion properties by spraying a solution of epoxy resin (EP), polydimethylsiloxane (PDMS) and SiO2 nanoparticles on

phosphate (AP), titanium dioxide (TiO2) and octadecyl trichlorosilane (OTS) on several substrates. The prepared AP was dissolved in water then added to the TiO2/OTS solution. The AP/TiO2/OTS solution was then sprayed onto the substrates under nitrogen gas and then heated at high temperature to obtain superhydrophobic coatings on substrates. The SEM showed that the coated substrates had a rough surface with dense and smaller nanoparticles coming together to form massive nanoparticles which help in water repellency. All the coated substrates exhibited superhydrophobicity with water contact angles above 150° and sliding angles below 10° (Fig. 2a). Sandpaper abrasion and sand impact tests were used to analyze the mechanical durability of the coated substrates. Sandpaper under 50 g weight was used for the abrasion. The water contact angle of the substrates remained greater than 150° after 100 abrasion cycles (Fig. 2b) and there were no visible scratches on substrates. 20 g sand from a funnel was made to fall on the coated substrates to see its effects. There was a slight drop in water contact angle after 50 cycles but also showed no visible scratches on substrates (Fig. 2c). The coated substrates were immersed in hot dichloroethane, hot water and hot acetone for a day. The water contact angle was above 150° after immersion. However, there was a slight decrease in water contact angle after the coated substrates were abraded with sandpaper after immersion in the hot solutions. The coated fabric became superhydrophilic after O2 plasma treatment but the fabric could regain its superhydrophobicity after heat treatment (Fig. 2d). The heat treatment caused the rapid migration of the low surface energy components to the surface which caused the coating to regain its superhydrophobicity. The coated fabric also became hydrophobic after treatment with an 536

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Fig. 6. (a) Ultraviolet-visible absorption spectra of solutions of AgNPs displaying different colors: 1) yellow, 2) orange, 3) red, 4) blue, 5) green, and 6) violet. (b) Digital photographs of solutions of AgNPs with different colors. (c) Cotton fabrics dyed with AgNPs of the above colors. (d) SEM image of F-POSS/AgNPs/PEI-coated cotton fabric (Blue cotton fabric was used). (e) Water contact-angle changes of the F-POSS/AgNPs/PEI-coated cotton fabric upon repeated O2 plasma etching and selfhealing. Inset: Shapes of water droplets (4 μL) of the prepared (top left), O2 plasma etched (bottom) and healed (top right) cotton fabric. Reproduced with permission from Ref. [64]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

polyethylenimine (bPEI) and fluorinated-decyl polyhedral oligomeric silsesquioxane (F-POSS). The SEM images showed that a weave-like structure was deposited on the fabric. Also, the F-POSS provided a micro-/nanoscale roughness which is required for superhydrophobicity (Fig. 5a). The APP/bPEI/F-POSS coated fabric had a water contact angle of 160° and a sliding angle of 4°. The flame-retardant property was tested by vertical flame with an uncoated cotton fabric acting as a control experiment. Upon subjection to the flame, the uncoated fabric quickly caught fire and was completely burnt after 14 s. However, only a small portion of the coated fabric caught fire and turned black after exposure to the flame. The flame went off on its own after the flame source was removed. This demonstrated the flame-retardant property of the coated fabric. The SEM of the black portion left by the flame showed no damage to the micro-/nanoscale structures but made the surface rough (Fig. 5b). The O2 plasma treatment was used to test the selfhealing superhydrophobic property of the coated fabric. The coated fabric became superhydrophilic after the plasma treatment but regained its superhydrophobicity after being left in a humid environment. The plasma treatment caused the F-POSS on the fabric surface to completely decompose hence the superhydrophilicity. However, F-POSS was not only deposited on the fabric surface but also embedded in the APP/bPEI coating. When the F-POSS on the surface got decomposed, the APP/ bPEI drove the embedded F-POSS to migrate to the surface to lower the surface energy which caused the coating to regain its superhydrophobicity. The O2 plasma treatment and healing were done for 10

several substrates. The SEM image showed rough hierarchical structure caused by SiO2 nanoparticles on the surface of the magnesium alloy (Fig. 4a). It had a water contact angle of 159.5° and sliding angle of 3.8°. The substrate was then immersed in boiling water and liquid nitrogen to check the effects of extreme temperature. There was barely any change in contact angle and sliding angle after immersion in the liquids (Fig. 4b, c). This showed the superhydrophobic coating can be used in extreme temperature conditions. Mechanical durability was also tested by a number of methods such as sand abrasion test, knife scratching, tape peeling and so on. The coating remained superhydrophobic after the various mechanical tests hence showing the mechanical durability of the coating. Self-healing ability was then tested by O2 plasma treatment. The coating became superhydrophilic after the plasma treatment but regained its superhydrophobicity after heating for some time. Superhydrophilicity occurred because of the accumulation of the hydrophilic oxygen-containing groups (–OH) on the surface. The heat treatment caused the hydrophobic PDMS to move to the surface which caused the coating to regain its superhydrophobicity. The plasma treatment/heating was done for repeated cycles and the coating could regain its superhydrophobicity in each case (Fig. 4d). However, the time taken to regain the superhydrophobicity increased as the healing cycles increased. Sun et al. [73] also embedded an intumescent flame-retardant and self-healing superhydrophobic coating on cotton fabric by dip-coating. The coating was made of ammonium polyphosphate (APP), branched 537

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Fig. 7. (a) Water contact angle of O2 plasma-treated coating and the coating after self-healing. (b) SEM image of the (PAH‐SPEEK/PAA)*60.5 superhydrophobic coating after 6 cycles of O2 plasma etching and self‐healing. (c) Time taken for O2 plasma-treated coatings to restore their original superhydrophobicity in different environmental humidities. (d) X-ray photoelectron spectra of prepared superhydrophobic coating (1), the same coating after Ar+ plasma etching for 2 (2), 7 (3), and 17 min (4), and an O2 plasma-etched coating after self-healing (5). (e) SEM image of the scratched coating. (f) Wetting characterization of the scratched coating before self-healing. Reproduced with permission from Ref. [74].

