Carbon nanostructure-based superhydrophobic surfaces and coatings

Researchanddevelopmenton superhydrophobic carbon nanostructures and their nanocomposites have high industrial significance. Here, a comprehensive review of the topic is provided. Reported works on superhydrophobic surfaces and coatings of carbonnanotubes, nanofibres, nanospheres/nanothorns/others, nanodiamond, fullerene and their various nanocomposites with metals, ceramics, and polymers are described. Superhydrophobic nanostructured carbon soot, graphitic carbon, and others are also presented. The section on superhydrophobic graphene is presented concisely at the end. Reports indifferent application areas, including anti-corrosion, anti-icing, oil separation, anti-biofouling, and sensors, are discussed separately. Superoleophobic and superamphiphobic surfaces are also discussed.

Carbon nanomaterials (CNMs) are typically hydrophobic [10,11]. SEs of CNTs could be at the range of 27-45.3 mJ/m 2 [12]. SEs of chemically exfoliated GR and graphene oxide (GO) have shown to be 46.7 and 62.1 mJ/m 2 , in that order, while that of normal graphite flake was ∼54.8 mJ/m 2 [13]. The higher surface roughness, nano/micro-hierarchical surface structures (nanoscale CNMs and their microscale aggregates), surface reduction processes (removal of hydrophilic surface groups), additional low SE treatments all could boost the hydrophobicity.
A significantly higher number of reports are available on SHPC carbon nanostructure (CNS)-based surfaces and coatings. Hitherto, no comprehensive review is available on the topic. Most of the published reviews focused on GR. Chen et al. in 2013 [36] and Wang et al. in 2015 [37] reviewed SHPC GR. Gupta et al. reviewed various CNMs in oil separation application [38]. Liu et al. [39], Khan et al. [40], and Li et al. [41] provided good accounts of photoreduced, chemical vapour deposited, and laser-structured GR, respectively. Jishnu et al. reviewed GR-based SHPC anti-corrosion coatings [42], whereas Sharma et al. provided a short account of CNT-based corrosion-resistant coatings [43]. Recent advances in laser-fabricated GR surfaces have been reviewed by Ma et al. [44]. A few recent reviews described SHPC CNSs as a part [10,45,46]. A considerably higher number of recent reports are available in this area. Hence, we made a systematic approach to comprehensively present the entire domain's available information under one roof. Single and multicomponent (with metals, ceramics, and polymers) SHPC surfaces and coatings based on CNTs, carbon nanofibres (CNFs), carbon nanospheres/thorns/others, nanodiamonds, fullerenes, nanoscale carbon soot/graphitic carbon/others, as well as GR are presented. Due to the available reviews, detailed descriptions of GR-based materials were not attempted; however, the literature carefully covered and presented concisely. Four application areas, viz., anti-corrosion, oil-separation, anti-icing, and anti-biofouling, are highlighted; other applications are briefly mentioned.

Carbon nanostructure-based superhydrophobic surfaces
Nearly 45% of the reports fall under GR, followed by CNTs (∼32%) (Figure 1a). The most investigated application area is oil separation, and the second most is the anticorrosion coatings (Figure 1b and c).

CNTs
SHPY was observed for both aligned and non-aligned CNT films, with and without an additional low SE component/modification. Typically, composite formation with polymers and ceramics help overcome the long-term durability issues of SHPC CNT surfaces [47][48][49].
Chemical vapour deposition (CVD)-fabricated 3D anisotropic ACNT film on the patterned Si template (with quadrate pillar array) displayed both SHPY and superhydrophilicity (SHLY) at varying structural parameters of the template. The SHPY of the as-formed CNT arrays was attributed to the vertically aligned organization and the copious fraction of trapped air. The as-formed arrays with pillar space of 20, 15, 10, and 6 µm have shown CAs of ∼22.1°, 142.9°, 25.5°, and 10°, respectively. The corresponding CAs of the surfaces after a vinyltrimethoxysilane modification were 21.2°, 153.3°, 27.2°, and 20.8° [63]. Lau et al. showed that a thin layer of polytetrafluoroethylene (PTFE) coating on VACNT forest could avoid the potential droplet seeping into the CNT's voids down to the microscopic level and created a stable SHPC surface with advancing and receding CAs of 170°and 160°, respectively. CVD was employed to deposit both the CNT array and the PTFE coating [52]. Wang et al. studied SHPY under dynamic conditions and observed that the droplet bounces off several times on an array with a CA of 163°, whereas for an array with a CA of 140°, the drop remained pinned [56]. For microfluidics applications, Qu et al. reported a variety of SHPC ordered CNT polyhedron structures with a CA of up to 162°via CVD (∼850°C) inside the microchannels of patterned SiO 2 substrate, where the confined space was decisive in the realization of polyhedral structures with hexa/hepta/octagonal cross-sections. CNT's self-ordering and the high-temperature deposition yielded SHPY where no low SE treatment was used [64]. Lu et al. showed that micropatterning of MWCNTs enhances hydrophobicity. They employed a laser pruning technique to make SHPC parallel microwall arrays from VA-MWCNTs. The optimal SHPC surface consisted of micro-wall arrays with a width of ∼13 μm and a channel width of ∼50 μm [65]. Ramos et al. showed that CO 2 laser treatment could re-establish superhydrophilic (SHPL) VACNT surface to SHPC by decreasing the polar components (oxygen terminations on the surface) due to the high local heating rate. The microwave (MW) plasma CVD-deposited VACNT film was initially pretreated with oxygen plasma to convert to SHPL and then subjected to the laser treatment [66]. Lepore et al. compared SHPY of cabbage leaf and CVD-grown VACNT carpet and showed that cabbage-like morphologies could be helpful to achieve better SHPY for nanofluidic applications [67]. Hierarchical CNT assembly fabricated on a Si micro-pillar array displayed slippery SHPY (CA of ∼155°and SA of ∼5°) with excellent durability against water ingression. The corresponding CNTs grown on planar Si wafer lose the SHPY once exposed to tiny water droplets [68]. Aligned crystalline carboxyl-functionalized MWCNTs grown on SiC cellular skeleton (ceramic pillars with ∼700 μm porous microchannels and ∼250 μm diameter) by catalyst CVD displayed long-term stability against water-droplet ingression [69].
To enhance the surface durability against wateringression and surface tension-assisted tie-up, various surface treatments for SHPC VACNT forests were investigated. In an earlier study, Journet et al. proposed a thiol modification for CVD-grown (planar Si substrate, 750°C) CNT forests. The surface first covered with a thin sputtered Au layer and subsequently, thiol-modified (CA ∼ 164 ± 2°) [70]. Studies on liquid flow slippage over SHPC thiolmodified VACNT forests (CVD, microchannels) disclosed that the slip lengths varied linearly with the lateral roughness scale [71]. Santhagopalan et al. showed that a high-voltage electrophoretic deposited (EPD) and low SE PTFE-coated VACNT film displayed a CA of ∼160° [72]. Jeong et al. employed a simple contact transfer micropatterning technique to fabricate VACNT micro-pillar arrays with different inter-pillar spacings extending from 45 to 160 μm (width ∼65 μm) (Figure 2a-c). A thin hydrophobic CVD silicone layer coating was helpful to enhance the SHPC robustness, even under pressurized conditions. The CAs of the as-fabricated VACNT arrays (without silicon coating) displayed a steady decrease, whereas the CAs of silicone-coated arrays were stable regardless of the droplet volume ( Figure 2d). The CAs of the VACNTs gradually increased with the increase of the inter-pillar spacing, reaching a maximum for 160 μm spaced sample (CA of 168 ± 0.3°, CAH of 2.64 ± 0.4°, and SA ∼ 5°) [73]. Sojoudi et al. have shown that the top-gathering and elastocapillary densification of the porous CNTs could be prohibited by conformal deposition (CVD, 80°C) of an ultrathin film of poly(1H,1H,2H,2Hperfluorodecylacrylate) [74]. Yung et al. investigated CF 4 plasma modification on VACNTs using CVD (800°C) to fabricate ultra-low reflectance SHPC surface [75].
Aria and Gharib showed that SHPC CNT arrays could be made by exposing hydrophilic CNT arrays to a suitable vacuum annealing (via removal of oxygenated hydrophilic groups). Alternate vacuum pyrolysis and UV/ozone treatments allowed easy switching of ACNT arrays between SHPY and SHLY [76]. The authors in a later work studied droplet-impact dynamics of SHPC CNT arrays and revealed that no droplet pinning happened during a wide range of critical Weber number (W n ) [57]. Babu et al. showed that SHPC VACNTs could be fabricated via water-assisted CVD by a regrowth process (by a second-time catalyst-assisted deposition). The regrown CNTs displayed high CA (due to increased surface roughness); however, the CAH was ∼60°(attributed to the surface hydrophilic groups). A subsequent vacuum annealing (350°C) or polydimethylsiloxane (PDMS) modification could further increase the CA with a significant CAH reduction ( Figure 3). Both the high-temperature annealing and the low SE treatment were helpful to eliminate the hydrophilic groups [77]. Later, the authors confirmed a flat continuous reflecting air layer at the VACNT surface a few moments after submerging in water [78]. Hsiao et al. fabricated SHPC and self-cleaning CNT forest on a quartz surface by CVD at 850°C (CA ∼ 154°). The lower CA (∼115°) obtained for a higher temperature (950°C) processed sample was attributed to the widened outer tube diameters [79].
Several studies reported composites of ACNTs with other CNSs. Maziar et al. synthesized self-assembled ACNT/ carbon-nanosphere hybrid film. Here, the ACNT array was first fabricated by CVD (800°C), and then amorphous carbon nanospheres were deposited by cathodic vacuum arc (negative substrate bias of 100 V). The wettability was closely related to the size of the deposited nanospheres. Increasing the deposition time increased the spheres' size, decreased the air gaps between the CNTs, and lowered the surface hydrophobicity [80]. SHPC CNT arrays capped with amorphous carbon NPs were fabricated by  [73]. Reproduced with permission from ref. [73]; © 2014 Elsevier Ltd. Figure 3: Schematic of the process. The regrowth CVD process increased the roughness of the CNT surface. Hydrophilic groups in the regrown CNTs are removed by either vacuum annealing or silane modification (θ h corresponds to CAH) [77]. Reproduced with permission from ref. [77]; © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. plasma immersion ion implantation (PII, 800°C). The CA of unprocessed CNT forests was ∼0°, whereas the nanocomposite displayed a CA of ∼180°. The excellent SHPY was attributed to the unique overhang structure in which drops could penetrate only under a higher hydrostatic pressure [81]. Han et al. synthesized CNT forests (CVD, 800°C) and then processed them by Ar PII so that the graphitic sidewalls of CNTs were partly shattered and sphere-shaped amorphous carbon NPs were shaped on top. The surface displayed a CA of ∼160°and an SA of ∼5°. Their electrowetting studies explored slippery-sticky transformation with applied potentials [82].
Huang et al. demonstrated SHPC ZnO-coated CNTs (CA ∼ 159°) where ZnO thin film was deposited (by cathodic vacuum arc) on CVD-made ACNTs. Contrary to the bare CNT surface, the coated surface showed no water ingression even after an extended period [83]. A report is available on room temperature EPD-made CNT-Ni and CNT-Zn. The study showed that CAs of the nanocomposite could be adjusted from 60°to >150°by altering the EPD voltage from 50 V to 250-550 V. The SHPY was attributed to the combined effect of the multi-scale roughness and the low SE attributed to the adsorbed hydrocarbon groups [84].
Most of these studies categorically showed that SHPC VACNT arrays suffer from long-term durability issues due to the potential water ingression. A subsequent low SE surface coating or a desirable composite formation could alleviate the shortcoming to the desired extent.

