A review on superhydrophobic polymer nanocoatings: Recent

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Review

A review on superhydrophobic polymer nanocoatings: Recent development, application Sonalee Das, Sudheer Kumar, Sushanta Kumar Samal, Smita Mohanty, and Sanjay Kumar Nayak Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 12 Feb 2018 Downloaded from http://pubs.acs.org on February 12, 2018

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Industrial & Engineering Chemistry Research

A review on superhydrophobic polymer nanocoatings: Recent development, application Sonalee Das, Sudheer Kumar*, Sushanta K. Samal, Smita Mohanty, Sanjay K. Nayak Laboratory for Advanced Research in Polymeric Materials (LARPM),Central Institute of Plastics Engineering & Technology (CIPET), B/25, CNI Complex, Patia, Bhubaneswar 751024, Odisha, India, Fax: +91-674-2743863; Tel: +91-674-2742852 *Corresponding Author, E-mail: [email protected]

Abstract In recent years, the worldwide coating industries and scientific communities has introduced superhydrophobic coatings with exceptional water repellency for marine, automotive and medical applications. Various research works has been carried to create superhydrophobic surface. This review attempts to highlight the recent development and technical breakthrough on superhydrophobic coatings and its potential application in various fields. Moreover, emphasis has also been led to provide an insight regarding the recent development in superhydrophobic coatings using nanotechnology focusing both on novel preparation strategies and on investigations of their distinctive properties. Keyword: Superhydrophobic coating, Self-repairing, Water repellency, Nanotechnology.

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1. Introduction Recently, increasing attention has been concentrated towards the development of superhydrophobic coatings with unique structure, properties and extended applications in the field of anti-corrosion,1-4 anti-icing,5-7 anti-fogging,8-10 self-cleaning,11-13 anti-fouling,14-16 and others sectors17-21 etc., as depicted in Figure 1. During 2006 and 2010 superhydrophobic surface has been ranked 7th in the top 20 research fronts in material science by essential science indicators.22

Figure 1. Superhydrophobic coatings market report.23 In general, wetting behaviour can be classified into 4 different regimes on the basis of water contact angle (WCA). WCA in the range of 0° < θ < 10°, 10° < θ < 90°, 90° < θ < 150° and 150° < θ < 180° can be termed as superhydrophilic, hydrophilic, hydrophobic and superhydrophobic respectively as shown in Figure 2.24 The superhydrophobic coatings being nearly non-wetting results in easy rolling of water droplets from its surface.


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Water 90O Hydrophilic surface

Water 90O

Hydrophobic surface

Water Water 150 O Superhydrophobic surface

Substrate

Figure 2. Schematic illustrations of hydrophilic, hydrophobic and superhydrophobic surfaces. In recent years, there has been increased interest in fabricating superhydrophobic surface

of micro-to nanoscale

architecture. According to the

literature reports

superhydrophobic coatings has been present in nature from time immemorial. Various natural materials like lotus leaf, animal species and their specific parts as depicted in Figure 3 exhibit superhydrophobic properties.25 It has been well established that the use of nano-fillers such as nanosilica,26,27 titanium dioxide,28-30 zinc oxide,30,31 etc. can generate superhydrophobic coatings for large scale industrial applications.32-34 By definition superhydrophobic nanocoatings contain at least one nanoscale ingredient that plays a pivotal role in the coating properties or the morphology of the superhydrophobic coating is at nanoscale dimension.35 Superhydrophobic nanocoatings have emerged to be an important technique in generating surface superhydrophobization with remarkable change in surface and interfacial field. Amongst the various materials polymers with unique features like flexibility, good processability and low cost have emerged to be candid matrix material for fabricating



superhydrophobic coatings. Moreover, polymer composed of large molecular units with sophisticated branches, different structural and functional groups interconnected by covalent bonds can produce various configurations that can be employed in applications where low adhesion and friction are desired.36 Hence, based on the above features polymeric superhydrophobic coatings with nano functionalized smart surface can be designed for various end use applications which include self-cleaning, self-healing, anti-icing, antiadhesion and oil/water separation fields.

Figure 3. Superhydrophobicity in nature. Various research has been carried out determine the various factors responsible for attaining superhydrophobic surface. These factors include nanoscale roughness, surface 4 ACS Paragon Plus Environment

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energy, coating thickness, mechanical stability, and binding strength between the substrate and coating.37,38 In this regard, different models such as Wenzel and Cassie’s have also been used to correlate and understand the relationship between surface roughness and superhydrophobicity.39,40 Moreover, different approaches and techniques such as ultra-violet technique, photo-polymerization and spray technique etc. have also been used to modify the substrate surface to attain superhydrophobic surface with tuneable properties. In this review, the recent advances related to the application of different nano-filler into the polymer matrix for developing superhydrophobic coatings using different techniques and substrates has been highlighted. This review mainly deals with the fabrication techniques; methodologies adapted and will cover the different nano-fillers used to develop the superhydrophobic coatings. Further, section will deal with the potential application of these superhydrophobic polymer nano coatings in the field of medical, oil/water separation and other industrial areas has also been reported. Finally, this review will provide the concluding remarks and the challenges in the field of superhydrophobic nanocoatings that needs to be attained in future. 3. Development of artificial superhydrophobic polymer nano coatings This section will deal with the recent progress in the field of superhydrophobic polymer nano coatings using different nano-fillers. In addition, this section will also provide detailed insight regarding the methodologies adapted and the different techniques involved in developing superhydrophobic nanocoatings. 3.1 CNT based superhydrophobic nano coatings Gu et al.41 prepared carbon nanotube (CNT)-based superhydrophobic hybrid membranes

with

fire-retardant

properties

by


attaching

1H,1H,2H,2H-


perfluorodecyltriethoxysilane (PFDTS) onto -OH functionalized CNTs. The PFDTS/CNT membranes were found to be superhydrophobic with high separation oil/water efficiency under extreme conditions. De et al.42 designed super-hydrophobic multi-walled carbon nanotube (MWCNT) coatings for stainless steel by chemical vapour deposition (CVD) technique at lowtemperature ( 0.3. The coating with MR ratio < 0.3 indicated contact angle 128.3º due to lack of nanoscale roughness structure and hair like protrusions. The hair like protrusions arises due to the CNT ends and the skeleton of the coating layer constituted by CNTs, bonded and reinforced with the crosslinked PDMS. The coating with MR ratio < 0.3 exhibited a sliding angle of < 5º with low adhesion strength. It was also observed that the droplet changes its shape tremendously owing to the transformation of kinetic energy into surface energy. The coatings also showed greater long term stability which was validated through optical method. Thus, the authors concluded that the technique could be used to fabricate superhydrophobic coatings with suitability for engineering applications.