The rubbed fabric regained its superhydrophobicity after being left in a humid environment. It was realized that it took almost 4 times the time it took the non-rubbed coated fabric to regain its superhydrophobicity. The O2 plasma treatment and healing of the rubbed fabric were done for 8 cycles. The fabric regained its superhydrophobicity in each case but took more time after each plasma treatment. The flame-retardant property of the rubbed fabric was also tested. 20 cm of the fabric was rubbed and marked (Fig. 5d). The flame spread for only 13 cm after

cycles without any decrease in water contact angle. It was noted that the healing took more time as the number of plasma treatment increased. The durability of the fabric was also tested by rubbing cylindrical copper for 1000 cycles. The SEM images showed breakages in the weave-like structures (Fig. 5c). The water contact angle of the rubbed coated fabric was 153° with a sliding angle of 6°. The rubbed fabric was then subjected to O2 plasma treatment which made it superhydrophilic. 538

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Fig. 8. (a) SEM images of coated cotton fabric, (b) Contact angles and sliding angles of coated cotton fabric after different abrasion cycles, (c) Healing ability of superhydrophobic coatings against plasma treatment and contact angle of coated cotton fabric with repeated plasma treatment and healing cycles (d) SEM images of the char residues after vertical flame test of coated cotton fabric. Reproduced with permission from Ref. [75].

ignition and went off on its own after the flame source was removed (Fig. 5e). The other part of the fabric remained untouched by the flame (Fig. 5f). This showed the coated fabric had excellent durability against mechanical damage. Sun et al. [64] also fabricated a durable self-healing superhydrophobic coated cotton fabric with tunable colors by deposition in branched poly(ethylenimine) (PEI), silver nanoparticles with different colors (AgNPs) and fluorinated-decyl polyhedral oligomeric silsesquioxane (F-POSS). The coated fabric also had an antibacterial property. UV–vis absorption spectra confirmed the different colors of aqueous AgNPs used (Fig. 6a, b). Fig. 6c also showed the dyed cotton fabric after deposition in the aqueous AgNPs solution. The coated fabric was then inserted in an ethanol solution of F-POSS and then dried to obtain PEI/AgNPs/F-POSS coated cotton fabric. The fabrics still maintained their colors. The coated cotton fabric had a water contact angle of 169°and a sliding angle of 3°. The SEM image showed micro-/nanoscale structures with a surface roughness (Fig. 6d). Escherichia coli (E. coli) and Bacillus subtilis (B. subtilis) were used to test the antibacterial property of the PEI/AgNPs/F-POSS coated cotton fabric. Uncoated, PEI/F-POSS coated and AgNPs/PEI coated cotton fabrics were used as control experiments. The uncoated and PEI/F-POSS coated cotton

fabrics supported the growth of E. coli and B. subtilis whilst the AgNPs/ PEI coated and PEI/AgNPs/F-POSS coated cotton fabrics did not support any growth. This showed that silver nanoparticles were responsible for the antibacterial property and proved PEI/AgNPs/F-POSS coated cotton fabric had an antibacterial property. The self-healing ability was tested using O2 plasma treatment. The fabric became superhydrophilic after plasma treatment but regained its superhydrophobicity after being placed in a humid environment. The fabric could regain its superhydrophobicity after 16 plasma treatment/healing cycles (Fig. 6e). However, the time taken for each healing increased as the plasma treatment cycles increased. Mechanical durability was also tested by abrasion with a cylindrical copper rod. The water contact angle decreased while the sliding angle increased after 5000 abrasion cycles. Layer by layer deposition is a technique that involves forming alternate layers of oppositely charged particles on substrates. Sun et al. [74] fabricated a self-healing superhydrophobic surface using a layer by layer assembly of polyallylamine hydrochloride (PAH) and sulfonated polyether ether ketone (SPEEK) with polyacrylic acid (PAA) and then deposited onto substrates. Polydiallyldimethylammonium chloride (PDDA) was first predeposited on the silicon wafer which helps the superhydrophobic coatings to adhere 539

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Fig. 9. (a) Roll-off angle of the FPU + 15 wt% F-POSS coating after O2 plasma treatment versus recovery time at 80 °C. The insets show dyed blue water droplets after O2 plasma treatment and after thermal recovery. (b) 10 continuous O2 plasma/recovery cycles, which show that the self-healing ability of the FPU/F-POSS coating was quite robust. (c) The roll-off angle of the FPU/F-POSS coating versus temperature after 1 h. The inset shows thermogravimetric analysis (TGA) of the same coating at different temperature points. Reproduced with permission from Ref. [76].

with POTS hence confirming the presence of POTS molecules on the coating surface (Fig. 7d). Fluorine signal was still seen after Ar plasma treatment which proved that PAH-SPEEK/PAA coating can harbor an abundance of POTS molecules. This showed the superhydrophobic coatings can preserve and smoothly aid in the migration of POTS molecules. The mechanical durability of the coating was also tested using sandpaper. The water contact angle of the coating did not change after it was scratched with sandpaper. No scratch was even seen. When the scratching pressure was increased, few scratches were seen on the coating (Fig. 7e). The water contact angle reduced to 154° but greatly affected the sliding angle which caused water droplets to stick to the surface (Fig. 7f). After the coating was taken to an environment with 100% relative humidity, the water contact angle became 156° with a sliding angle around 5°. This showed the coating could also self-heal even after mechanical damage. Zeng et al. [75] fabricated a self-healing superhydrophobic and flame-retardant coating on cotton fabric using branched poly(ethylenimine) (b-PEI), ammonium polyphosphate (APP) and fluorinated silica@polydimethylsiloxane composite (F-SiO2@-PDMS) using dipcoating and layer-by-layer assembly. The SEM images showed a uniform and rough surface (Fig. 8a). It was also realized that increasing the mass ratio of F-SiO2 to PDMS caused an increase in the contact angles. The coated cotton fabric also showed good thermal stability. It retained its superhydrophobicity after heating at high temperatures. The fabric was then treated with acid/alkali solutions. There was no change in the

onto the surface. The substrate is then immersed in PAH-SPEEK complex and then in an aqueous PAA solution which then produces one layer of PAH-SPEEK/PAA coating. The procedure is repeated to get the desired number of layers. The superhydrophobic coatings were finally acquired after chemical vapor deposition of perfluorooctyltriethoxysilane (POTS) on cross-linked PAH-SPEEK/PAA coating. The superhydrophobic coatings had a water contact angle of 157° and a sliding angle of 1° which ensured that water droplets easily roll off the coatings. The self-healing property of the superhydrophobic coatings was tested by treating with O2 plasma. The coatings became superhydrophilic after the O2 plasma treatment. The POTS layer on the coating surface was destroyed while the plasma treatment also provided the surface with hydrophilic oxygen-containing groups. The coatings were then taken to an environment with a relative humidity of 40% for about 4 h. The coatings regained its superhydrophobicity with a water contact angle of 157° and a sliding angle below 2°. The hydrophobic POTS molecules stored in the PAH-SPEEK/PAA coating migrate to the surface and restore the superhydrophobicity. After several O2 plasma treatments and healing cycles, the water contact angle remained the same (Fig. 7a). SEM images also confirmed the coatings still had micro-/nanoscale structures even after several plasma treatments and healing process (Fig. 7b). Another factor which was found was that the self-healing process was humidity dependent (Fig. 7c). The more humid the environment, the faster the rate of healing. X-ray photoelectron spectrum (XPS) showed the presence of fluorine which is associated