CNTs (non-aligned)
Randomly laid CNT films would deliver a more accessible and cost-effective method for developing SHPC coatings [62,. Many fabrication techniques, including CVD, vacuum filtration, spray coating, and drop coating, were reported. Typically, as-purchased pristine CNTs or low SEmodified CNTs are used.
Xu et al. reported SHPC non-aligned CNT coating (drop coated on glass) by using alkyl (COOC 18 H 37 )n)-modified MWCNTs. The CAs of alkyl-modified, carboxylic-functionalized, and as-purchased MWCNTs were 155°(SA > 5°), 63°, and 39°, respectively. The SHPY was attributed to the double-structured surface roughness and the low SE grafted alkyl chains [62]. Hong et al. prepared SHPC CNT powder by low-pressure NF 3 glow-discharge plasma. The fluorinated CNTs with very low SE (0.12 mJ/m 2 ) displayed CAs at the range of 153-158°for polyethylene glycol (PEG), glycerol, and water. The corresponding untreated CNT powder was SHPL (CA ∼ 0°) [85,86]. SHPC MWCNTs were derived via a three-step procedure involving oxidation, cycloaddition, and silane modification. The study showed that incorporating fluorosilane-coated CNTs in polymers or textiles in small proportions could render them SHPC [119]. Hsieh et al. employed catalytic CVD to decorate CNTs on a carbon fabric. The resulted two-tier roughness along with a spin-coated perfluoroalkyl top coating yielded CA and CAH of 169.5°and 4.7°, respectively [120]. Zhang and Resasco demonstrated dramatic changes in hydrophobicity with different types of SWCNT films made on Si wafers, namely, random network (grass), vertically aligned (forest), and bundled (pillars). The CNT pillars prepared under optimized conditions exhibited the best SHPY with a CA of ∼160°, which is attributed to the typical nano/microscale surface roughness [88].
Sunny et al. have grown CNTs on acid-etched steel by CVD (750°C), and subsequently, a thin layer of PDMS (0.5 g of Sylgard in 1 mL of xylene) was spin coated and dried at 70°C (to facilitate PDMS cross-linking). The coating improved the abrasion resistance of the surface while the SHPY and the conductivity were unaffected [127]. Li et al. described a highly flexible SHPC (CA of ∼155°) carpet structure composed of long (∼300 μm), roughly aligned, and pure cup-stacked CNTs synthesized by catalytic CVD at 850°C [89]. Wang et al. developed a one-step template-free CVD approach, and the fabricated CNT film effectively avoided the capillary-induced liquid spreading. The SHPC surface displayed excellent air exposure and chemical durability ( Figure 4) [93]. Recently, Yin et al. made SHPC CNT film composed of short CNT strands via CVD at 700°C. The fraction of the water/air contact area was calculated to be 95.7%, demonstrating a significant extent of the air pocket existed. The surface showed excellent durability during 4 weeks of wettability test and superb chemical robustness over a wider pH range (0-14) [94]. A few earlier studies attempted CNT only-coatings on steel meshes by CVD [90,91].
Several works employed vacuum filtering. Wu et al. used a mixture of 0.1 g of MWCNTs with the desired amount of stearic acid (STA) in 80 mL of deionized water. The mixture was further diluted with water (5 mL diluted to 100 mL), CH 3 COOH (1 mL) was added, vacuum filtrated, and dried (70°C) to obtain the SHPC MWCNT hybrid [95]. A film fabricated from octadecylamine (ODA) (∼14%) functionalized MWCNTs by vacuum filtration (MWCNT-ODA dispersion was sieved through a filter paper, peeled off, and dried at 60°C) exhibited a CA of 165°and an electrical conductivity of 860 S/m [96]. Chen et al. reported MWCNT/SWCNT hybrid film with a CA of 152°and an SA of 2°. CVD-made MWCNTs were ultrasonically dispersed in EtOH (25 mg/mL) and vacuum filtrated through an SWCNT film, dried at 80°C, and annealed at 700°C. The MWCNTs do not detach from the SWCNTs during bending or twisting studies, displaying a firm hybrid structure [97]. Su et al. reported layer-by-layer (LbL) SHPC CNT film by vacuum-assembling multilayer carboxylated/aminated MWCNTs, followed by transferring onto ethylene-vinyl acetate (EVA) copolymer and modifying with ODA [99]. An ultrathin nanocomposite was fabricated on polyethylene via LbL self-assembly of aminated-MWCNTs (nucleophilic) and polyacrylic acid (PAA) or Gantrez (electrophilic). Ionic assembly of aminated-MWCNT/PAA or covalent assembly of aminated-MWCNT/Gantrez resulted in SHPY. A fivetimes covalent LbL-deposited surface showed a CA of 165°and an SA of >5°, whereas the corresponding ionic LbL film resulted in CA ∼ 155°with water pinning [98]. Kakade et al. reported electrowetting transition (from SHPC to hydrophilic) in MWCNTs buckypaper fabricated by ozonolysis and vacuum filtration [87].
Spray-coated SHPC CNTs were widely investigated. Yang et al. showed that simple spray-coated MWCNT film (as-purchased, 20 mg, dispersed in 10 mL of chloroform) on Cu displayed SHPY (CA ∼ 155°, SA ∼ 3.1°) without any chemical modification. SHPC-SHPL transition was also demonstrated via alternating UV irradiation and dark storage [100]. The authors in a later work showed that SHPC spray-coated film changed to SHPL after heating at 300°C, and the transition was attributed to the change of electronic structure of CNTs [101]. Li et al. also reported SHPC spray-coated CNT film without any low SE polymer coating (CA ∼ 160°and SA ∼ 3°). Aspurchased CNTs (20 mg) dispersed in 10 mL of EtOH were used, and the coating was dried at room temperature. The reversible switchable transition between high and low water adhesion was presented by alternating heat treatment and UV illumination [102]. An extremely durable SHPC coating (CA 153.1 ± 2°and SA < 5°) was fabricated by spraying an aqueous dispersion of CNTs and perfluoroalkoxy resin [131]. Yoon et al. reported a two-step process where CNT solution was first spray coated on stainless steel (SS), and subsequently, a PTFE suspension (waterbased, 60% w/w PTFE) was spin coated. A coating annealed at 360°C displayed a CA of 154.6 ± 6° [124]. Wang et al. reported spray-coated and pressure-proof flexible CNT/silane coating where PDMS and as-purchased MWCNTs were used. First, the substrate was spin coated with a solution of 0.1 g of PDMS and 0.01 g of curing agent in 10 mL of toluene or n-hexane. Subsequently, MWCNT/EtOH suspension was spray coated, and the surface heated at ∼90°C and oven-cured at 150°C. The coated Al foil displayed high CAs even after subjecting to uniaxial pressing under 3.56-32 ksi pressure. The film displayed excellent chemical (pH [1][2][3][4][5][6][7][8][9][10][11][12][13][14] and aggressive-air-exposure (2 weeks) durability [103]. Zhu et al. fabricated SHPC coating by spraying MWCNT dispersion (0.2 g in 30 mL acetone + formic acid) followed by a subsequent fluorination step. The spraycoated sample was oven-dried (120°C) and placed in a sealed desiccator (having ∼0.1 mL of fluorosilane) under vacuum for the surface fluorination. The study also showed that a thermal treatment (400°C) could retain the damaged (due to mechanical abrasion) SHPY. However, annealing above 400°C resulted in the loss of SHPY due to fluorosilane degradation [104]. Ogihara et al. demonstrated three types of dispersions, namely, CNT/EtOH, dodecyl-functionalized-CNT/ EtOH, and CNT/trimethylsiloxysilicate (TMSS)/EtOH. With the unmodified CNTs, the aggregation due to π-π interaction reduced the hydrophobicity, whereas the dodecyl functionalization screened the π-π interaction. The TMSS performed like glue, barred CNTs from aggregation, and lowered the required CNTs for the SHPY [105]. A hot/cold water-repelling superamphiphobic (SAPC) surface was fabricated by spray coating a suspension of MWCNTs, prepared via hydrolytic condensation of tetraethoxysilane (TEOS) and perfluorodecyltrichlorosilane (PFTS). The introduction of PFTS was beneficial for enhancing the coating's durability, whereas the TEOS addition was helpful to reduce the SE. Representative SEM images of the SAPC coating displayed a random cross-linked network structure with two-tier roughness composed of modified MWCNTs (Figure 5b-d). At 1 mg/mL of CNTs, the fluorosilane wrapped most of the CNTs resulting in lower surface roughness. As the concentration increased to 2 and 4 mg/mL, the fraction of exposed CNTs increased with an associated enhancement of micro/nanoscale roughness resulting in superior superamphiphobicity (SAPY) (Figure 5e). The wettability varied considerably with the CNT's diameter ( Figure 5f) [106].
The authors also reported a transparent, hot liquidrepelling durable SAPC coating from a suspension of polysiloxane-modified MWCNTs in toluene, followed by calcination to form SiO 2 nanotubes (SNTs) and subsequent PFTS modification. The SNT/PFTS coating presented high chemical and mechanical durability during liquid immersion (in water, toluene, EtOH, and 1M HCl), UV light exposure, and water jetting tests [107]. Wang et al. reported spray-coated SHPC coating with enhanced wear resistance from an aqueous dispersion containing CNTs and PTFE. Thin film fabricated on Si (CA of 154.1 ± 2°and SA < 2°) retained the SHPY even after 500 times of abrasion test under 50 g/cm 2 [134]. Belsanti et al. fabricated SHPC film via spraying CNT suspension on Al alloy for anti-corrosion application [118]. A flexible, SHPC (CA of 165 ± 2°) and transparent (>70%) PDMS/CNT strain sensor was fabricated by spray coating CNT solution onto the PDMS nanowrinkle substrate [109].
Li et al. presented an approach to creating an SHPC surface from hierarchically combined polystyrene (PS) microspheres and CNTs. First, a monolayer of PS colloidal crystals was spin coated on a glass substrate and dried at 130°C; subsequently, Au layer was sputter coated, and the sample was dipped into 0.1 M mercaptoethylamine solution to facilitate linking of -COOH-terminated CNTs on the PS spheres (CNT solution was drop coated on the modified colloidal layer). The surface was then immersion coated with EtOH solution of 20 mM PFTS and dried at 120°C. The CA before and after the PFTS modification was ∼33°and 165°, respectively. The CA decreased to 158°a fter 5 days of continuous immersion in water; however, no significant change was observed during long-term air exposure studies [110]. Meng and Park employed as- (e and f) CAs and SAs of n-decane on the spraycoated surface as a function of (e) concentration and (f) diameter of MWCNTs [106]. Reproduced with permission from ref. [106]; © 2017 Elsevier Inc.
purchased and functionalized MWCNTs (by refluxing in H 2 O 2 ), which was first dispersed in CHCl 3 (0.5 mg/mL) and further mixed with a fluoropolymer (1 wt%), and dip coated on glass. A seven-time dip-coated film with 10:3 (v/v) MWCNT/fluoropolymer displayed the highest CA of 160.2°, along with 83.5% transmittance and 1.38 × 10 4 Ω/sq sheet resistance [121]. Dispersions of CNTs, GR, and carbon black (CB) in water/organic solvents were investigated as water-repellent agents for hydrophilic wood by simple drop/dip coating [114]. Several works made use of laser in getting patterned/ transparent SHPC surfaces. Kinoshita et al. achieved SHPC/SHPL micropatterning on CNT film using a laserassisted process where the areas exposed to hyperthermal F-atom and O-atom beams, respectively, became SHPC and SHPL [111]. To obtain high transparent SHPC patterned CNT clusters, femtosecond laser micro-machining was employed by Tang et al. [112].
Rajiv et al. reported long-term-durable SHPC MWCNT-CNF composite coatings on fibre-reinforced polymer (FRP) sheets. The supercritical fluid processing method was used to reduce the composite flaws and the CNT's aggregation. The composite layer displayed better SHPY (CA ∼ 171.6°and SA ∼ 2.7°) than a corresponding coating made by physical mixing (160°and 7.12°, respectively) [115].