Figure 4. Fabrication of MWCNT-PDMS coatings. 3.2 Graphene based superhydrophobic nano coatings Yang et al.45 studied about the synthesis and characterization of superhydrophobic epoxy coatings on copper substrate modified by fluorographene (FG) nanosheets. In addition, GOc (graphene oxide-coated epoxy coating) and BLc (blank epoxy coating) were also prepared and coated onto the copper substrate (Figure 5). These nanosheets get spatially stuck onto the surface of epoxy resin to lower the surface energy and to build rough surface with


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random arrangement of micro/nano structure. The FG sheets prepared by modification of GO by Hummers method were characterized through transmission electron microscopy (TEM) and atomic force microscopy (AFM). From the results obtained it could be observed that the prepared FG sheets were exfoliated with the thickness of 3.5 nm. X-ray photoelectron spectroscopy (XPS) indicated the presence of evident peak at 688.5 eV representing fluorine species in the prepared FG sheets. The contact angle of FG coated copper substrate was found to be 154° whereas, that of BLc and GOc coating was found to be 116° and 82° respectively. The prepared surface with FG nanosheets maintain the water contact angle above 150° even after 60 abrasion cycle depicting good superhydrophobic feature. The superhydrophobicity characteristic of FG coated sheet was attributed to the irregular microstructure constructed by the randomly stacked FG nanosheets. Moreover, the FG coated nanosheets also exhibited self cleaning properties as compared to BLc and GOc coatings. Hence, the authors concluded that the prepared FG coatings demonstrated excellent protection performance characteristics owing to its self-cleaning properties, mechanical abrasion resistance and chemical stability both in acidic and alkaline aqueous solutions.



Figure 5. Schematic representation of superhydrophobic epoxy coating. Lin et al.46 developed superhydrophobic silane surface functionalized with graphene aerogels. Graphene as matrix are novel candidates for fabricating superhydrophobic surfaces owing to their intrinsically high surface area and non-polar carbon structure. On the other hand aerogels possess high porosity, large surface area, and extremely low bulk density, which render them a promising candidate for creating superhydrophobic surface for various novel applications. Graphene aerogels (GA) can be obtained through simple process with low density as compared to other hydrophobic surfaces produced using vertically aligned CNT or silica aerogels. As such GA possesses hydrophobicity owing to its high surface roughness and microscale roughness from existing macropores on its surface. This hydrophobicity behaviour can be further improved through the application of a fluorinated silane, wherein, a water contact angle of 160° can be obtained. It was observed that the water contact angle first increases and then decreases with an increment in the aerogel density for both modified and unmodified samples. This can be attributed to the progress of relative fraction of pores and graphene sheets. At low concentration, the surface has a high fraction of pores as compared to graphene sheets on the aerogel surface. On the other hand at high concentration, the surface roughness and porosity that traps air beneath the droplet is reduced resulting in an optimal density aerogel, providing maximum hydrophobicity. The author concluded that the developed superhydrophobic graphene aerogel can be a strong candidate for self-cleaning or water repelling applications where low bulk density is important. 3. 3 Silica based superhydrophobic nano coatings Junaidi et al.47 developed superhydrophobic coating based upon silica with photoluminescence properties synthesized from rice husk ash (RHA). In this work the RHA was calcined at 550 °C and 650 °C to form silica particles with a small amount of carbon


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residue. RHA calcined at 550 °C was denoted as RHA 550 while RHA calcined at 650 °C was denoted as RHA 650. The RHA-SiO2 was further chemically modified using 1H,1H,2H,2H perfluorodecyltriethoxysilane (HFDS) to form superhydrophobic coatings. Fluorescent images and photoluminescence spectra indicated successful grafting of different hydrophobic groups such as −CH2, −CH3, −CF2 and −CF3. However, the developed superhydrophobic coating could only be casted on glass slide and concrete by spray coating of RHA 550 HFDS on a layer of commercial adhesive. The coated concrete showed a water contact angle of 157.7°. Meng et al.48 synthesized and characterized water vapor permeable and superhydrophobic coating film via spray coating in combination with vapor-induced phase separation technique (VIPS). The VIPS process is a technique that is used for the preparation of porous materials especially porous membranes by casting the polymer solution in an atmosphere of non-solvent vapor usually water. The polymer solution surface is cooled down through solvent evaporation which results in condensing the non-solvent into the atmosphere. As a consequence, phase separation takes place with heat and mass transfer of the system, resulting in the formation of a porous structure after the complete evaporation of solvent. In addition, VIPS has been also used for the fabrication of superhydrophobic surfaces with specific surface morphologies. On the other hand spray-coating technique possess advantages like easy handling and convenience for large-scale preparation. Thus, keeping in view of the above statements the author utilized both VIPS and spray coating techniques to prepare superhydrophobic coatings. The authors used poly(styrene-block-butadiene-block-styrene) polymer reinforced with hydrophobically modified fumed silica nanoparticles i.e. HMFS, Aerosil R-812. The contact angle test results indicated higher hydrophobic characteristics for the nanocoatings loaded with HMFS as compared to the pure coating which was due to the improvement in surface roughness after addition of HMFS. The obtained nanocoating films 11 ACS Paragon Plus Environment


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via. VIPS and spray coating method exhibited contact angle of about 145°, while those prepared using alcohol vapors indicated a contact angle greater than 150°. In order to further improve the coating properties thermal treatment was carried out to increase the mechanical properties of the coatings for practical application. The thermal treatment led to decrease the pore size resulting in reduction of water permeation through collapsing and combination of the microparticles in the coating film matrix. Caldona

et

al.49

studied

about

the

superhydrophobic

rubber-modified

polybenzoxazine/SiO2 nanocomposite coatings (PBZ/SiO2) with anti-corrosion, anti-ice and superoleophilicity properties. PBZ a new class of fluorine- and silicon-free, thermosetting polymeric material exhibiting low surface energy, chemical resistance, low water absorption and UV absorption ability was taken as the matrix. However, the major drawback of PBZ is its brittleness. Thus, the author incorporated rubber i.e. hydroxyl-terminated, and epoxidized polybutadiene (HTBD) into the PBZ matrix in order to overcome this disadvantage. Moreover, the inclusion of rubber improved the fracture toughness without sacrificing other properties. The hydroxyl groups produced upon benzoxazine ring opening can aid in crosslinking the PBZ matrix. In addition, the authors reinforced PBZ matrix with SiO2 nanoparticle to improve the hardness, refractive index and superhydrophobic character. The disappearance of the peak at 3287 cm-1, for the hydroxyl group in PBZ, indicates that the rubber has been successfully grafted onto the PBZ matrix. The thermogravimetric results reveal improvement in thermal stability of PBZ composite through the incorporation of SiO2 nanoparticle. The incorporated SiO2 nanoparticle acted as superior insulators and mass transport barriers to the volatile products generated during decomposition. The wettability of the PBZ/rubber coatings indicated contact angle of 101° which was due to the strong intramolecular hydrogen bonding between the hydroxyl and the amino groups in the Mannich bridges. This resulted in lowering the surface energy of PBZ matrix. On the other hand 12 ACS Paragon Plus Environment