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Fig. 10. (a) The changes in WCA and SA of SOMs encapsulated with different types of oils. (b) Rhodamine B water drops on HSMs coatings. (c) Rhodamine B water drops on SOMs coatings. (d) WCA of different coatings. (e) WCA of composite coating with flushing cycles. (f) Digital photograph of composite coating (inset, SEM image) before abrasion. (g) Digital photograph of composite coating (inset, SEM image) after 200 cyclic sandpaper abrasions. Reproduced with permission from Ref. [77].

superhydrophobicity even after 6 plasma treatment/healing cycles (Fig. 8c). The flame-retardant property was also tested while using uncoated cotton fabric as a control experiment. The uncoated fabric instantly caught fire after exposure to flame. The flame spread quickly until the fabric was almost completely burnt. For the coated fabric, the fire extinguished on its own after the flame source was removed and the fabric was still intact. The SEM of the charred area showed no significant change to the morphology but made the surface rougher with bubble-like swellings (Fig. 8d). Golovin et al. [76] fabricated numerous superhydrophobic surfaces by combining polymeric binders and hydrophobic fillers via spray coating. They developed a miscibility parameter denoted as S*. The parameter helped to predict if the binder would split from the filler

contact angle after immersion in pH = 2 solution. The SEM also showed no change in surface morphology. However, there was a decrease in contact angle after immersion in pH = 12 solution. The SEM showed cracks and peeling of micro/nanostructures. This shows coating cannot be used in alkali conditions. The abrasion test was also done to test mechanical durability. The contact angle decreased gradually when the abrasion cycles increased while the sliding angle increased drastically (Fig. 8b). This shows the coating is not resistant to mechanical damage. The O2 plasma treatment was used to test the self-healing ability. The fabric became hydrophilic after plasma treatment but regained its superhydrophobicity after heating for some time. The heat treatment caused the hydrophobic components to move to the surface and restore superhydrophobicity. The fabric was able to regain its 541

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Fig. 11. (a) SEM photograph PU/SiO2@HD-POS coating. (b) Changes of superhydrophobicity in the abrasion test against sandpaper. (c) SEM photograph of the coatings after 200 cycles (40 cm per cycle) under 9.8 kPa. (d) Changes of superhydrophobicity with tape-peeling cycles. (e) SEM photograph of the coatings after 200 cycles under 90.5 KPa. (f) Variation of water CA with plasma/self-healing cycles. Reproduced with permission from Ref. [78].

hydrolyzed F-POSS and made the coating hydrophilic. However, due to the heating, the remaining F-POSS stored in the coating migrated to the surface to reduce the surface energy and hence restore superhydrophobicity. Another factor observed was that the coating regained its superhydrophobicity faster when the temperature was increased. The coating still regained its superhydrophobicity even after 10 O2 plasma treatments and a heating process (Fig. 9b). The coating was then subjected to high temperatures of about 400 °C (Fig. 9c). However, the abraded surface still maintained its superhydrophobicity even though the abrasion causes some damage to the texture. The coating still showed a roll-off angle less than 15° at high temperatures and maintained its superhydrophobicity. Wu et al. [77] in their work decided to fabricate for the first time a

during spray coating. The water contact angle was measured for over 50 combinations of fluorodecyl polyhedral oligomeric silsesquioxane (FPOSS) and several binders. Hydrophobic fillers such as eicosane, octaisobutyl polyhedral oligomeric silsesquioxane (IB-POSS) and fluorooctyl polyhedral oligomeric silsesquioxane (FO-POSS) were used. During the fabrication process, for each binder that was used the amount of filler was varied. Fluorinated polyurethane elastomer (FPU) combined with 15 w% F-POSS was the most promising surface. The surface remained superhydrophobic even after 800 abrasion cycles using a rotary taber abrader. This showed the mechanical durability of the surface. The coating was also subjected to O2 plasma treatment. The surface became hydrophilic within minutes but regained its superhydrophobicity upon heating (Fig. 9a). The plasma treatment 542

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Fig. 12. (a) SEM images of the FD–POSS/FAS coated polyester fabric (inset SEM image with increased magnification). (b) Contact angle changes after the first 10 cycles of plasma-and-heat treatments. (c) Contact angle changes after plasma-and-heat treatment cycles. (d) SEM images of 1) freshly-coated fiber, 2) after 1st plasma treatment, 3) after 1st heat treatment (135 °C, 3 min), 4) after 50th plasma and heat treatment cycles. Reproduced with permission from Ref. [80].

Fig. 13. SEM images of superhydrophobic coatings on (a) brick, (b) marble and (c) glass substrates. (d) Water contact angles after emery paper abrasion. Reproduced with permission from Ref. [81].

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energy material (HD-POS) to move to the oxidized surface which caused the coating to regain its superhydrophobicity. The plasma treatment/heat treatment was done for nine cycles and the coating could recover in each case (Fig. 11f). The coating was also able to selfheal after damages such as immersion in pH = 14 solution.