As filler in organic coatings/composites with polymers
Several works employed CNTs as fillers in the polymer matrix to enhance mechanical durability. The incorporated CNTs could also provide the required hierarchical surface roughness for the SHPY. The CA and SA of the composite coating could be fine-tuned by regulating the aspect ratio or concentration of the CNTs. Their inclusion could also provide additional functionalities, such as conductivity, photothermal effect, EMI shielding, and self-cleaning. immobilized on a PDMS/adhesive multilayer. The exposed CNTs at the lower surface of the layer helped to enhance the coating/substrate adhesion, whereas those exposed on the top surface contributed to SHPY. A CNT loading of 2.5 wt% was found to be essential for the SHPY. Tape tests showed that the SHPC surface was robust against adhesive material's induced damages. The surface well preserved the SHPY even at 300°C (Figure 6a). Until 200°C, the SA remained < 2°, while at 300°C, it becomes ∼3.7°. The CA of the bare PDMS sheet (116°at 20°C) displayed an obvious decline with the rise of temperature (∼100°at 300°C). Even at 10,000 kPa, the CA of the SHPC composite coating was reduced by ∼8°only ( Figure 6b). The surface also presented excellent durability during bending, adhesion, water jet, and chemical tests [139]. Han et al. fabricated SHPC surface on a flexible substrate via screen printing with a paste of PDMS-PEG copolymer and MWCNTs. Superior SHPY with >1,000 S/m of conductivity was attained at 25 wt% of PDMS-PEG [143].
Highly conductive nanocomposite coatings of fluoropolymer dispersion with CB, CNTs, or GR nanofillers were studied. At 50 wt% of CNTs or CB, the SHPC surface resisted water-drop impalement at 3.7 m/s. GR-based surface displayed the most inferior dynamic impact resistance; however, the best conductive one [138]. Bayer et al. fabricated electrically conducting SHPC polymer-CNT coating via an emulsion-based spray process. The emulsion was composed of fluoro-acrylic latex solution (capstone ST-100), toluene-MWCNTs, and MtOH. The spraycoated film was cured at a temperature above the melting point of the fluoro-acrylic latex (>160°C) to yield the SHPY (CA >165°, SA < 5°). However, increasing the CNT loading beyond 17 wt% caused coating flaking and poor substrate adhesion [132]. Zhang et al. described highly transparent SAPC coating by CVD of PFTS onto SNTs. MWCNTs were employed as the template, and the SNTs were made via hydrolytic condensation of silanes followed by calcination. The fabrication steps include preparation of polysiloxane-modified MWCNT suspension, spray coating, and preparation of SNTs and SNTs/PFTS. The SAPY and the film transparency were highly dependent on the solvents used; n-octane was optimal [136]. Wu et al. fabricated MWCNT-PVDF hybrids through a facile phase-separation method. PVDF crystallization was almost unaffected with the extent of MWCNT loading, and the β-phase crystals dominated the process. The neat PVDF exhibited a CA of ∼144°, whereas the hybrids with 8 and 16 wt% of MWCNTs displayed CAs of 163°and 166°, respectively [130]. An SHPC GR/POSS/MWCNT hybrid coating prepared by a two-step casting process on a glass substrate displayed a CA of 155°without compromising the conductivity. MWCNT/POSS was coated on the GO/ THF layer, and thermally treated at 200°C [125]. Core-shell-structured polyaniline (PANI)/CNT composite, SiO 2 NPs, and 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFOS) were integrated synergistically into the ethylene tetrafluoroethylene (ETFE) matrix. Here, ETFE powder (1 g in 30 mL ethyl acetate), SiO 2 (0.05 g), and PFOS (0.2 g) were ultrasonicated with a selected amount of PANI/CNTs and the welldispersed solution was used for the spray coating, followed by curing at 300°C. A composite layer made from 6 wt% of PANI/CNTs revealed superior SAPC property, with CAs of 159°, 163°, and 167°for ethylene glycol, glycerol, and water, respectively (with SAs < 5°). However, at concentrations >6 wt%, surface cracks were evident. The optimum coating demonstrated durable SHPY with droplets of a wider pH range (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14) and thermal stability at temperatures <400°C. SAPY was preserved even after 30 times bending test or 45,000 times of abrasion. Direct immersion studies in aggressive HCl (1 mol/L, 60 days) and NaCl (3.5 wt%, 90 days) revealed excellent chemical durability [128].
A CNT-organic silicone resin (SiR) composite displayed outstanding mechanical robustness and hot water repellency. The wettability variation was closely dependent on the CNT concentration. Coating with 15 wt% or more CNTs repelled hot water effectively (Figure 7a). The coating displayed excellent mechanical durability during the abrasion test ( Figure 7b) and waterfall/jet testing (Figure 7c and d) [140]. Cold-curable silicone sealant was used along with MWCNTs. Here, the required amount of silicone matrix was added into a dispersion of CNTs in hexane and that was used for spray coating. The CA monotonically increased with the increase of CNT content in the polymer matrix, reaching 158.4°for 50 wt% loading [141]. Lv et al. prepared robust and F-free SHPC coating from polyphenylene sulphide (PPS)-SiR composite. The SiR also acted as the low SE component, whereas the reinforced fillers (CNTs or GR) provided the required hierarchical surface structure; a silane-coupling agent was also employed. The sprayed and heat-treated (320°C) coating exhibited a CA of ∼161°and an SA of 2°. The wettability varied both as a function of SiR and CNT content, and the optimized coating was composed of 10 wt% of SiR and 1 wt% of CNTs. The evenly distributed CNTs with self-lubricating properties were helpful to reduce the abrasion loss significantly. After 2,500 times of friction (1,000 sandpaper, 500 g, 37.5 circles/min), the sample continued to be SHPC with a CA of 150 ± 2°. After 5,000 cycles, the CA becomes ∼139°. Compared with the PPS coating (0.2106 g), the SHPC coating's weight loss (0.063 g) considerably decreased [142].
Song et al. reported nano-hybrid membrane based on chitosan (as the matrix), cationic chitosan (chitosanmodified by a triazine derivative), MWCNTs, and a silicon coupling agent, which was further modified by low SE perfluorooctanesulphonyl fluoride. The membrane with 3 wt% MWCNTs showed excellent SHPY [144]. Wang et al. fabricated nanofibre composite membrane of CNTs and PVDF-co-hexafluoropropylene (PVDF-HFP) using electrospinning and pressure-driven filtration processes.
After a low SE fluorosilane modification (by CVD), the CNT/PVDF-HFP membrane displayed SHPC and superoleophilic (SOPL) properties [145]. SHPC and SOPL porous CNTs/PVDF composite was prepared via gel formation and freeze drying. The study proved that even a small amount of CNTs could considerably enhance the porosity and surface roughness of PVDF [146]. Kousalya et al. made a mixed wetting surface with interchanging parallel SHPC/SHPL bands through graphitic petal-decorated CNT coatings via PTFE deposition, shadow mask, and oxygen plasma treatment [147].
Wettability transitions in a spray-coated coating of polyfluo 150 wax and fluorocarbon-modified MWCNTs (1:4 wt ratio, acetone-ethyl acetate solvent) were studied as a function of temperature. SHPY was attained at 150, 200, and 360°C, whereas SHLY resulted at 300°C. The variation was associated with the melting/decomposition of the wax and the related phase separation [122]. Rungraeng et al. revealed a slippery liquid infused porous surface consisting of SHPC MWCNT-PTFE layer filled with a low surface tension liquid. Water, honey, ketchup, oil, and bacterial biofilms freely slipped off the surface without leaving any apparent wreckage [148]. Peng et al., in earlier work, reported a way for transforming common polymers into SHPC conductive surface by simple pressing of a layer of MWCNTs on a polymer melt ( Figure 8). Under appropriate conditions, the CNTs were partly inserted inside and partly exposed outside the coatings' surface, creating a carpet-like structure, providing both SHPY and conductivity [126].