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inclusion of nano SiO2 particles, lead to achievement of higher contact angle values i.e. 110° to 132°. This was due to the roughness brought about by the presence of these densely packed nanoparticles. In another work Liao et al.50 fabricated patterned superhydrophobic polybenzoxazine hybrid surfaces reinforced with SiO2 nanoparticle. In the present study the author introduced direct ultraviolet (UV)-assisted replica molding method to produce superhydrophilic regions on the superhydrophobic PBZ hybrid surface through an optical mask or shadow mask. Good adhesion of the water droplet on the superhydrophobic PBZ-silica hybrid surface could be obtained after 5 min of UV exposure with a contact angle of 161.1º. The major reason behind the increase in the adhesion force is the change of chemical composition of the surface. On UV irradiation the presence of polar quinone C=O functional groups led to stronger polar forces between the PBZ film and polar liquid resulting in good adhesion. This behaviour can explore the application of PBZ in the field of liquid transportation without any loss of liquid samples. Cao et al.51 developed photo-crosslinked polyurethane (PU) reinforced with organo fluoro group-functionalized SiO2 nanoparticles (F-SiO2 NPs) shown in Figure 6, which were used for preventing micro leakage in a dental composite restoration. The F-SiO2 nanoparticle possess low surface energy and PU improves mechanical stability and also promotes F-SiO2 nanoparticles to form multiscale structure. This facilitates development of superhydrophobic coating by synergetic effect. The wettability test results indicated superhydrophobic coatings with high contact angle of 160.1° and low sliding angle (< 1°) at a relatively low molar ratio of PU: F-SiO2 ratio i.e. 1:3. This is due to the existence of hierarchical papillae structure with trapped air pockets ideal for rendering superhydrophobic character to the coatings. Thus, the obtained superhydrophobic coatings can be effectively used to prevent water permeation in resin composite restoration evaluation. Thus, the authors concluded that the present method 13 ACS Paragon Plus Environment


can be used to solve the problem of micro leakage that will efficiently increase the success rate of dental composite restorations.

Figure 6. Synthesis route of PU/F-SiO2 superhydrophobic coatings. 14 ACS Paragon Plus Environment

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Sparks et al.52 fabricated superhydrophobic hybrid inorganic-organic thiol-ene coatings via sequential spray-deposition and photo polymerization process under ambient conditions. The coatings were obtained by spray-deposition of UV-curable hybrid inorganicorganic thiol-ene resins which consists of pentaerythritol tetra (3-mercaptopropionate) (PETMP),

triallyl

isocyanurate

(TTT),

2,4,6,8-tetramethyl-2,4,6,8-

tetravinylcyclotetrasiloxane (TMTVSi), and hydrophobic fumed silica nanoparticles depicted in Figure 7. The spray deposition technique and nanoparticles agglomeration/dispersion creates surface with hierarchical morphologies exhibiting both micro and nanoscale roughness typically necessary for superhydrophobic wetting behaviour. The wetting behaviour of the developed films and coatings were evaluated by measuring the static water contact angle, dynamic contact angle, contact angle hysteresis and roll off angle. The authors observed increment in static CA with steady increase in TMTVSi concentration that reflects the transition to a more porous microstructure. It was also observed that the wetting behaviour depends upon the concentration of TMTVSi and hydrophobic silica nanoparticles. The obtained coatings had a broad range of static water contact angles i.e. >150°, low contact angle hysteresis, and low roll off angles i.e. 150°. This was due to the flower-like ZnO microstructure which possesses UV-durable superhydrophobicity. In addition, it was also revealed that this micro structured ZnO coated surface can be used for underwater oil capturing Zhang et. al.57 also fabricated non-fluorinated Zinc Oxide/Polydimethylsiloxane composite coatings with oil-fouling-resistant superhydrophobic property. SEM micrographs reveal nanostructure morphology that is responsible for inducing superhydrophobic as well as superoleophilic characteristics. It was observed that the contact angle was 160⁰ with water droplets rolling at a tilting angle of 3⁰. This result implies that the ZnO/PDMS surface is resilient in terms of oil fouling. It was assumed that the reason behind the anti-oil-fouling performance is the swelling of the underlying PDMS substrate, which rapidly draws of oil 18 ACS Paragon Plus Environment

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from the nanostructured coating. The anti-fouling behaviour of these coatings was due to the ability of the polymeric substrate to absorb the oil. 3.5 TiO2 based superhydrophobic nano coatings Lai et al.58 fabricated transparent superhydrophobic/superhydrophilic TiO2-based coatings for self-cleaning and anti-fogging application using electrodeposition technique (EPD) (Figure 9). The functionalization of TiO2 was carried out using 1H,1H,2H,2Hperfluorooctyltriethoxysilane (CF3(CF2)5CH2CH2Si(OCH2CH3)3. The authors used EPD approach for bonding hydrogen group functionalized titanium dioxide nanobelts (TNBs) to produce stable, transparent and superhydrophobic films. The authors observed that the optimized coating exhibited excellent superhydrophobicity with contact angle > 160̊ and also high transparency of 76%. The high contact angle was attributed to the strong adhesion of the horizontally aligned TNB particles. In addition, the developed coatings possessed superior chemical stability with self-cleaning performance characteristics. The high chemical stability of the coatings was due to the inert chemical nature of FAS coating which has strong –CF3 terminal groups. In addition, the air trapped within the pores of the TNB/FAS surface layer decreases the direct contact between liquid and solid, resulting in good stability of the TNB/FAS coatings.

Figure 9. Schematic route of fabrication of TNB/FAS superhydrophobic coatings. 19 ACS Paragon Plus Environment


Holtzinger et al.59 fabricated superhydrophobic TiO2 coatings via non-fluorinated wet chemistry route based on nanosphere lithography (NSL) method using polystyrene (PS) spheres grafted with hexadecyl trimethoxysilane (C16). This technique enables the formation of homogenous coatings exhibiting remarkable superhydrophobic behaviour. The wettability of the TiO2 coating grafted with the C16 precursor was examined. It was observed that the morphological feature imparted by C16 precursor and the increase in the number of PS layers resulted in producing superhydrophobic coatings with contact angle of 160°. Xu et al.60 fabricated the multifunctional TiO2−high-density polyethylene (HDPE) nanocomposite surfaces. The synthesized surface exhibited roughness ranging from micro- to nanoscale. This feature could be achieved by using wire mesh template to emboss the HDPE surface by creating an array of polymeric material with partial embedding of untreated TiO2 nanoparticles on the top surface. As a consequence the surface demonstrates outstanding superhydrophobic properties instantly after lamination without any chemical surface modification of TiO2 nanoparticles. The surface exhibits hydrophilic behaviour on exposure to UV light and superhydrophobic on heating the surface. The decrease in contact angle on UV exposure is due to the photosensitive nature of TiO2 nanoparticles and not due to HDPE polymer. The contact angle of TiO2-HDPE superhydrophobic coatings on heating was found to be 158º using water droplet of 5 µL. From the results obtained it can be concluded that the unique multifunctional surface can be created through the combination of superhydrophobic and photoactive features.


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3.6 Clay based superhydrophobic nano coatings Bayer et al.61 fabricated of water repellent polyurethane-organoclay nanocomposites coatings by dispersing moisture curable polyurethane (PU) and fatty amine/amino-silane modified montmorillonite clay (MMT) in cyclomethicone in water emulsion as depicted in Figure 10. PU and organo-clay dispersed emulsions were spray coated onto aluminium substrates to obtain water repellent self-cleaning coatings. The degree of superhydrophobicity was measured by determing the water contact angle which was found to be 155°. Owing to bio-compatibility of cyclomethicones and PU the developed coatings could be used for a specific bio-medical applications.