mesoporous silica-shell/oil-core mesospheres (SOMs) which would be used for preparing non-silicon and non-fluorine superhydrophobic waterborne coating systems. SOMs were prepared by dissolving Cetyl trimethyl ammonium bromide (CTAB) in water and ethanol solution and then stirred while adding a mixture of ethanol and silicone oil. 1 ml of ammonia solution and 0.67 ml of tetraethoxysilane (TEOS) were added into the mixture and then stirred for 6 h. A white suspension was obtained which was then centrifuged and washed a few times with distilled water and ethanol to remove any residue. The resulting product was dried in a hot air oven at 30 °C. Four different types of oils were used namely alkyl silicone oil, amino silicone oil, mineral oil and hydro-silicone oil. KH550-SOMs were prepared by the same procedure but except 0.33 ml 3-aminopropyltriethoxy silane (KH550) was added. Mesoporous hollow silica microspheres (HSMs) without oil were also fabricated to act as a control. SOMs coatings were obtained by dispersing SOMs in a water/ethanol solution and then spray coated on glass substrates and dried. Waterborne polyurethane (WPU)/SOMs composite coatings were obtained by dispersing 0.5 g of KH550-SOMs in water/ethanol solution and then sonicated. The resulting product was mixed with WPU under magnetic stirring for 2 h and then spray-coated on glass substrates and dried. WPU/HSMs coatings were also fabricated with the same procedure to serve as a control. SOMs with alkyl silicone oil had the largest WCA of 168° with a sliding angle of 1.4° (Fig. 10a). This was because it had the lowest surface tension of the four different oils used. HSMs coated glass could be wetted by water but the SOMs coated glass could not (Fig. 10b, c). WPU/SOMs coatings also showed superhydrophobicity with a WCA of 158° while WPU and WPU/HSMs did not (Fig. 10d). The durability of the superhydrophobic coating was tested by treating the coating under flushing water. There was no significant change in the WCA after 500 flushing cycles (Fig. 10e). The superhydrophobic coating was also subjected to sandpaper abrasion test. Scratches were observed on the coating after 200 abrasion cycles (Fig. 10f, g). This led to a decrease in the WCA. The self-healing was achieved by releasing the hydrophobic oils from the mesoporous shells to cover the coating surface and the hierarchical structure of SOMs which restored the superhydrophobicity. Zhang et al. [78] in their work fabricated a waterborne, self-healing superhydrophobic coating by spray coating aqueous polyurethane (PU) and polysiloxane-modified SiO2 (SiO2@HD-POS) onto a glass substrate. The main application of their superhydrophobic coating is anti-icing. PU was used as an adhesive to hold the SiO2@HD-POS firmly on the substrate. SiO2@HD-POS was obtained by adding SiO2 to an aqueous HCl solution which was then stirred then ultrasonicated. Hexadecyltrimethoxysilane (HDTMS) and tetraethoxysilane (TEOS) were then added and the solution was stood at room temperature for a day to obtain SiO2@HD-POS suspension. SEM image showed a rough nanostructure with the SiO2 nanoparticles coming together to form a compact nanostructure (Fig. 11a). The coated glass substrate exhibited superhydrophobicity with a WCA of 163.9° and a sliding angle of 3.7°. The PU/SiO2@HD-POS coating was also coated on other substrates such as polyester fabric, wood plate and PU sponge and they all showed superhydrophobicity with a WCA greater than 150° and a sliding angle below 10°. Sandpaper abrasion was used to test the mechanical durability of the coating. It was done for 200 cycles. Three different pressures were applied to see the effect. It was noticed that increasing the applied pressure did not have a significant impact on the contact angle (Fig. 11b). SEM showed some cracks and also some of the nanostructures were destroyed (Fig. 11c). This caused a decrease in the contact angle. Tape peeling test was also done for 200 cycles under three different pressures. It was found that increasing the pressure and the abrasion cycle greatly affected the contact angle (Fig. 11d). This is because some of the SiO2@HD-POS peeled off by the tape. This was confirmed by the SEM (Fig. 11e). The coating was also subjected to O2 plasma treatment. The coating became superhydrophilic after plasma treatment. However, it regained its superhydrophobicity after heat treatment. This was because the heat treatment caused the low surface

2.2. Fabrication of self-healing superhydrophobic surfaces by regeneration of the topographic surface Zhang et al. [79] fabricated superhydrophobic and self-healing polydimethylsiloxane (PDMS) film by replicating the microstructure of shark skin on PDMS films and surface-initiated atom transfer radical polymerization (SI-ATRP) of 2-perfluorooctylethyl methacrylate (FMA). The PDMS films were immersed in DMF or ethanol. The water contact angle was 158° and 166° after immersion in DMF and ethanol respectively whilst the shedding angle remained below 15° for both solvents. The sand abrasion test was used to test the mechanical durability of the films. The PDMS films lost its superhydrophobicity after 20 abrasion cycles. The SEM images showed the surface had become flat and also a loss of some hierarchical structures. Mechanical abrasions (fingerwiping test) with and without water were also done to check the selfhealing property. The PDMS films were able to maintain their superhydrophobicity after 8 cycles of mechanical damage with water and healing by immersion in DMF. The surface became flat and possessed less hierarchical structure after finger wiping with water due to the abrasion. The hierarchical structures started to appear again after immersion in DMF. PFMA brushes self-assembled into micro/nanostructures again. The films also kept their superhydrophobicity after 6 cycles of mechanical damage without water and healing with DMF immersion. After second damage and healing, the morphology changes were the same as those in first damage and healing. The same results were obtained for finger wiping without water. The SEM images showed a slight decrease in the microstructures after each damage but the surface had rougher hierarchical structures after immersion in DMF. The PDMS films also showed excellent drag reduction effect. Dip coating means immersing a substrate into a container containing coating solutions then removing then drying to obtain a coated substrate. Dip coating is normally used to obtain film coatings on substrates. Lin et al. [80] made a superhydrophobic and superoleophobic surface by coating fluorinated-decyl polyhedral oligomeric silsesquioxane (FD-POSS) and fluorinated alkyl silane (FAS) onto polyester fabric. The coating was prepared by combining FAS and FDPOSS in a 5:1 mass ratio. The solution is then put onto the surface via a dip-coating method and then dried. The SEM image showed a uniform distribution of the coating on the fabric (Fig. 12a). Water, hexadecane and tetradecane were used as the liquids to test the contact angles. The contact angles were 171°, 155° and 152° for water, hexadecane and tetradecane respectively. To show the self-healing property of the coated fabric, the coated polyester fabric was then damaged by O2 plasma treatment. The surface became superhydrophilic with a contact angle of 0° for all 3 liquids. After the plasma treated fabric was heated, the surface regained its superhydrophobicity with contact angles of 171°, 155° and 151° for water, hexadecane and tetradecane respectively. The O2 plasma treatment and the healing process were done for 10 cycles (Fig. 12b). The surface was still superhydrophobic without any change in its contact angle. However, after 100 plasma treatment and healing process cycles, the surface was still superhydrophobic but lost its superoleophobicity (Fig. 12c). The SEM images showed that after numerous plasma treatment and healing process, cracks were discovered on the surface of the coating (Fig. 12d). This was the reason behind the reduced superhydrophobicity and superoleophobicity. The coated polyester fabric was treated with strong acid and base solutions. The coated fabric was inserted into a potassium hydroxide (KOH) solution with a pH of 14. The fabric became superhydrophilic with a contact angle of 0° for water and hexadecane. The fabric though regained its superhydrophobicity 544