With polyurethane/epoxy/PS
Several studies explored CNTs as filler/hydrophobicity promoter with polyurethane (PU), epoxy (EP), and PS resins. Hejazi et al. described a single-step pressing method to fabricate SHPC, self-cleaning, and mechanically durable PU/CNT composite. After placing the PU sheet into a pre-designed mould, 15 mg of CNT powder (∼2 vol%) was distributed, and the surface was pressed under 4 MPa for 10 min and heated to 180°C. The pressing time had a significant effect on the type of SHPC surface. A longer processing time (60 min) resulted in sticky SHPY [149]. A report on spray-coated SAPC coating used fluorinated MWCNTs as a nanoroughness promoter and fluorinated PU as a low SE binder and acetone/toluene cosolvents [150]. Reports on PU-CNT-based aerogels/sponges are described in Section 2.1.5.
Besides SHPY, the CNT fillers in the EP matrix could help to overcome their inherent brittleness and poor wear resistance. The high mechanical strength and abrasion resistance of a spray-coated CNT-EP composite were attributed to the even dissemination and strong bonding of CNTs on the EP resin. The suspension for the spray coating was prepared by adding EP resin (200 mg) to a dispersion of CNTs (200 mg) in acetone (100 mL). A subsequent curing step at 120°C facilitated melting and ingression of EP resin to CNT/substrate interface and ensured coating with exposed CNTs (CA ∼ 166°and CAH ∼ 4°) [151]. Hsu et al. reported amphiphilic polyamine-modified CNT/EP SHPC film by using a dispersion of as-purchased CNTs (0.05 g), polyisobutylene-amine copolymer (PBamine, 0.1 g), THF, and EP resin (0.071 mmol, 0.025 g). The SHPY (CA > 152°and CAH ∼ 7°) of the drop coated and cured (80-150°C) surface was primarily attributed to the well-ordered orientation of CNTs. The CA augmented to ∼158°at an optimized PB-amine/CNT wt ratio of 1/2; however, it declined with a further increase of PB-amine content ( Figure 9a). The corresponding variation of sheet resistance is also shown ( Figure 9a). Their time-dependent CA variation studies displayed excellent durability for PB-amine/CNT and PB-amine/CNT/EP coatings. On the other hand, the pristine CNT film failed within 1 h (Figure 9b) [152]. A robust SAPC EP/PVDF/FEP spray coating with incorporated CNTs and SiO 2 (2.5 wt% each) was reported. The synergistic combination of SiO 2 and CNTs was utilized to create the required multilayer structure. Fluorinated ethylene propylene (FEP, 17.8-18.8 mJ/ m 2 ) was employed to reduce the SE [153]. The authors also reported an SAPC and electroactive bilayer composite coating by integrating EP, PANI, FEP, SiO 2 , and CNTs by air and electrostatic spray methods. The brilliant cross-linking effect of EP resin and the nanofillers' reinforcement enhanced the mechanical properties considerably and provided a durable SHPC surface with excellent adhesion and wear resistance [154].
Zhang et al. have studied spray-coated SHPC MWCNT/ EP nanocomposite coating on carbon steel (CS), where the agglomerated CNTs mainly constituted the hierarchical microstructure. Different amounts of CNTs (0.125, 0.25, 0.5, and 0.75 g) were first dispersed in the EP resin (1.875 g in 40 mL of acetone), to which jeffamine D230 (0.625 g) was added and stirred to prepare the dispersion. The coated/cured (at 60°C, 2 days) sample with 30 wt% CNT loading displayed slippery SHPY. However, a lower (10 wt%) or a higher (40 wt%) loading does not yield SHPY and that was, respectively, attributed to the insufficient CNT content and the wider spacing between the neighbouring agglomerates [155]. An SAPC coating was fabricated by spraying a suspension of fluorinated MWCNTs (hydroxylated MWCNTs dispersed in dry THF with added 1H,1H,2H,2H-perfluorodecyltriethoxysilane [PFDTS]) and EP adhesive. The F content was as high as 47.4%. The natural crowding of the CNTs and the solvent evaporation created the required surface roughness and the microstructure. The surface maintained the SAPY even after 40 abrasion cycles. Excellent durability was observed during finger wiping, sand dropping, and sandpaper abrasion tests ( Figure 10 [156]. Reproduced with permission from ref. [156]; © 2020 Elsevier. expanded graphite) fillers, uniformly dispersed in EP resin. First, E51 EP resin (2 g) and THF (3 g) were stirred and added to a solution comprising MWCNTs (0.16 g), graphite powder (0.5 g), EG (0.04 g), PFOS (0.1 g), and THF (5 g); subsequently, 0.2 g of diethylenediamine was also added. The SHPY was not destroyed during the abrasion testing (200 sandpaper, 500 g, 20 cm, 50 cycles), even if several powders have been worn out and the coating thickness reduced [157].
Several studies explored PS/CNT coatings [158][159][160][161][162][163][164]. Yang et al. reported one-step spray-casted SHPC and transparent (78%, visible) CNT/PS coating. PS capped with 4-hydroxy-2,2,6,6-tetramethyl-piperidinooxy was used [158]. The authors, in a later work, described a versatile strategy for fabricating SHPC PAA-block-PS-functionalized MWCNT (by nitroxide-mediated living free-radical polymerization) film by spray coating (CA ∼ 166°, SA ∼ 5°) [159]. Song et al. also reported a spray-coated PS/MWCNT film. SHPY was achieved after incorporating 1 wt% of γ-aminopropyl trimethoxysilane-modified MWCNTs. The surface displayed excellent durability during 1 week of water immersion, and 1 month of high-humidity air-exposure studies [160]. An SHPC conductive surface with a maximum CA of 165°(SA < 3°) was fabricated via drop coating by utilizing α-methylstyrene-butylmethacrylate copolymergrafted MWCNTs. The CA displayed only a marginal decrease during 210 days of indoor air-exposure study [161]. Kim and Cho employed MWCNTs with PS and THF, and the coating was made on a cover-glass by spraying. The highest SHPY with a CA of 163.8 ± 2.5°a nd an SA of 5 ± 0.9°was recorded for a film coated from a mixture of 1 wt% of PS and 0.08 wt% of CNTs [162]. An SHPC anti-static coating was fabricated from a blend of poly(styrene-alt-maleic anhydride), SiO 2 , and MWCNTs. γ-Aminopropyl-triethoxysilane was utilized to bond the inorganic and organic components. A sharp transition from hydrophobic to SHPC was noted at ∼4.6 wt% of CNT's loading. High-temperature curing (220°C) was found to be essential to enhance the coating's durability. The hybrid coating exhibited CA as high as 180°. The surface remained SHPC even after 20 days of immersion in deionized water [164]. Gu et al. presented an SHPC PS/CNT hybrid membrane through covalent attachment of PS to CNT network [163].