Figure 10. (a) Polyurethane dispersion with cyclic siloxane polymer network and (b) Formation of semi-interpenetrating polymer network (SIPN) after crosslinking with polyurethane and cyclomethicone polymer



Steel et. al.62 investigated about the fabrication of superhydrophobic coatings on aluminium substrates from polyurethane modified with waterborne perfluoroalkyl methacrylic copolymer (PMC) and fatty amine filled with montmorillonite clay nanofiller. Since organoclay exhibits strong compatibility with rubber and fluoroacrylic and thus, it was chosen to be compatible nanofiller for inducing suitable nanoscale structure and superhydrophobic property in fluoropolymer-MCPU matrix. Water contact angle of 167 º with an average contact angle hysteresis of 4º was measured for the optimized modified MCPU coatings. MCPU coatings with 10 % loading of organoclay with 1:1 weight ratio of MCPU and PMC was found to be the optimized composition with strong adhesion to aluminium substrate. Improved adhesion strength results due to the formation of interpenetrating polymer network (IPN) structure obtained after postcuring of MCPU and PMC as shown in figure 11. On the other hand higher weight ratio of MCPU led to reduction of antiwetting properties before and after tape test. The contact angle of the optimized coatings was around 160º with low contact angle hysteresis below 10º.


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Figure 11.(a) Perfluoroalkyl methacrylic copolymer (PMC) dispersion within the moisture cured polyurethane (MCPU) network and after solvent evaporation from the coating and (b) Polymer crosslinking to form an IPN network 3.7 Hybrid system based superhydrophobic nano coatings Lee et al.63 fabricated transparent superhydrophobic/translucent superamphiphobic coatings based on silica−fluoropolymer hybrid nanoparticles (SFN) via spray technique as shown in Figure 12. The nanoparticles imparted both microscale and nanoscale roughness while, fluoropolymer served to lower the surface energy of the binder. The authors investigated the role of varying SFN sol concentration on the water repellency behaviour and on the transparency of the coated glass substrates. It was observed that an increase in the concentration of the SFN sol facilitates the transition from superhydrophobic/transparent to superamphiphobic/translucent states. This transition arises due to an increment in the discontinuities of the three-phase (i.e. solid-liquid-gas) contact line. In addition, the light scattering properties also varied due to change in the concentration of the SFN sol. Moreover, with increase in the concentration of SFN sol there was an overall increase in the micro- and nanoscale roughness. Scanning electron microscopy (SEM) of the surface topography of the coated sample indicated that with an increase in the sol concentration from 0.05 wt % to 0.1 wt % there is full coverage of SFNs over the substrate with only nanostructure morphology. Further, increase in the sol concentration led to an increase in superamphiphobic characteristic. Thus, the authors concluded that the concentration of sol influences the surface morphology of the coatings.

Contact angle results indicated that an increase in the sol

concentration resulted in an overall increase in the water contact angle hysteresis to reach a saturation value. It was observed that the coated film containing 0.05 wt % solution of SFNs is hydrophilic with contact angle of 35°. On the other hand, with an increase in SFN sol concentration equal to or higher than 0.1 wt %, the coated surface becomes superhydrophobic 23 ACS Paragon Plus Environment


with water contact angle exceeding 150°. The authors also demonstrated the versatility of the methodology adopted by investigating the transparency and superhydrophobicity of the coated SFNs on various substrates including glass, metal, and polymer. For glass substrate 0.1 wt % sol exhibited good transparency and superhydrophobicity. Coated steel substrates revealed higher contact angle of 163° and 151°. The coatings containing 0.1 wt % sol also demonstrated durability of over 6 months without any noticeable change. Hence, from the above observations the authors concluded that the approach used in the work was versatile and controllable which can be used for various substrates for applications ranging from selfcleaning coatings of optoelectronics, liquid-repellent coatings and microfluidic systems.

Figure 12. Schematic route of the spray-deposited SFNs coatings on substrates with (a) low and (b) high sol concentrations. Franciso et al.64 fabricated superhydrophobic luminescent polyfluorene/organosilica (PFO/Si) hybrid coatings deposited onto glass, whatman paper and cellulose-based substrates by varying the amount of PFO:Si concentration. The authors investigated the photophysical 24 ACS Paragon Plus Environment

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properties and water repellency of the coatings. It was observed that all the composites irrespective of the content of PFO/organosilane display fluorescence. Amongst the various compositions the coatings synthesized with ratio PFO/Si of 50/50 display strongest chain confinement with highest surface roughness and superhydrophobic characteristics. This behaviour might be due to the increase in surface roughness brought about by the incorporation of organosilica. The main advantage of the present technique was the simplicity of the synthesis methodology adapted which can be utilized for large industrial applications. Wang et al.65 studied about the preparation of superhydrophobic hybrid coatings via a simple solidification-induced phase separation technique from liquid polysiloxane (PSO) containing Si–H and Si–CH=CH2 groups as the precursor and methyl-terminated polydimethylsiloxane (PDMS) as porogens depicted in the Figure 13. Different coating compositions were prepared by altering the viscosity of PDMS i.e. 10, 15, 20, 30 and 50 cP respectively. As investigated in earlier studies the roughness factor which dictates the transparency and superhydrophobicity behaviour of the hybrid coatings could be manipulated at the nanometer scale by altering the viscosity parameter of PDMS. With increase in viscosity of PDMS there was an increment in surface roughness resulting in shifting of contact angle from 97.5⁰ to 155⁰. It was also observed that the transparency of the coating decreased with an increase in surface roughness. Wenzel state and Cassie state were used to explain the effect of surface roughness and the improvement in behaviour of the coatings from hydrophobic to superhydrophobic. In the present study it was also revealed that phase separation plays a dominant role behind the superhydrophobic behaviour. The SEM images of the hybrid coatings synthesized using PDMS revealed the presence of microstructures consisting of interconnected microdomain and nanoporous structures. It was analysed that the existence of these nanoporous microstructures arises due to the phase separation between PSO and PDMS. The solidification process of PSO results in the cross-linking reaction of Si– 25 ACS Paragon Plus Environment


H and Si–CH=CH2 groups. This leads to gradual transformation of PSO through diffusion to form a homogeneously dispersed linear polymer with interconnected solid microdomains. The highly viscous porogen PDMS lowers the diffusion rate of PSO molecules, thereby reducing the cross-linking reaction rate of PSO resulting in greater phase separation. The AFM images also reveal the existence of microdomains, nanopores with rough surface thereby corroborating the SEM observations. The authors concluded that the simple and economical approach adapted in the present study could be used for practical applications i.e. for safety goggles, windshields, solar cell panels and windows for electronic devices.