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Acknowledgments

and superoleophobicity after heat treatment with a contact angle of 171° and 154° for water and hexadecane respectively. The process was repeated for 7 cycles without any change in contact angles. The coated fabric was also immersed in a sulphuric acid solution (H2SO4) with a pH of 1 which resulted in no change in contact angle for water and hexadecane. The coated fabric also showed no change in superhydrophobicity and superoleophobicity after irradiation for a day using a middle-pressure Hg lamp. Zulfiqar et al. [81] made self-healing superhydrophobic coatings on brick, marble and glass substrates that were able to regain its superhydrophobicity even after damage by an acetone treatment. An amount of sodium silicate solution was diluted with distilled water and methanol was added to the resultant solution. Trimethylchlorosilane was then added to the solution and stirred to obtain the hydrophobic silica nanoparticles which were recovered by centrifugation. The marble, glass and brick substrates were sprayed with multipurpose spray adhesive which acted as a binder. The hydrophobic silica nanoparticles suspension was dropped on the sprayed substrates and then allowed to dry at room temperature. The SEM images (Fig. 13a–c) showed the hydrophobic silica nanoparticles are completely embedded in the adhesive coatings. It also displays the micro-/nanoscale structures and roughness on the substrates which are essential for superhydrophobicity. The water contact angle for marble, glass and brick were 166°, 163° and 168° respectively. Abrasion tests using emery paper and sand abrasion tests were done to check the durability of the coatings. All 3 substrates maintained their superhydrophobicity even after 5 cycles of abrasion with emery paper (Fig. 13d). Minimal drop in water contact angle was observed during the sand abrasion test. However, with an increase in the weight of the sand applied, the water contact angle reduced significantly. The acetone treatment was used to restore the superhydrophobicity of the substrates. The brick substrate was subjected to three sand abrasions and healing by acetone treatment cycles. After the third cycle, the water contact angle 146°. This shows the substrate cannot be used over a very long period of time as it loses its superhydrophobicity easily. Also, no talk if the substrates can withstand chemical damage or heal after chemical damage.

We acknowledge the financial supports of National Natural Science Foundation of China (21607063), Jiangsu Collaborative Innovation Center of Technology and Material of Water Treatment. Declaration of Competing Interest Authors have no competing interests to declare. References [1] K. Koch, B. Bhushan, Y.C. Jung, Self-cleaning efficiency of artificial superhydrophobic surfaces, Langmuir 25 (2009) 3240–3248, https://doi.org/10.1021/ la803860d. [2] T.T. Isimjan, T. Wang, S. Rohani, A novel method to prepare superhydrophobic, UV resistance and anti-corrosion steel surface, Chem. Eng. J. 210 (2012) 182–187, https://doi.org/10.1016/j.cej.2012.08.090. [3] F. Zhang, L. Zhao, H. Chen, S. Xu, D.G. Evans, X. Duan, Corrosion resistance of superhydrophobic layered double hydroxide films on aluminum, Angew. Chem. Int. Ed. 47 (2008) 2466–2469, https://doi.org/10.1002/anie.200704694. [4] Z. Wang, Y. Su, Q. Li, Y. Liu, Z. She, F. Chen, L. Li, X. Zhang, P. Zhang, Researching a highly anti-corrosion superhydrophobic film fabricated on AZ91D magnesium alloy and its anti-bacteria adhesion effect, Mater. Charact. 99 (2015) 200–209, https://doi.org/10.1016/j.matchar.2014.12.004. [5] R.J. Daniello, N.E. Waterhouse, J.P. Rothstein, Drag reduction in turbulent flows over superhydrophobic surfaces Drag reduction in turbulent flows over superhydrophobic surfaces, Phys. Fluids 21 (2009), https://doi.org/10.1063/1.3207885. [6] N.J. Shirtcliffe, G. Mchale, M.I. Newton, Y. Zhang, Superhydrophobic copper tubes with reduction, Appl. Mater. Interfaces 1 (2009) 1316–1323, https://doi.org/10. 1021/am9001937. [7] G. Mchale, M.I. Newton, N.J. Shirtcliffe, M. Curie, Immersed superhydrophobic surfaces: gas exchange, slip and drag reduction properties, Soft Matter 6 (2010) 714–719, https://doi.org/10.1039/b917861a. [8] N. Wang, L. Tang, Y. Cai, W. Tong, D. Xiong, Scalable superhydrophobic coating with controllable wettability and investigations of its drag reduction, Colloids Surf. A 555 (2018) 290–295, https://doi.org/10.1016/j.colsurfa.2018.07.011. [9] S.J.C. Xue, X. Guo, M. Zhang, J. Ma, Fabrication of robust superhydrophobic surfaces by modification of chemically roughened fibers via thiolene click chemistry, J. Mater. Chem. A 3 (2015) 21797–21804, https://doi.org/10.1039/C5TA04802H. [10] X. Gao, J. Zhou, R. Du, Z. Xie, S. Deng, R. Liu, Robust superhydrophobic foam: a graphdiyne-based hierarchical architecture for oil/water separation, Adv. Mater. 28 (2016) 168–173, https://doi.org/10.1002/adma.201504407. [11] H.H. Bay, D. Patino, Z. Mutlu, P. Romero, M. Ozkan, C.S. Ozkan, Scalable multifunctional ultra-thin graphite sponge: free-standing, oleophilic architecture with ferromagnetic properties for environmental cleaning, Sci. Rep. 6 (2016) 1–9, https://doi.org/10.1038/srep21858. [12] Y. Wu, S. Zhou, B. You, L. Wu, Bioinspired design of three-dimensional ordered tribrachia-post arrays with re-entrant geometry for omniphobic and slippery surfaces bioinspired design of three-dimensional ordered tribrachia-post arrays with re-entrant geometry for omniphobic and slipper, ACS Nano 11 (2017) 8265–8272, https://doi.org/10.1021/acsnano.7b03433. [13] B. Li, J. Zhang, Polysiloxane/multiwalled carbon nanotubes nanocomposites and their applications as ultrastable, healable and superhydrophobic coatings, Carbon NY 93 (2015) 648–658, https://doi.org/10.1016/j.carbon.2015.05.103. [14] B. Li, J. Zhang, Durable and self-healing superamphiphobic coatings repellent even to hot liquids, Chem. Commun. 52 (2016) 2744–2747, https://doi.org/10.1039/ C5CC09951J. [15] N. Wang, L. Tang, W. Tong, D. Xiong, Fabrication of robust and scalable superhydrophobic surfaces and investigation of their anti-icing properties, Mater. Des. 156 (2018) 320–328, https://doi.org/10.1016/j.matdes.2018.06.053. [16] Y. Tian, B. Su, L. Jiang, Interfacial material system exhibiting superwettability, Adv. Mater. 26 (2014) 6872–6897, https://doi.org/10.1002/adma.201400883. [17] B.X. Tian, T. Verho, R.H.A. Ras, Moving superhydrophobic surfaces toward realworld applications, Science 352 (2016) 142–143, https://doi.org/10.1126/science. aaf2073. [18] R. Crawford, L.K. Koopal, J. Ralston, Contact angles on particles and plates, Colloids Surf. 27 (1987) 57–64, https://doi.org/10.1016/0166-6622(87)80328-1. [19] R. Good, Contact angles and the surface free energy of solids, Surf. Colloid Sci. 11 (1979) 1–29, https://doi.org/10.1016/B978-0-12-800722-8.00002-3. [20] M. Ashraf, C. Campagne, A. Perwuelz, P. Champagne, A. Leriche, C. Courtois, Development of superhydrophilic and superhydrophobic polyester fabric by growing zinc oxide nanorods, J. Colloid Interface Sci. 394 (2013) 545–553, https:// doi.org/10.1016/j.jcis.2012.11.020. [21] N.J. Shirtcliffe, G. Mchale, S. Atherton, M.I. Newton, An introduction to superhydrophobicity, Adv. Colloid Interface Sci. 161 (2010) 124–138, https://doi.org/ 10.1016/j.cis.2009.11.001. [22] D. Qu, Non-sticking drops, Rep. Prog. Phys. 68 (2005) 2495–2532, https://doi.org/ 10.1088/0034-4885/68/11/R01. [23] P. Dimitrakellis, E. Gogolides, Hydrophobic and superhydrophobic surfaces fabricated using atmospheric pressure cold plasma technology: a review, Adv. Colloid Interface Sci. 254 (2018) 1–21, https://doi.org/10.1016/j.cis.2018.03.009. [24] T. Wong, T. Sun, L. Feng, J. Aizenberg, Interfacial materials with special wettability, MRS Bull. 38 (2013) 366–371, https://doi.org/10.1557/mrs.2013.99. [25] N.A. Patankar, Transition between superhydrophobic states on rough surfaces, Langmuir 20 (2004) 7097–7102, https://doi.org/10.1021/la049329e.