Others
Earlier works by Han et al. [165], Li et al. [166], and Men et al. [167] have used CNTs along with polycarbonate (PC), poly(4-azidophenylmethacrylate-co-methyl acrylate), and poly(furfuryl alcohol), respectively. Men et al. used poly(furfuryl alcohol) as adhesion and low SE agents, along with fluorocarbon-modified MWCNTs and PTFE. The spray-coated and cured (70-180°C) coating displayed superior SHPY when the three components' mass ratios were 1:1:1 [167]. Han et al. employed CNTs and reduced-GO (RGO) sheets to nucleate PC crystallization through phase separation. A 10 s of dipping of the polymer sheet in the MWCNT solution was enough to shape the SHPC surface with a CAH of <5°. An optimized solvent mixture of methyl ethyl ketone and isopropyl alcohol (poor solvent) was used. The study also showed that MWCNT was a better nucleation agent than SWCNT or RGO [165].
A few works explored polybenzoxazine (PBZ)-based SHPC coatings [168][169][170]. PBZ is also known for its low SE (21 mJ/m 2 ) [169]. Zhang et al. presented an immersion coating comprising pristine MWCNTs and PBZ for ramie fabric [168]. Wang et al. sonicated MWCNTs (10 mg) with a BZ solution (10 mg in 10 mL THF) and then poured onto a glass slide, dried, and oven-(at 240°C) or MW cured. Although the as-fabricated surface was SHPC, the curing step, as expected, was essential to improve the thermal and mechanical durability [169]. The authors later developed different CNTs/PBZ coatings and showed that a fluorinated surface with a micro/nanoscale structure possessed the most robust SHPY [170].
The non-solvent-assisted phase separation was utilized to create self-cleaning SHPC porous film based on polyvinyl chloride (PVC) loaded with CNTs. On the other hand, the coatings fabricated in the absence of the nonsolvent (EtOH) exhibited sticky behaviour [171]. Su et al. reported a robust 3D porous SHPC composite with good cyclic shape memory performance with poly(ethyleneco-vinyl acetate)/MWCNT as the skeleton and NaCl as a sacrificial template [172]. Mokarian et al. reported a highly durable SHPC nanocomposite comprising silicone rubber and MWCNTs. Outstanding durability was observed during soaking tests in boiling water, 5 wt% NaCl, and condensed HCl [174]. An SHPC conductive paper was conceived by successive dip coatings in CB/CNT/methylcellulose and fumed-SiO 2 suspensions [175].
All these reports unanimously proved that CNT's incorporation is highly beneficial to enhance mechanical durability, surface hydrophobicity, electrical conductivity, and self-cleaning properties of polymer-based nanocomposites. The concentration and aspect ratio of the CNTs are decisive. None of the studies revealed inferior durability during chemical, mechanical, or thermal stability studies. However, a few works showed compromised CAs after more prolonged water immersion or higher abrasion cycles.
Several works employed SiO 2 . Hsieh et al. decorated polyacrylonitrile-based carbon fibre (CF) fabric with solgel-made SiO 2 microparticles and CNTs. A wet chemical impregnation approach was used to disperse SiO 2 onto the fabric, and then CVD (900°C) was employed to grow CNTs. A CA > 170°was observed in the CF/SiO 2 /CNT threetier structure [178]. Peng et al. fabricated SHPC conductive coating by air spraying using a mixture of functionalized MWCNTs and aqueous SiO 2 sol, followed by fluorosilane modification. The study explored two types of CNTs: hydroxylated (h-MWCNTs) and copolymer/silane-wrapped (w-MWCNTs). The threshold concentration for achieving the SHPY varied with the type of the CNTs, i.e., 2.7 vol% for w-MWCNTs and 4.8 vol% for h-MWCNTs. The nanocomposite coatings retained SHPY for more than 1 year in outdoor weathering. The study also showed that the w-MWCNTs/SiO 2 coatings' durability increased with coating drying temperature. During continuous water immersion studies, the coatings dried at 160°C and 240°C retained SHPY for 20 and 27 days, respectively. However, for a coating without high-temperature curing, the SHPY was lost within 3 days [179]. Li et al. reported spray-coated SAPC CNT-SiO 2 hybrid coating. Here, h-MWCNTs were first ultrasonically dispersed in EtOH and then 5 mL of 25 wt% aq. NH 3 and TEOS-EtOH solution were added. The welldispersed solution was used for spraying, and the coated glass was vacuum dried and subjected to CVD of PFOTS [180]. Wang et al. demonstrated SHPC spray-coated h-CNT-SiO 2 NP composite coating with a post-fluorination step [181]. SHPC SS mesh was fabricated via coating of oxidized MWCNT inks with post-modification in perfluorosilane/SiO 2 NP solution and curing at 150°C [183]. Yu [188].
A transparent SAPC coating was fabricated using a template approach where CNTs were sol-gel-coated with SiO 2 and the CNTs-SiO 2 suspension was spray coated onto glass slides and cured at 600°C and further subjected to a fluorination step. The coated surface displayed many protrusions composed of SiO 2 NPs and SNTs. The transparent coating sustained SAPY even at 400°C [189]. Li et al. fabricated semitransparent SOPC SNT coatings on glass with a post-PFTS modification. The SNT layer was spray coated using a dispersion of PDMS-modified MWCNTs and subsequently calcined to remove the CNT template. The SNT layer's microstructure and hence the SAPY could be controlled by optimizing the diameter and concentration of MWCNTs. The surface displayed excellent superwettability for water, n-decane, n-hexadecane, toluene, and several hot liquids [190].
Zhu et al. fabricated an SHPC film by spray coating a dispersion of ZnO NPs and MWCNTs in PDMS solution [191]. Barthwal et al. reported ZnO/MWCNT coating (by sol-gel and dip coating) for Cu mesh, with a subsequent PDMS modification. The SHPY was maintained during direct immersion in 3.5 wt% NaCl for 15 h and ambient air exposure for 2 months. A composite coating with 2.5 wt% MWCNTs presented a CA of 156°and an SA of 4°. The CAs recorded for coatings with 1 and 5 wt% MWCNTs were 151°and 145°, respectively [192].
SWCNT/GR composite was fabricated via covalent crosslinking through two different coupling strategies: carbodiimide and Sonogashira. The results showed that the composite assemblies obtained by Sonogashira coupling exhibited higher surface areas and greater CAs (159-163°) when compared to the carbodiimide coupling [193].