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Figure 13. Fabrication routes of the hybrid nanoporous coatings corosslinked in the presence of Pt-catalyzed hydrosilylation reaction of Si−H and Si−CH=CH2 groups in the solidification process of PSO. Zhou et al.66 fabricated stable superhydrophobic porous coatings from hybrid ABC triblock copolymers wherein A, B and C implies to poly(dimethylsiloxane) (PDMS), polystyrene (PS), and poly(methacrylolsobutyl polyhedral oligomeric silsesquioxane) (PiBuPOSSMA), respectively (Figure 14). Block copolymers (BCPs) have attracted huge attention for developing superhydrophobic structure since they have an ability to selfassemble into nanoscopic structures with ease of microphase separation under various conditions. The authors used solvent evaporation induced micelle fusion-aggregation assembly and non solvent vapor-induced phase separation technique to design and develop hybrid ABC triblock copolymers with superhydrophobic/porous surfaces and unique nano/micro scale hierarchical morphology. The aim behind the use of polyhedral oligosilsesquioxanes (POSS) with cubic siloxane cages was to have “hybrid” properties in between organic silicone polymers and inorganic ceramic materials. In addition, POSScontaining block, resulted in producing hierarchical stable nano/microscale surfaces. It was found that after solution casting PDMS-PS-PiBuPOSSMA triblock micellar solutions onto substrate, superhydrophobic state was achieved with ultra-low water adhesion property. The presence of PiBuPOSSMA led to an increase in surface roughness resulting in superhydrophobic coating with a contact angle of 153 ± 1°. This phenomenon was due to the PiBuPOSSMA block which induces both nano and micro scale morphology within the coating. The synthesized coatings present unique twin-scale structure wherein, the bumpy nanospheres

create

nanoroughness

while,

the

nanosphere

assemblies

produce

microroughness. Thus, it was concluded that the due to the presence of both



superhydrophobic surface and underlying porous morphology, the developed coatings could be used for anticorrosive paints for metal protection.

Figure 14. Synthesis route of hybrid ABC triblock copolymer. Amigoni et al.67 prepared superhydrophobic organic-inorganic hybrid coating films by assembling covalently different layers of amino-functionalized silica nanoparticles of


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diameter 295 nm and epoxy-functionalized smaller silica nanoparticles of diameter 20 nm. This arrangement led to the formation of nanoscale textures. The functionalization of the last layer of the amino-functionalized silica particles was performed by grafting newly designed fluorinated aldehydes via imine function, thereby producing a monomolecular layer. The authors postulated that hydrophobicity can be controlled through topography and chemical composition. They proposed that regularly structured surfaces can impart better wettability as compared to irregular ones. Thus, the authors used layer by layer technique to produce nano/microparticulate structure assembled on solid substrate. It was observed that with an increase in the fluorinated layers there was an increase in the superhydrophobic behaviour with stable water repellent surface bearing contact angle of 150⁰. Moreover, the presence of silica particles of different sizes resulted in increasing the roughness of the surface that led to the enhancement of the hydrophobic properties with low surface energy and stable high surface area. Thus, from the results obtained it was concluded that the developed coating can be used for fabricating coating material for antibacterial activities. Zhang et al.68 also fabricated highly transparent superhydrophobic coatings of silica nanotube (SNT) on a glass substrate by employing polymethylsiloxane (PDMS) and multi-walled carbon nanotube (MWCNTs) (Figure 15). The author reported hydrophobization through chemical vapour deposition (CVD) of cured PDMS. The high transparency and superhydrophobicity of the surface can be achieved by various the concentration of PDMS to SNT. The optimize coatings exhibit higher transmittance greater than 83% in visible light range (400-780 mm) higher water contact angle (WCA) of 165°, and slide angle also lower than 3°. They also found that the development superhydrophobic coatings could be applied on glass substrate and also employed as a protective cover over solar cells. The developed superhydrophobic coating can also be used to for self-cleaning properties.



Figure 15. Schematic representation fabrication of superhydrophobic coatings. 3.8 Other superhydrophobic coatings Shah et al.69 developed superhydrophobic coatings on wood substrate based on polydimethylsiloxane (PDMS) modified alumina nanoparticles. The coatings developed were analyzed through energy paper abrasion, contact angle and water impact test. The SEM micrographs revealed switching of Al2O3-NPs surface from hydrophilic to hydrophobic after treatment with PDMS. It was also observed that the contact angle of the coatings increased from 132 ± 4° to 165 ± 4° an with increase in coating layers. This was related to the considerable increase in the size and quantity of micro-scale features of the PDMS treated Al2O3-NPs surface. It was revealed that the hierarchical structure obtained after three cycles of coatings was the key reason behind the water-repellent properties of the nanocomposite coatings. The emery paper abrasion test indicated decrease in water contact angle with increase in sliding distance against emery paper. However, the superhydrophobic character was maintained up to a distance of 150 mm. The durability of the coatings was evaluated 30 ACS Paragon Plus Environment

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using two different kinds of wood i.e. China Kail and Cedrus Deodara. The coatings on the wood also showed excellent water-repellent properties after three cycles of coatings. From the above results the author concluded that the methodology adapted could be used to fabricate lab scale superhydrophobic wood coatings. 4. Applications of superhydrophobic polymer nanocoatings in various fields Many research have been carried out to develop strategies for fabrication of superhydrophobic surface which can be commercialized for use in various fields.70-74 Thus, in this section, we will generally address the applications of the superhydrophobic coatings for anti-corrosion, anti-icing, anti-fogging, anti-fouling, self-cleaning, oil/water separation, medicine, and other fields as depicted in Figure 16.

Figure 16. Superhydrophobic coatings applications with various fields. 31 ACS Paragon Plus Environment


4.1 Anti-corrosion In recent time corrosion has become one of the most serious problems in the world. As a consequence hundreds of billion dollars is being spent to increase the research and development activities for developing corrosion resistant material. In the present era, chromium-containing compounds are being employed as corrosion protection material, but this material provides a negative impact on environment and human being. Another process of improving their anticorrosion properties is through the direct fabrication of superhydrophobic coatings on the metal surfaces.75 The basic mechanism behind the development of corrosion resistant materials is the existence of air layer between the substrate and solution, which inhibits the movement of corrosive ions. Recently, superhydrophobic coatings have been extensively employed on various substrate surfaces such as Al, Cu, Fe, Ti, Zn, alloy, and steel, to enhance their corrosion resistance. Liu et al.76 prepared fluoroalkylsilane and Mg alloy surfaces with stable superhydrophobic coatings via solution immersion process for improved corrosion resistance. In author reported, Xu et al.77 fabricated superhydrophobic Mg alloy surface with corrosion resistance by using an electrochemical machining method in the presence of fluoroalkylsilane. In another paper a series of superhydrophobic Mg alloys were fabricated by microwave plasma-enhanced chemical vapor deposition and immersion process as reported by Ishizaki et al.78-80 In this work the corrosion resistance and chemical stability of the superhydrophobic Mg alloys was examined. The authors also reported about the fabrication of color-tuned superhydrophobic Mg alloy with better corrosion resistance and damping capacity using the chemical free immersion approach in ultrapure water.81


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4.2 Anti-icing The ice formation and accumulation on exposed substrates can create difficulties in the operational performance of airplanes, highways, power lines, ships, etc.,82 Currently, superhydrophobic surfaces have been recommended to be exploited in the field of anti-icing application owing to its excellent water repellent properties that can delay the accumulation/adhesion of wet snow, ice, or frost onto the surface. Many scientists have developed physical/chemical deicing approaches to remove ice for both energy and resource dissipation applications. In the last few years, scientists have been inspired by biomaterial surface fabricated artificial superhydrophobic surface with high static contact angles (CAs) and low contact hysteresis by utilizing diverse synthesis mechanism. Tourkine et al.83 studied about the development of water repellent materials with delayed freezing ability. In the present work the author studied about the two states of responsible for inducing superhydrophobicity i.e. Cassie state and Wenzel state. The Wenzel state refers to the conformation of liquid onto the contours of the solid surface leaving the cavities below filled with air. On the other hand the Cassie state often referred to as the Fakir state refers to the conformation of liquid on the bed of the cavities. The later state generates remarkable properties like reduced adhesion, large slip and antifogging properties. The authors fabricated rough copper surface by immersing in 0.1 M solution of silver nitrate via using technique proposed by Larmour et al.84 The copper plates were then dipped in a bath of ethanol for 30 min containing fluorinated thiols. These fluorinated thiols chemisorb onto the copper plates resulting in the creation of rough superhydrophobic surface with contact angle of 165° and 155°. The authors concluded that the presence of air in the cavities results in delaying the anti-icing property.