3. Perspective and conclusion The above studies show that in the past few years the level of interest in the field of self-healing superhydrophobic surfaces has increased. Novel methods are being introduced and discussed each year. Although good methods are being reported, some issues still need to be addressed for future applications. Cost-effective, safe and environmentally friendly materials should be used. Using perflourinated materials as the source of self-healing is bad for the long term as the materials can easily be decomposed or evaporated. Another issue has to do with the characterization of the materials as a durable, self-healing superhydrophobic surface. Standardized tests must be proposed for checking the self-healing property and durability of a superhydrophobic surface. Different papers are reported whereby tests used for checking the durability and self-healing property of a surface are different. Also, correct characterization is needed when checking the durability and self-healing property. For example, recording only the water contact angle after an abrasion test gives very little information because the abrasion normally increases the contact angle hysteresis which makes water droplets stick to the surfaces and not roll. Having a low contact angle hysteresis is a criterion for a surface to be superhydrophobic so it needs to be tested. Also, the effect of extreme conditions such as temperature needs to be tested on the surfaces. All the list above make it difficult for its transfer into real-life applications. More efforts must be directed in that way. A lot of ventures are emerging from self-healing superhydrophobic surfaces.

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[26] A. Lafuma, D. Quéré, Superhydrophobic states, Nat. Mater. 2 (2003) 457–460, https://doi.org/10.1038/nmat924. [27] H. Butt, I.V. Roisman, M. Brinkmann, P. Papadopoulos, D. Vollmer, C. Semprebon, Characterization of super liquid-repellent surfaces, Curr. Opin. Colloid Interface Sci. 19 (2014) 343–354, https://doi.org/10.1016/j.cocis.2014.04.009. [28] M. Callies, D. Quéré, On water repellency, Soft Matter 1 (2005) 55–61, https://doi. org/10.1039/b501657f. [29] M. Spaeth, W. Barthlott, Lotus-effect®: biomimetic super-hydrophobic surfaces an their application, Adv. Sci. Technol. 60 (2008) 38–46, https://doi.org/10.4028/ www.scientific.net/AST.60.38. [30] X. Zhang, F. Shi, J. Niu, Y. Jiang, Z. Wang, Superhydrophobic surfaces: from structural control to functional application, J. Mater. Chem. 18 (2008) 621–633, https://doi.org/10.1039/b711226b. [31] H. Bellanger, T. Darmanin, E. Ta, Chemical and physical pathways for the preparation of superoleophobic surfaces and related wetting theories, Chem. Rev. 114 (2014) 2694–2716, https://doi.org/10.1021/cr400169m. [32] Y. Wang, Y. Shi, L. Pan, M. Yang, L. Peng, S. Zong, Y. Shi, G. Yu, Multifunctional superhydrophobic surfaces templated from innately microstructured hydrogel matrix, Nano Lett. 14 (2014) 4803–4809, https://doi.org/10.1021/nl5019782. [33] Y. Lu, S. Sathasivam, J. Song, C.R. Crick, C.J. Carmalt, I.P. Parkin, Robust selfcleaning surfaces that function when exposed to either air or oil, Science 347 (2015) 1132–1135, https://doi.org/10.1126/science.aaa0946. [34] D. Aslanidou, I. Karapanagiotis, C. Panayiotou, Progress in organic coatings superhydrophobic, superoleophobic coatings for the protection of silk textiles, Prog. Org. Coatings 97 (2016) 44–52, https://doi.org/10.1016/j.porgcoat.2016.03.013. [35] Q. Gao, Q. Zhu, Y. Guo, C.Q. Yang, Formation of highly hydrophobic surfaces on cotton and polyester fabrics using silica sol nanoparticles and nonfluorinated alkylsilane, Ind. Eng. Chem. Res. 48 (2009) 9797–9803, https://doi.org/10.1021/ ie9005518. [36] D. Bazin, C. Faure, Superhydrophobic, highly adhesive arrays of copper hollow spheres produced by electro-colloidal lithography, Soft Matter 13 (2017) 5500–5505, https://doi.org/10.1039/C7SM01256J. [37] J. Gao, X. Huang, H. Xue, L. Tang, R.K.Y. Li, Facile