Sponges, foams, aerogels, fabrics, and meshes
Several works investigated CNT-based sponge/foam/ aerogel/fabric/mesh for different applications including oil separation and sensors. Polymer/CNT composite sponges are mainly studied .
A few works reported SHPC CNT-only sponges/bundles [198][199][200], whereas a few others studied CNTs + GR sponges/monoliths [201][202][203]. CNT/polymer composite sponges are attractive for practical applications due to their improved mechanical durability. Several works employed composite structures of CNTs with commercial PU foams. Ge et al. presented a dip-coating method for building SHPC and SOPL CNT/SiO 2 -coated PU sponge where the as-purchased PU sponge was consecutively dipped in EtOH suspensions of PVDF-HFP and CNTs/ SiO 2 , and cured at 140°C, blew off the loose particles and subjected for further fluorination in EtOH solution of perfluorotetradecanoic acid, and dried at 80°C [204]. Sultanov et al. also employed a dip-coating method where commercial PU sponges' walls were coated with RGO and MWCNTs [205]. PU/MWCNT composite with a nano/microscale hierarchical porous structure with copious air holes was fabricated via non-solvent-assisted thermal phase separation. Morphology analysis revealed uniform dispersion of the carboxylated-CNTs in the porous structure, facilitated by strong hydrogen bonding interaction. The composite monolith displayed excellent mechanical elasticity and chemical durability [206]. Hong et al. reported a flexible, conductive SHPC PU/CNT/silane aerogel composite microfibre. CNTs (2.5 wt%) were mixed with PU first and then with PFTS, and further, SiO 2 aerogel particles were embedded in the matrix [207].
A few studies employed melamine (ML) sponges [205,208,209]. Mechanically robust 3D SHPC composite sponges were prepared using ML as a scaffold, MWCNTs, GR, or activated carbon as SHPC and robust coating material, and a polyphenol-Fe 3+ complex as a low-cost adhesive. The optimized loading of MWCNTs for the SHPY was 3% [208].
Cellulose-based materials are attractive in terms of abundance and eco-friendliness. Lu et al. made SHPC/ SOPL sponge by cross-linking epichlorohydrin with ethyl cellulose (CL) sponge and further complexing with silanized CNTs and modifying with SiO 2 /HDTMS. The SHPC/ SOPL material (water CA ∼ 158.2°, oil CA ∼ 0°) displayed superb mechanical strength (withstand 28.6 kPa pressure) due to the facilitated chemical cross-linking and the incorporated nanofillers. The sponge also revealed excellent thermal (up to 330°C) and chemical durability (acid/alkali/salt solutions) [197].
Several works explored SHPC CNT coating on cotton fabrics. Makowski et al. fabricated SHPC cotton woven fabrics covered with silane-modified MWCNT layer [210]. Zheng et al. manufactured a conductive SHPC cotton fabric (CA of 162°) by LbL assembling carboxylated and aminated MWCNTs and further modifying with PDMS. The assembly was repeated for different cycles and subsequently dipped in 5 wt% hexane solution of PDMS and vacuum dried at 135°C [211]. SHPC, flame-retardant, and conductive cotton fabric was fabricated by LbL assembly of poly(ethylenimine), ammonium polyphosphate, and CNTs, followed by PDMS treatment [212].
A few studies explored CNT coating on SS mesh. Lee et al. described a technique to directly synthesize VACNTs on commercial SS mesh by CVD for oil separation [91]. Lu et al. used a spray-coating method for CNT layer deposition onto steel wire mesh. Poly(methylmethacrylate) (PMMA) was used to deliver robust bonding between CNTs and the mesh surface [213].

Carbon nanofibres 2.2.1 CNFs only
This section discusses CNF-based SHPC surfaces . A few works on microscale CFs are also included.
demonstrated the influence of the F/C ratio on the SHPY of CNFs prepared by a template-assisted method and showed that the CA increases with the increase of F/C ratio [234]. Wang et al. employed an alumina template to fabricate aligned SHPC CNFs (CA of 153.1 ± 2.2°) via dipping in hydrophilic PVA polymer followed by a carbonization process (at 600°C) and subsequent partial template removal. No low SE modification was used. The CA of the corresponding disordered CNFs was only 126.3 ± 3.7° [235]. Hima et al. studied three different types of CNSs (caterpillar-like fibres, tubes, and interwoven spheres) by CVD (700°C, morphology varied with the growth duration) and showed that the caterpillar-like CFs and interwoven carbon spheres exhibited high CAs of 163 ± 2°and 168 ± 2°, respectively, and that was attributed to their unique surface structures. The corresponding tube structure displayed CA ∼ 140°only [244].
Tsai et al. provided a detailed account of drop-impact dynamics on SHPC surfaces made up of CNF forest and microscale-patterned polymers. The study showed that the multiscale nanoroughness had only a negligible effect on the impact dynamics when W n was ≤120; however, it was significant at W n ≥ 120 [236]. Ogihara et al. demonstrated EPD-made SHPC coloured films using different hydrophobic pigment particles, including vapour-grown CNFs and CB [237].
An SHPC CF coating with boosted corrosion protection capability was developed on Zn via CVD at 350°C. The CAs measured on Zn-CF and bare Zn surfaces were 153.3 ± 1°and 68.2 ± 1°, respectively [238]. Durable CNF coatings were developed on mild steel (MS) and AZ31 Mg alloy by subsequent plasma sputtering and CVD. The CA of the bare MS (∼69°) increased to ∼150°(SA ∼ 7°) after the CNF coating. The corresponding transition for the Mg alloy was ∼66.7°to ∼145° [239].
Several works employed SHPC fabrics made of CFs [241][242][243][244][245]. Meng et al. explored the role of CNFs in preparing SHPC and electroconductive surface on glass fabrics. Homogeneous CNFs were grown by CVD, and the surface modified with a fluoropolymer [243]. Ko et al. utilized preferential oxygen plasma etching to fabricate high-aspect-ratio CF-network-structures with morphology ranging from nanopillar to hairy. A subsequent siloxane modification increased CA from 147°(pristine CFs) to 163°( 30 s of siloxane vapour treatment) with a reduction of CAH from 71°to <5°. Under super-saturated vapour conditions, the pristine CFs were wet with condensation between fibres. They led to flooding, whereas dropwise condensation was dominant on the SHPC surface, allowing the interstitial spaces to remain dry [245].
Siddiqui et al. employed a two-step plasma-sputter/ CVD (300°C) process to fabricate SHPC CNFs on activated CF (CA ∼ 146°) and glass (CA ∼ 156°) substrates without the use of fluorosilanes. The CAs of the corresponding bare CF and glass surfaces were ∼0°and 36° (Figure 11a-d). SEM images displaying the surface morphology of the SHPC surfaces are also shown (Figure 11e-h). The SHPY was well-maintained during the abrasion test with different sandpapers and applied loads. An already abraded surface (using P1500 for nine cycles) retained excellent durability even after subjecting a second round of abrasion studies (P800, 100 g) (Figure 11i). The surface presented outstanding chemical durability over a wider pH and hot water (95°C). The SHPY was well-maintained during an ultrasonication test for 300 min (Figure 11j) [248]. Xu et al. employed an SHPC CF sponge without any chemical modification for oil separation [247].