Mishchenko et al.85 designed ice-free nanostructure surfaces based on repulsion of impacting water droplets. The authors stated that the ability to fend off water droplets that can result in the prevention of icing on the surface via. chemically as well as geometrically is through an array of nanoscale bristles. The approach made by the authors was inspired by the technology used by many organisms such as mosquitoes defogging their eyes to water striders keeping their legs dry. The authors presented a comprehensive theoretical and experimental study of dynamic droplet freezing mechanism on closed-cell surface microstructures by analyzing a broad range of geometries, impact angles, droplet temperatures and comparing how wetting behaviour can affect the hydrophilic, hydrophobic, and superhydrophobic surfaces. The authors observed a novel mechanism highlighting the importance of dynamic wetting behaviour that can lead to full retraction and repulsion of impacting water droplets from a cooled superhydrophobic surface before ice nucleation occurs. Ice accumulation at -25 to -30 °C could be easily avoided due to unique non-wetting freezing transition of the superhydrophobic surface. This results reveal that the presence of nano- and microstructured superhydrophobic surface can be promising anti-icing materials that requires no active energy inputs. The authors pointed that the closed-cell surface microstructures can provide better mechanical and pressure stability and feasibility of large scale fabrication with opportunities for manipulating their material and chemical properties. Cao et al.86 fabricated anti-icing superhydrophobic coatings inspired by the selfcleaning properties of lotus leaves by using nanoparticles. The authors found that the size of the nanoparticles plays exposed on the surface plays a pivotal role in determining the superhydrophobic and anti-icing characteristic of the coatings. The authors synthesized a series of polymer nanocomposite coating using acrylic polymer as matrix and silica nanoparticles with diameter ranging from 20 nm to 20 µm on various substrates including metal and glass. The polymer nanocomposite with unmodified silica indicated a contact angle 34 ACS Paragon Plus Environment

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of 107º while, the silica particle modified with organosilane molecules revealed higher contact angle of nearly 150º. The authors observed that varying the particle size of the silica nanoparticles

affected

the

superhydrophobic

characteristic

of

the

coatings.

The

nanocomposites synthesized using particle size of 20 nm, 50 nm, 100 nm, 1 µm, and 10 µm possess superhydrophobicity with contact angle of greater than 150º. However, nanocomposite synthesized using particle size of 20 µm indicated contact angle of less than 150º. To test the anti-icing characteristic super cooled water of temperature -20 ºC was poured onto two Al plates. The right side of the plate was untreated while, the left side was coated with nanoparticles of dimension 50 nm. It was observed that the right side of the plate revealed ice formation as soon as it came to contact with the supercooled water. On the other hand the left side of the plate did not reveal any ice formation upon contact with supercooled water. The above observation was found to be hold good for nanocoatings with particle size of 20 nm and 50 nm. However, icing probability of nanocoatings was found to increase with particle size greater than 50 nm. The authors demonstrated that the icing of super cooled water occurs through a heterogeneous nucleation process when there is contact between water and the particles exposed on the surfaces. It is a complex phenomenon, which depends on ice adhesion, hydrodynamic conditions, structure of the water film on the surface and the size of the nanoparticles. 4.3 Anti-fogging Fogging commonly arises, in mirrors, glass and other substrate creating various difficulties in daily life and for many industrial applications.87-89 This problem can be controlled by investigating the interaction between the substrate and the liquid. Recently, superhydrophobicity induced anti-fogging coating for automobile application for mirrors has been commercialized by many companies.



Antifogging strategies can be divided into the three approaches: (1) Superhydrophilic θ=0° (2) Superhydrophobic WCA> 150° and low slide angles (3) Hydrophilic (hydrophilicity for antifogging and contamination resistance). Superhydrophilic coatings can significantly prevent the fogging behaviour. It was found that the condensed water droplets immediately spread flat to form a thin water layer on the substrateas compared to the dispersed droplets.90 However, three different approaches have been developed in case of superhydrophilic surface to attain anti-fogging. First, the use of titanium dioxide (TiO2) functionalized based materials for antifogging properties owing to their photoinduced superhydrophilicity.91,92 The second process involves the utilization of hydrophilic polymer to obtain superhydrophobic surface and anti-fogging properties.93,94 The third approach deals with the design of textured or porous surface which can improve the surface wettability via manipulation of roughness at various length scale.95 Watanabe et al.96 fabricated TiO2 coated glass with anti-fogging properties under UV illumination. Thus, TiO2 functionalized coating can not only be employed in glass, but also for various other substrates such as polymer, metal and ceramic materials. Gao et al.97 studied mosquito compound eyes which demonstrated superhydrophobicity and efficient protective mechanisms for sustaining clear vision in humid environment. Mosquito compound eyes inspired by superhydrophobic and antifogging properties were formulated by a soft lithography approach followed by low-surface-energy fluoroalkylsilane modification.


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4.4 Self-cleaning In the last few decades a large number of synthesis processes have been developed to prepare various self-cleaning coating materials for potential application in daily life for industry, agriculture and military fields.

98-100

At present several self-cleaning coatings have

been commercialized for example cement, tile, glass in the global market. Self-cleaning materials are divided into 4 parts: (I) TiO2 based (II) bio-inspired (lotus effect) superhydrophobic self-cleaning (III) gecko setae-inspired dry self-cleaning and (IV) marine organisms inspired antifouling self-cleaning such as shark skin and pilot whale skin. Titanium oxide is one of the most utilized functional materials which can be used to prepare self cleaning coatings due to its unique physical and chemical properties exhibiting both photocatalytic and photoinduced superhydrophilicity.101 These unique characteristic properties are sufficient enough to induce the self-cleaning properties. TiO2 coated selfcleaning glass has been used in the National Opera Hall102 in China. In 2003 Japan also employed self-cleaning tiles and windows for eco-life house.103 Cyranoski et al. prepared self-cleaning nanotiles which are both water-repellent and oil-repellent.104 4.5 Anti-fouling and anti-scaling Antifouling coatings for marine application are specially important topic in the coating fields.105,106 These marine coatings are divided into two main categories: (i) micro and (ii) macrofouling. In case of microfouling, a bio-film is formed and bacteria starts to stick while in macrofouling larger organisms start to adhere.107 Antifouling coatings for ship hulls protect the exterior surface of the ships exterior as well as reduced the growth of the organism. Ships derived of antifouling coatings leads to adherence of microorganism onto the surface resulting in gradual increase in fuel consumption.108-110



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Figure 17 demonstrate ship hull without antifouling coating resulting in smooth of barnacles and other marine organism on the exterior surface of the ship.