preparation of hybrid microspheres for super-hydrophobic coating and oil-water separation, Chem. Eng. J. 326 (2017) 443–453, https://doi.org/10.1016/j.cej.2017.05.175. [38] J.O. Wolff, M. Seiter, E. Alexander, S.N. Gorb, Numerical simulation of colloidal self-assembly of super-hydrophobic arachnid cerotegument structures, J. Theor. Biol. 430 (2017) 1–8, https://doi.org/10.1016/j.jtbi.2017.07.001. [39] L. Deng, X. Kang, Y. Liu, F. Feng, H. Zhang, Characterization of gelatin/zein films fabricated by electrospinning vs solvent casting, Food Hydrocolloids 74 (2017) 324–332, https://doi.org/10.1016/j.foodhyd.2017.08.023. [40] M. Zahid, J.A. Heredia-guerrero, A. Athanassiou, I.S. Bayer, Robust water repellent treatment for woven cotton fabrics with eco-friendly polymers, Chem. Eng. J. 319 (2017) 321–332, https://doi.org/10.1016/j.cej.2017.03.006. [41] Y. Li, X. Men, X. Zhu, B. Ge, F. Chu, One-step spraying to fabricate nonfluorinated superhydrophobic coatings with high transparency, J. Mater. Sci. 51 (2016) 2411–2419, https://doi.org/10.1007/s10853-015-9552-5. [42] N. Bai, Q. Li, H. Dong, C. Tan, P. Cai, L. Xu, A versatile approach for preparing selfrecovering superhydrophobic coatings, Chem. Eng. J. 293 (2016) 75–81, https:// doi.org/10.1016/j.cej.2016.02.023. [43] K. Lum, D. Chandler, J.D. Weeks, Hydrophobicity at small and large length scales, J. Phys. Chem. A 103 (1999) 4570–4577, https://doi.org/10.1021/jp984327m. [44] W. Barthlott, C. Neinhuis, H. Verlot, C.L. Schott, Purity of the sacred lotus, or escape from contamination in biological surfaces, Planta 202 (1997) 1–8. [45] K. Koch, B. Bhushan, C. Jung, W. Barthlott, Fabrication of artificial Lotus leaves and significance of hierarchical structure for superhydrophobicity and low adhesion, Soft Matter 5 (2009) 1386–1393, https://doi.org/10.1039/b818940d. [46] J.B. Boreyko, C.H. Baker, C.R. Poley, C. Chen, Wetting and dewetting transitions on hierarchical superhydrophobic surfaces, Langmuir 27 (2011) 7502–7509, https:// doi.org/10.1021/la201587u. [47] C.-H. Xue, J.-Z. Maa, Long-lived superhydrophobic surfaces, J. Mater. Chem. A 1 (2013) 4146–4161, https://doi.org/10.1039/c2ta01073a. [48] A.C.C. Esteves, Y. Luo, M.W.P. Van De Put, C.C.M. Carcouët, G. De With, Self-replenishing dual structured superhydrophobic coatings prepared by drop-casting of an all-in-one dispersion, Adv. Funct. Mater. 24 (2014) 986–992, https://doi.org/10. 1002/adfm.201301909. [49] X. Li, D. Reinhoudt, M. Crego-calama, What do we need for a superhydrophobic surface? A review on the recent progress in the preparation of superhydrophobic surfaces, Chem. Soc. Rev. 36 (2007) 1350–1368, https://doi.org/10.1039/ b602486f. [50] B.L. Feng, S. Li, Y. Li, H. Li, L. Zhang, J. Zhai, Y. Song, B. Liu, L. Jiang, Superhydrophobic surfaces: from natural to artificial, Adv. Mater. 14 (2002) 1857–1860, https://doi.org/10.1002/adma.200290020. [51] G. Wu, J. An, X. Tang, Y. Xiang, J. Yang, A versatile approach towards multifunctional robust microcapsules with tunable, restorable, and solvent-proof superhydrophobicity for self-healing and self-cleaning coatings, Adv. Funct. Mater. 24 (2014) 6751–6761, https://doi.org/10.1002/adfm.201401473. [52] K. Chen, Y. Wu, S. Zhou, L. Wu, Recent development of durable and self-healing surfaces with special wettability, Macromol. Rapid Commun. 37 (2016) 463–485, https://doi.org/10.1002/marc.201500591. [53] Q. Liu, X. Wang, B. Yu, F. Zhou, Q. Xue, Self-healing surface hydrophobicity by consecutive release of hydrophobic molecules from mesoporous silica, ACS J. Surf. Colloids 28 (2012) 5845–5849, https://doi.org/10.1021/la300187q. [54] D. Zhu, X. Lu, Q. Lu, Electrically conductive PEDOT coating with self-healing superhydrophobicity, Langmuir 30 (2014) 4671–4677, https://doi.org/10.1021/ la500603c. [55] X. Deng, L. Mammen, H.-J. Butt, D. Vollmer, Candle soot as a template for a

[56]

[57]

[58]

[59] [60] [61]

[62]

[63]

[64]

[65]

[66]

[67]

[68]

[69]

[70]

[71]

[72]

[73] [74] [75]

[76]

[77]

[78]

[79]

[80]

[81]