With polymers
Several works explored CNFs with silanes/fluorosilanes [250][251][252][253][254][255][256][257][258]. In 2011, Das et al. reported a spray-coated SHPC self-cleaning CNF/PTFE/composite-polymer coating. CNFs were employed mainly to adjust the conductivity without compromising the SHPY, whereas PTFE particles were used as hydrophobic fillers. A solution blend of PVDF and acrylic PMMA was used as the composite-polymer matrix. The optimized CNF loading amount was 1.1 wt% [250,252]. The authors also reported a water-based spray coating of commercial fluoroacrylic copolymer and hollow CNFs. The study showed that replacing lengthy CNFs with short solid nanowhiskers would help produce more stable fluoropolymer-nanocarbon dispersion. At a CNF concentration of >30 wt%, self-cleaning SHPY was observed, whereas oil-droplet mobility was experienced at 60 wt% loadings only [251].
CNF-PDMS systems were explored. Seo et al. prepared CNFs by CVD and surface modified by PDMS (CA ∼ 170°). A corresponding STA-modified film showed a CA of ∼150°only [253]. Abdulhussein et al. employed onestep vacuum filtering to fabricate SHPC (CA > 163°and SA < 5°) and SOPL (oil CA ∼ 0°) CNF/PDMS composite block with high mechanical and chemical stability [254]. A robust SHPC (CA ∼ 163°) and SOPL CNF/PDMS-modified SS mesh was fabricated by vacuum filtration, followed by PDMS coating. Well-dispersed CNFs with lengths at the range of 20-200 μm and diameters of 100 nm were used, and SS/CNF mesh with a pore diameter of <1 μm was assembled by random cross-linking ( Figure 12). Most of  the CNFs were well-placed inside the SS mesh, whereas a few stayed exterior. The SHPC surface displayed extreme chemical durability to 1 M NaCl, pH 2 solution, and toluene solvent. The durability was preserved even after continuous immersion of the membrane for a longer duration (Figure 12) [255].
Yang et al. in a recent work presented SHPC and SOPL CNF-reinforced PDMS-metal rubber (MR) composite. CNFs were embedded into MR via vacuum filtration, and subsequently, PDMS coated. PDMS acted as an effective binder between CNF fillers and MR. The composite remained SHPC even after 100 cycles of compression at 200 N. Excellent chemical durability against pH 2, pH 10, and 3.5 wt% NaCl solutions was observed [256]. Guo et al. reported an SHPC/SOPL foam composite in which CNFs were uniformly distributed on PDMS foam under ultrasonication, followed by an extra PDMS modification to improve the interfacial adhesion [257]. An approach for designing hemostatic wound-dressing material favouring rapid blood clotting and easy clot removal utilized SHPC CNF/PTFE or CNF/PDMS networks [258].
A few works explored CNFs with PUs [259][260][261]. Zhang et al. prepared SHPC/SOPL and electrically conducting PU/CNF network. Hollow CNFs were ultrasonically integrated onto the PU nanofibre surface and subsequently subjected to PDMS modification. PDMS served as an interfacial adhesion agent and waterproof protective layer. The composite displayed excellent strain independent superwettability; original CA (153°) was retained even at 100% applied strain. The superb durability was attributed to the robust interfacial (CNFs/PU-nanofibres) adhesion [260]. Baig et al. synthesized CNF-grafted PU sponge by dip coating. Besides enhancing the hydrophobicity, the CNF's grafting helped increase the surface area and decrease the average pore size of the PU matrix (improved capillary action) [261].

With metals/ceramics
Li et al. employed CVD grown herringbone CNFs along with commercial carbon felt and SiO 2 . Here, fumed SiO 2 NPs were distributed onto the carbon felt and then exposed to CNF growth condition. The CNF/SiC/carbon-felt hybrid composite formation happened at high temperatures (1,650°C, Ar) via carbothermal reduction at SiO 2 /carbon interfaces. The composite was superbly mechanically robust [240]. Aljumaily et al. synthesized two types of hierarchical SHPC CNMs on Ni-doped activated carbon surface, namely, carbon sphere-free CNFs (CA ∼ 167°) and carbon sphere-mixed CNFs (CA ∼ 177°), via CVD at 650 and 750°C, respectively. The study suggested that such mixed structures could be an attractive way to enhance hydrophobicity. The spheres on the CNF network divulged a rougher surface and minimized the cavities [246].
Earlier reports explored SHPC SiO 2 NP-coated CFs [263] and hydrothermal (HT)-synthesized ZnO/CNF hybrid [264]. Wu et al. in a recent work presented nanocomposite of CFs, PVDF, CB, and CeO 2 NPs with excellent SHPY (CA ∼ 156°and SA ∼ 5°), mechanical properties (tensile strength and tensile modulus were ∼109 MPa and 10 GPa, respectively), and electrical conductivity (∼6.8 S/cm). The SHPY remained stable even after 60 min of strong acid and 24 h of strong base immersion, and 200 cycles of sandpaper abrasion [265]. SHPC fluoroalkylsilane-modified Ni-electrodeposited CFs displayed a water CA of ∼159.1°and an oil CA of ∼0°. The surface remained SHPC with a CA of ∼158°even after 24 months of air exposure [266]. A few works on SHPL and underwater SOPC membranes/monoliths are also available [267,268].

Carbon nanosphere/nanothorn/others
Micro/nanoscale carbon spheres can be synthesized by various methods, including CVD, pyrolysis, HT/solvothermal, and chemical routes. Qu et al. fabricated SHPC carbon nanosphere film by depositing the soot of burning rapeseed oil without any surface modification [271]. The authors also reported an SAPC film via an additional fluorosilane modification where the fluorosilane acted as a glue to the loose carbon nanospheres [272]. Joula et al. reported a method for preparing SHPC surface by spin coating EtOH colloidal solution of HT-synthesized carbon nanospheres on glass substrate [273]. EPD at 30 V, followed by heat treatment at 250-350°C, was utilized to obtain SHPC carbon micro/nanosphere thin film on FTO glass. HT (glucose solution, 160°C) prepared carbon spheres with different diameters were used [274].
A multifunctional composite coating based on mesoporous carbon nanocapsules and PVDF displayed a CA of ∼160°and an SA of ∼5°. Excellent chemical durability was achieved over wider pH (1.29-13.54), and humidity (35-83%) ranges. The film displayed thermal durability up to 350°C. The surface maintained the SHPY after 30 s of abrasion with a CA of ∼155°and an SA of ∼15°(150 rotations, 200 g, dragged while rotating at 300 rpm). After 60 s, even though the SHPY maintained, the SA increased to ∼43°. The surface lost SHPY after 90 s of abrasion [275,276]. A few other studies also described different carbon nanosphere-based SHPC coatings [41,80,277,278].
In recent work, Li et al. designed an SHPC carbon nanothorn array by a single-step reaction in a liquid-phase environment containing 1,3,5-triethynyl-2,4,6-trimethylbenzene, pyridine, acetone, and N,N,N′,N′-tetramethylethylenediamine. The fabricated SHPC surface on a commercial Cu form displayed excellent long-term durability during 800 days of water immersion study. When the experiment was conducted at pH 5 or 9 solutions, the SHPY marginally compromised (CA of ∼145°after 800 days) [279].
SHPC powder sample composed of hierarchical carbon microflowers (CMFs) and dispersed-MoO 3 -NPs was fabricated by utilizing the wet chemistry of (NH 4 ) 6 Mo 7 O 24 4H 2 O and dopamine, followed by annealing (700°C) and PFDTS modification (0.5 g of CMF/MoO 3 was added to 40 mL of 0.5% PFDTS-hexane solution). An SHPC coating was fabricated by using the powder, EP resin, and a curing agent via sieve-deposition technique. The surface displayed excellent mechanical durability until 50 abrasion cycles (400 Cw sandpaper, 200 g, moved back and forth for 10 cm). A corresponding coating without EP resin performed poorly in the abrasion testing. Good acid/alkali resistance (pH 1 and 14) was also noted. The addition of MoO 3 was found to be helpful to enhance the chemical durability [280]. Ghosh et al. deposited SHPC vertically aligned tree-like carbon nanospheres by CVD. The robust sticky SHPY was attributed to the highly roughened porous surface [281].
Onion-like carbon microspheres comprised of nanoflakes were made from waste polyethylene terephthalate via pyrolyzing (650°C, supercritical CO 2 ) and vacuum annealing (1,500°C). The authors fabricated an F-free SHPC coating on polyester fabric by using the prepared carbon microspheres with PDMS [282].