Figure 17. Anti-fouling castings of ship hull. Junaidi et al.47 fabricated rice husk ash (RHA) contain by SiO2 antifouling superhydrophobic coatings

prepared

via

chemical

modification

using

1H,1H,2H,2H-

perfluorodecyltriethoxysilane (HDFS) and stearic acid. The results demonstrated that the commercial adhesive spray mount and HFDS (3M-HFDS) coating displayed the highest contact angle (CA) value in this work with 63% compared to the virgin 3M coatings. Conversely, 3M and stearic acid (SA) based coating attained higher contact angle CA as 50% compared to virgin spray adhesive coatings (3M) sample. In the anti-fouling test, the coating with HFDS modification could avoid the stain of kaolin slurry with dye. 38 ACS Paragon Plus Environment

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4.6 Oil/water separation The oil/water mixture separation is one of the most challenging research topic worldwide owing to increasing rates of industrial oily waste water and oil spill accidents. The environment concern and economic demands emphasize the requirement of functionalized membrane to successfully separate oil and water.111-114 There are lots of traditional methods used for oil/water separation such as flotation, centrifugation and filtration.115-117 However, these are very complicated operation and time consuming processes.118 Recent years has witnessed momentous attention for fabricating polymer coatings for oil/water separation as shown in Figure 18. At present, porous superhydrophobic and superoleophilic materials containing fabrics, membrane, sponge and meshes prepared by various chemical and physical methods have been confirmed to be promising candidate for oil/water separation.119-121

Figure 18. Published research articles on oil/water separation materials each years from 2007- to 2016 (based on search result from Web science) Feng et al.122 fabricated superhydrophobic/superoleophilic functional materials for oil/water mixture separation. On the basis of lotus leaf structure, teflon coated mesh films with superhydrophobic and superoleophilic properties were prepared via spray dry method



for water and oil mixture separation. Based on this approach industries could efficiently separate oil/water separation, and which can be also commercialized for marine ships. Zhang et al.123 prepared nanoporous polydivinylbenzene materials by solvothermal process exhibiting both superhydrophobicity and superoleophilicity properties. However, the nanoporous materials revealed special selectivity for several organic compounds, as compared to conventional activated carbon absorbent. Zhang et al.124 also reported polyester textile based superhydrophobic and superoleophilic materials prepared via chemical vapour deposition. These materials exhibited superwetting and flexibility characteristics with high oil/water separation efficiency and reusability. Recently, many authors focused to develop oil/water separation materials which can exhibit superhydrophobic and oleophilic surfaces. For instance, Varshney et al.125 synthesized novel hydrogel coated mesh with superhydrophilicity and underwater superoleophobicity for oil/water separation. These unique materials can successfully separate water from oil/water mixtures successfully, with controlled recyclability and resistant oil fouling characteristics.126 However, there is a new challenge to develop next generation materials for oil/water separation since, traditional separation technology is very difficult to separate micrometer scale oil droplets from water. Thus, scientists are focussing on the incorporation of stimuli responsive polymeric materials for controlling the oil/water separation which can be a new innovation in near future. Tu et al.127 synthesized oleophilic conical needle like structure which can be utilized for the collection of micrometer size oil droplets from the water. As a result, the cone like


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arrangements structure can capture micrometer size oil droplets and convey them towards the bottom of conical needle, with high flow and throughput. Recently, Wen et al.128 prepared zeolite coated mesh films with superhydrophilic and underwater superoleophilic characteristics exhibiting gravity oil/water separation behaviour wherein, water can penetrate through the film with oil retention on the film surface. Xiao et al.129 fabricated highly efficient conjugated microporous polymer-coated stainless steel and sponge for separation of oil/water and trace organic contaminants based on superhydrophobic conjugated microporous polymer coated devices. Conjugated microporous polymers (CMPs) have been explored as a new class of porous materials that can be easily synthesized by common organic reactions such as Suzuki coupling, Sonogashira coupling and condensation reaction. CMPs owing to their easily adjusted surface nature, good stability, and large surface area have gained momentous attention for their physicochemical properties. In the present work the author reported about the preparation of superhydrophobic CMPs based on iron (III) porphyrin unit coated on stainless steel mesh for oil/water separation. The surface morphology of coated and uncoated CMPs was analyzed using scanning electron microscopy. It was observed that the surface morphology of the uncoated mesh is perfectly smooth and clear. On the other hand, the coated mesh indicate rough surface with dendritic nano branches of diameter between 50-80 nm indicating successful coating of the stainless steel with CMPs. As a consequence water does not penetrate through the coating mesh resulting in higher contact angle of 152 º. In addition, the coated mesh floats in water while, the uncoated ones sink. It was also observed that oil can easily pass through coated mesh owing to the oleo phobic characteristic of CMPs. The coatings could be easily coated on the framework of sponge to form superhydrophobic devices with rapid efficient oil/water separation efficiency.



Cho

et

al.130

reported

about

the

preparation

of

Page 42 of 63

robust

multifunctional

superhydrophobic coatings with enhanced water/oil separation, self-cleaning, anti-corrosion, and anti-biological adhesion features. In this study the authors used silica ormosil suspension and siloxane polymer to prepare highly transparent and superhydrophobic nanocomposites. To further enhance the superamphiphobic properties the hybrid nanocomposites were modified with n-alkylsilane (C8, C16), phenylsilane and fluorosilane. The suspension on account of its good

solution processability could be spin coated on various substrates. Since, the wetting behaviour of superhydrophobic surface arises from the synergistic effect of low surface energy and rough surface hence, the authors blended poly (methylhydroxysiloxane) (PMHOS) nanoparticles with silica ormosil suspension to increase the surface roughness within nanoscale as depicted in Figure 19. It was observed that the synthesized hybrid nanocomposite film exhibits superhydrophobic surface with contact angle of 168° and transparency of 90 %. The high transmittance signifies the antireflective property of ormosil film due to its low refractive indices and porous structure. In addition, the smaller particle size and better dispersability of ormosil/PMHOS leads to reduced scattering resulting in better antireflective property. This anti-reflective property can be utilized for self-cleaning applications and solar cell panels. The observed superhydrophobicity of ormosil/PMHOS film arises due to its hierarchical surface morphology. It was also found that the superhydrophobic surface of ormosil/PMHOS composite exhibits good non-sticking properties. This observation was attributed to the low surface energy and hierarchical surface morphology of ormosil/PMHOS film. The prepared nanocomposites also showed improved barrier property owing to the crosslinked structure of PMHOS. The hydroxylation and condensation of methyl and hydroxyl groups in siloxane and ormosil results in dense aggregation leading to better barrier properties. Thus, from the above findings it was


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concluded that the process of fabricating simple and cost effective F-ormosil/PMHOS composites can be sued for barrier, separation, optoelectronics and other applications.