546

transparent robust superamphiphobic coating, Science 335 (2012) 67–70, https:// doi.org/10.1126/science.1207115. T. Verho, C. Bower, P. Andrew, S. Franssila, O. Ikkala, R.H.A. Ras, Mechanically durable superhydrophobic surfaces, Adv. Mater. 23 (2011) 673–678, https://doi. org/10.1002/adma.201003129. N. Puretskiy, G. Stoychev, A. Synytska, L. Ionov, Surfaces with self-repairable ultrahydrophobicity based on self- organizing freely floating colloidal particles, ACS J. Surf. Colloids 28 (2012) 1–4, https://doi.org/10.1021/la204232g. U. Manna, D.M. Lynn, Restoration of superhydrophobicity in crushed polymer films by treatment with water: self-healing and recovery of damaged topographic features aided by an unlikely source, Adv. Mater. 25 (2013) 5104–5108, https://doi. org/10.1002/adma.201302217. Y. Li, L. Li, J. Sun, Bioinspired self-healing superhydrophobic coatings, Angew. Chem. Int. Ed. 122 (2010) 6265–6269, https://doi.org/10.1002/ange.201001258. J.A. Burdick, G.D. Prestwich, Hyaluronic acid hydrogels for biomedical applications, Adv. Mater. 23 (2011) 41–56, https://doi.org/10.1002/adma.201003963. K. Koch, B. Bhushan, H.-J. Ensikat, W. Barthlott, Self-healing of voids in the wax coating on plant surfaces, Philos. Trans. R. Soc. A 367 (2009) 1673–1688, https:// doi.org/10.1098/rsta.2009.0015. C. Neinhuis, K. Koch, Movement and regeneration of epicuticular waxes through plant cuticles, Planta 213 (2001) 427–434, https://doi.org/10.1007/ s004250100530. L. Zhang, B. Tang, J. Wu, R. Li, P. Wang, Hydrophobic light-to-heat conversion membranes with self-healing ability for interfacial solar heating, Adv. Mater. 27 (2015) 4889–4894, https://doi.org/10.1002/adma.201502362. M. Wu, T. Pan, S. Chen, J. Sun, Silver-nanoparticle-colored cotton fabrics with tunable colors and durable antibacterial and self-healing superhydrophobic properties, Adv. Funct. Mater. 26 (2016) 569–576, https://doi.org/10.1002/adfm. 201504197. Y. Cong, K. Chen, S. Zhou, L. Wu, Synthesis of pH and UV dual-responsive microcapsules with high loading capacity and their application in self-healing hydrophobic coatings, J. Mater. Chem. A 3 (2015) 19093–19099, https://doi.org/10. 1039/C5TA04986E. K. Chen, S. Zhou, S. Yang, L. Wu, Fabrication of all-water-based self-repairing superhydrophobic coatings based on UV-responsive microcapsules, Adv. Funct. Mater. 25 (2014) 1035–1041, https://doi.org/10.1002/adfm.201403496. X. Lü, Z. Cui, W. Wei, J. Xie, L. Jiang, J. Huang, J. Liu, Constructing polyurethane sponge modified with silica/graphene oxide nanohybrids as a ternary sorbent, Chem. Eng. J. 284 (2016) 478–486, https://doi.org/10.1016/j.cej.2015.09.002. J. Liu, W. Wei, X. Lv, W. He, Z. Cui, L. Jiang, J. Huang, Fabrication of polysiloxanemodified polyurethane sponge as low-cost organics/water separation and selective absorption material, Water Sci. Technol. 74 (2016) 1936–1945, https://doi.org/10. 2166/wst.2016.348. K. Tu, X. Wang, L. Kong, H. Guan, Facile preparation of mechanically durable, selfhealing and multifunctional superhydrophobic surfaces on solid wood, Mater. Des. 140 (2018) 30–36, https://doi.org/10.1016/j.matdes.2017.11.029. M. Liu, Y. Hou, J. Li, L. Tie, Y. Peng, Z. Guo, Inorganic adhesives for robust, selfhealing, superhydrophobic surfaces, J. Mater. Chem. A 5 (2017) 19297–19305, https://doi.org/10.1039/c7ta06001g. R. Zhao, Y. Chen, G. Liu, Y. Jiang, K. Chen, Fabrication of self-healing waterbased superhydrophobic coatings from poss modified silica nanoparticles, Mater. Lett. 229 (2018) 281–285, https://doi.org/10.1016/j.matlet.2018.07.040. D.W. Li, H.Y. Wang, Y. Liu, D.S. Wei, Z.X. Zhao, Large-scale fabrication of durable and robust super-hydrophobic spray coatings with excellent repairable and anticorrosion performance, Chem. Eng. J. 367 (2019) 169–179, https://doi.org/10. 1016/j.cej.2019.02.093. S. Chen, X. Li, Y. Li, J. Sun, Intumescent flame-retardant and coatings on cotton fabric, ACS Nano 9 (2015) 4070–4076, https://doi.org/10.1021/acsnano.5b00121. Y. Li, L. Li, J. Sun, Bioinspired self-healing superhydrophobic coatings, Angew. Chem. Int. Ed. 49 (2010) 6129–6133, https://doi.org/10.1002/anie.201001258. D. Lin, X. Zeng, H. Li, X. Lai, Facile fabrication of superhydrophobic and flameretardant coatings on cotton fabrics via layer-by-layer assembly, Cellulose 25 (2018) 3135–3149, https://doi.org/10.1007/s10570-018-1748-9. K. Golovin, M. Boban, J.M. Mabry, A. Tuteja, Designing self-healing superhydrophobic surfaces with exceptional mechanical durability, Appl. Mater. Interfaces 9 (2017) 11212–11223, https://doi.org/10.1021/acsami.6b15491. T. Wang, Y. Bao, Z. Gao, Y. Wu, L. Wu, Synthesis of mesoporous silica-shell/oil-core microspheres for common waterborne polymer coatings with robust superhydrophobicity, Prog. Org. Coat. 132 (2019) 275–282, https://doi.org/10.1016/j. porgcoat.2019.03.051. Y. Li, B. Li, X. Zhao, N. Tian, J. Zhang, Totally waterborne, nonfluorinated, mechanically robust and self-healing superhydrophobic coatings for actual anti-icing, Appl. Mater. Interfaces 10 (2018) 39391–39399, https://doi.org/10.1021/acsami. 8b15061. Y. Liu, H. Gu, Y. Jia, J. Liu, H. Zhang, R. Wang, B. Zhang, H. Zhang, Q. Zhang, Design and preparation of biomimetic polydimethylsiloxane (PDMS) films with superhydrophobic, self-healing and drag reduction properties via replication of shark skin and SI-ATRP, Chem. Eng. J. 356 (2018) 318–328, https://doi.org/10. 1016/j.cej.2018.09.022. H. Wang, Y. Xue, J. Ding, L. Feng, X. Wang, T. Lin, Durable, self-healing superhydrophobic and superoleophobic surfaces from fluorinated-decyl polyhedral oligomeric silsesquioxane and hydrolyzed fluorinated alkyl silane, Angew. Chem. Int. Ed. 50 (2011) 11433–11436, https://doi.org/10.1002/anie.201105069. U. Zulfiqar, M. Awais, S. Zajif, I. Hussain, S.W. Husain, Durable and self-healing superhydrophobic surfaces for building materials, Mater. Lett. 192 (2017) 56–59, https://doi.org/10.1016/j.matlet.2017.01.070.