Figure 19. Surface modification of ormosil/PMHOS. 4.7 Medical Superhydrophobic polymeric nanocoatings have been employed in medical field for drug delivery, self cleaning and for dental applications. Yohe et al.131 developed 3D superhydrophobic poly(ε-caprolactone) electrospun meshes containing poly-(glycerol monostearate-co-ε-caprolactone) as a hydrophobic polymer dopant. This superhydrophobic mesh was used in tunable drug release activity through displacement of air to control delivery rate. The entrapped air layer within superhydrophobic meshes demonstrated long-term stability in the existence of serum, showing efficacy against cancer cells in vitro for >60 days. High-aspect ratio TiO2 nano test tube can also be used for self-cleaning purpose with highsensitive immunoassays via immobilizing antibody for diagnosis of a target antigen reported 43 ACS Paragon Plus Environment


by Song et al.132 TiO2-based immune sensor exhibit reusable and self-cleaning properties arising from the photocatalytic character of TiO2. TiO2 demonstrate antibacterial effect owing to its unique photocatalysis and self-cleaning properties.133,134 The photocatalytic property was enhanced owing to the incorporation of sulfur into TiO2. In contrast with commercial products, the sulfur-doped TiO2 demonstrated higher photocatalysis and photoinduced superhydrophilicity. These films were found to be useful mediators for killing the bacterium Escherichia coli employing light sources generally found in UK hospitals. Similarly, Wang et al.135 also fabricated antibacterial Escherichia coli materials with photoinduced self-cleaning properties using 2, 6-anthraquinone sulfonate via a layer by layer process. Super-hydrophobic surface demonstrated self-cleaning properties that can reduce bacterial adhesion and also be used into coat, gloves, fabrics, and general equipments. First time Cao et al.13 developed superhydrophobic coatings through simple photocrosslinked method which can be used for dental composite restoration as shown in Figure 20 a,b and c. Superhydrophobic coatings exhibited high contact angle, low SA and outstanding cell feasibility and biocompatibility suitable for preventing microleakage.


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Figure 20. (a) Illustration of the microleakage appraisal specimen (b) structure of artificial restoring

cavity

with

superhydrophobic

coating

(c)

superhydrophobicity and superhydrophobic coating.13


Cassie-Baxter

model

of


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4.8 Others Superhydrophobic surfaces can also be used in other important applications for example,

in

automotive,

marine,

shielding, microfluid,

sensor and

solar cells.

Superhydrophobic surface exhibiting low adhesion is suitable for droplet-based microfluidic systems owing to the high mobility of liquids on such surface. Further, Mumm et al.136 fabricated copper based superhydrophobic surfaces which can be used in wire guided droplet microfluidic systems. Park et al.137 developed ordered microshell array on flexible and transparent polydimethylsiloxane elastomer surface for solar cell applications. Golovin et al.33 designed new parameter to develop mechanical durable superhydrophobic surface by spray coating in the presence of different binders and fillers. Further, self-healing SHSs were fabricated from fluorinated polyurethane elastomer and 1H,1H,2H,2H-heptadecafluorodecyl polyhedral oligomer silsequioxane (F-POSS).138 5. Conclusion In this review we have given a detailed insight regarding the recent progress in the synthesis of different superhydrophobic nanocoatings using various nano-fillers and fabrication techniques. On the basis of the above literature findings it is noticeable that the inclusion of nano-fillers improves the superhydrophobicity of the polymer coatings. In addition, the diverse applications of the superhydrophobic polymer nanocoatings have also been discussed in brief. As investigated ZnO based polymer nanocoatings can be exclusively used for oil-water separation and transportation application. On the other hand, clay based polymer nanocoatings exhibited superior anti-wetting properties with good self-cleaning ability. Both CNT and silica based polymer exhibited superior chemical resistance properties in both acidic and alkaline solution. Moreover, the different fabrication techniques involved in developing the superhydrophobic polymer nanocoatings would enable the scientist and


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young researchers the mechanism and the practical concepts to attain better properties in future. Although remarkable advancements have been carried out in past two decades in superhydrophobic polymer nanocoatings yet number of challenges still needs to be addressed for large scale industrial application. The current era demands the fabrication of green ecofriendly superhydrophobic polymer nanocoatings with low volatile organic emission (VOCs), longer shelf life and good adhesion strength. The technologies involved in designing and fabricating superhydrophobic polymer nanocoatings should be cheaper to be more productive on large scale. In addition, new theoretical model also needs to be developed apart from Cassie and Wenzel to gain deep insight regarding the mechanism of superhydrophobicity. Thus, it can be claimed that with ever increasing interest and scientific focus in this area much development could be attained with huge industrial commercialization and large production scale. ACKNOWLEDGEMENTS The authors are thankful to the Department of Chemicals and petrochemicals, Government of India for the finance support of the research work. REFERENCES (1) Wei, Y.; Hongtao, L.; Wei, Z. Preparation of anti-corrosion superhydrophobic coatings by an Fe-based micro/nano composite electro-brush plating and blackening process. RSC Adv. 2015, 5, 103000-103012. (2) Isimjan, T. T.; Wang, T.; Rohani, S. A novel method to prepare superhydrophobic, UV resistance and anti-corrosion steel surface. Chem. Eng. J. 2012, 210, 182-187. (3) Mo, C.; Zheng, Y.; Wang, F.; Mo, Q. A simple process for fabricating organic/TiO2 super-hydrophobic and anti-corrosion coating. Int. J. Electrochem. Sci. 2015, 10, 73807391. 47 ACS Paragon Plus Environment


(4) Chen, Y.; Chen, S.; Yu, F.; Sun, W.; Zhu, H.; Yin, Y. Fabrication and anti‐corrosion property of superhydrophobic hybrid film on copper surface and its formation mechanism. Surf. Interface Anal. 2009, 41, 872-877. (5) Boinovich, L. B.; Emelyanenko, A. M. Anti-icing potential of superhydrophobic coatings. Mendeleev Commun. 2013, 23, 3-10. (6) Li, J.; Zhao, Y.; Hu, J.; Shu, L.; Shi, X. Anti-icing performance of a superhydrophobic PDMS/modified nano-silica hybrid coating for insulators. J. Adhes. Sci. Technol. 2012, 26, 665-679. (7) Peng, C.; Xing, S.; Yuan, Z.; Xiao, J.; Wang, C.; Zeng, J. Preparation and anti-icing of superhydrophobic PVDF coating on a wind turbine blade. Appl. Surf. Sci. 2012, 259, 764-768. (8) England, M. W.; Urata, C.; Dunderdale, G. J.; Hozumi, A. Anti-fogging/self-healing properties of clay-containing transparent nanocomposite thin films. ACS Appl. Mater. Interfaces, 2016, 8, 4318-4322. (9) Howarter, J. A.; Youngblood, J. P. Self‐cleaning and anti‐fog surfaces via stimuli‐responsive polymer brushes. Adv. Mater. 2007, 19, 3838-3843. (10) Gan, W. Y.; Lam, S. W.; Chiang, K.; Amal, R.; Zhao, H.; Brungs, M. P. Novel TiO2 thin film with non-uv activated superwetting and antifogging behaviours. J. Mater. Chem. 2007, 17, 952-954. (11) Latthe, S. S.; Terashima, C.; Nakata, K.; Sakai, M.; Fujishima, A. Development of sol– gel processed semi-transparent and self-cleaning superhydrophobic coatings. J. Mater. Chem. A. 2014, 2, 5548-5553. (12) Nakajima, A.; Hashimoto, K.; Watanabe, T.; Takai, K.; Yamauchi, G.; Fujishima, A. Transparent superhydrophobic thin films with self-cleaning properties. Langmuir, 2000, 16, 7044-7047.


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Table of Contents Graphic A review on superhydrophobic polymer nanocoatings: Recent development, application Sonalee Das, Sudheer Kumar, Sushanta K. Samal, Smita Mohanty, Sanjay K. Nayak


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