Nanodiamond
In earlier work, Zhou et al. achieved self-cleaning SHPY on the two topmost hardest materials, diamond and cubic BN, via reactive ion etching (H/Ar plasma) and surface fluorination [283]. Coffinier et al. reported SHPC (CA ∼ 160°, CAH < 2°) B-doped diamond nanograss by reactive ion etching with oxygen plasma, followed by octadecyltrichlorosilane or PFTS modification [284]. The authors also fabricated heterogenous SHPC/SHPL patterns where the SHPL regions were generated by selective UV light exposure [285]. Yang et al. reported an SHPC/SOPL robust diamond mesh [286]. SHPC diamond microspheres were deposited on Si by MW CVD. The microspheres were then collected by blade scratching and employed in fabricating an EP-based SHPC film with excellent mechanical and chemical durability [287]. Deshmukh

Fullerene
A few works addressed SHPC fullerene assemblies [194,195,[289][290][291][292][293][294][295][296]. In earlier work, Nakanishi et al. prepared SHPC surfaces with two-tier nano/microroughness by the molecular assembly of a fullerene derivative bearing a long aliphatic chain, without using fluorinated compounds [290]. The authors also reported the formation of a selfassembled hierarchical fullerene derivative with long hydrocarbon chains and perfluoroalkyl tails [292]. An SHPC thin film was prepared via self-assembly of Au NPs and fullerene pyridyl derivatives [291]. Mansurov presented the formation of SHPC fullerenes and CNTs on Ni and Si supports in benzene-oxygen and propaneoxygen diffusion flames [194]. A supramolecular method was used to fabricate C60/tetracene flower-like microstructure comprised of nanoplates. FCC C60 microstructures were further derived via sublimation of tetracene component at 330°C. Thin films prepared with FCC C60 and C60/tetracene displayed SHPY with CAs of 156.3°and 150.2°, respectively [293].
A two-step self-assembly strategy was reported for the preparation of SHPC fullerene hierarchical architectures. The process was composed of a precipitation method to synthesize the initial fullerene microstructure and a subsequent drop-drying process to facilitate the selfassembled hierarchical structure [294]. Pérez-Ojeda et al. also reported hierarchical self-assembly of fullerene derivative with SHPY [296]. Partheeban and Sathish demonstrated the formation of SHPC flower and octahedron-like fullerene microcrystals using a liquid-liquid interfacial precipitation method [295]. Ayyappan et al. presented an SHPC surface by covalent functionalization of methacrylate polymers with CNTs and fullerenes [195].

Nanostructured carbon soot/graphitic carbon/others
This section briefs works reported with nanoscale-activated carbon, CB, carbon soot, carbon sponges etc. that are not mentioned in other sections. Carbon soot is one of the earliest known hydrophobic materials. Despite the advantages of easy availability and robustness [297][298][299][300][301], the amorphous nature and loose structural binding between the carbon NPs restrict their applications [298].

Graphene
SHPC surfaces and coatings based on GR and its derivatives are briefly provided here . A detailed discussion is out of the scope of this review.
Several works endorsed the SHPY primarily to the surface roughness and the reduction (removal of hydrophilic oxygen groups) processes. Different reduction methods were explored for achieving SHPY with GR/GO, including thermal [371], chemical [372], and photoreduction [39]. A significant extent of research has been performed on laser-assisted approaches since 2009 [373][374][375][376]. The wettability of hydrophobic GR is similar to that of graphite; however, dynamic wettability could significantly depend on the layered structure [36,377].
Liu et al. fabricated SHPC anti-corrosion GR film on Al alloy by spin coating EtOH-dispersed graphene sheets [382]. Zhong et al. reported a multifunctional paper where dopamine was concurrently used as a reductant for GO and a crosslinking adhesive for the adjacent GR [398]. Highly complex wrinkled/crumpled textures were created through extreme compression of GR coating [383]. Choi et al. reported a thermally controlled transfer printing technique for multiple patterned GR layers [384]. A shrinking method was used to generate strain-sensitive hierarchical RGO buckling patterns [408]. Multifunctional, rose-petal-like SHPC GR film was fabricated via self-assembly of GO. Here, vacuum filtration was used to make a hollow GO film on a PTFE membrane and subsequently reduced by HI vapour at 100°C [399]. Spark plasma sintering was shown to be a superior GO reduction method to achieve SHPC RGO in a single step. The SHPY was attributed to the sintering-assisted removal/ reduction of oxygenated functional groups in GO where the surface roughness was ∼10 times larger, with a higher C:O atomic ratio of ∼83 [401,402]. Lin et al. deposited vertical aligned GR nanosheet-embedded carbon film using an ultrasonic extrusion method to fabricate SHPC, photo-sterilize, and reusable mask [403]. Most of these studies, however, resulted in sticky SHPY.
Recent studies extensively employed laser processing [385][386][387][388][389][390][391][392][393][394][395][396][397]404,405]. Rough micropatterns and nanoscale layers shaped from laser reduction could readily lead to SHPY [385]. Shi et al. fabricated microflower structures comprised of GR nanoflakes via a single femtosecond laser pulse. The GR film was composed of LbL GR nanosheets with ∼10-50 nm gaps; GR monolayers with ∼0.37 nm interlayer spacing constituted each GR nanosheet [386]. Two-beam laser treatment of GO film was used to create microscale grating-like assemblies with fine nanoscale roughness. The hierarchical micro/nanostructuring and the elimination of oxygen-containing groups endowed the resultant GR film with unique SHPY [404,405]. Das et al. demonstrated a direct-pulsed laser approach to adjust the electrical conductivity and hydrophobicity of the inkjet-printed GR. The study showed that the laser writing converted hydrophilic (CA ∼ 47.6°) and resistive (sheet resistance ∼21 MΩ/sq) GR surface to SHPC (CA ∼157.1°) and conductive (∼1.1 kΩ/sq). Molecular dynamic simulations revealed that the laser-induced nanoscale GR flake alignment and the hydrophobic surface chemistry contributed to SHPY [389]. Hall et al. presented a method to make open microfluidic podiums by controlling the hydrophobicity of spin-coated GR on a flexible plastic substrate via laser-assisted patterning [392]. Jiang et al. first fabricated micro-grooved structures on a GO film via photolithography, and then two-beam laser treatment was performed on the corrugated surface to remove the hydrophilic groups and upsurge the nanoscale surface roughness [390]. Luong et al. investigated an infiltration process that enabled the devolution of laser-induced GR onto different commercial materials and produced SHPC composites with Portland cement, PDMS, EP, and wax for multipurpose applications [396].
A few works reported SHPC hybrid film of GO with metals, such as Ni/GR film on MS by ED followed by MA modification [456], Ni/RGO/MA coating on CS by ED [457], RGO/Ni coating on SS by ED [458], GR/amorphous carbon/Ni-based film by high-voltage ED [459], GR/ amorphous carbon/Co by ED [460], Co/Ni/GR composite coating on Cu by ED [461], and GO/Ag NW/fluoride polyvinyl butyral spray coating [462]. Most of these works employed ED as a means to incorporate metallic NPs in the composite.
Several works used GR with steel [598][599][600][601][602][603][604] and Cu/ wire [605][606][607]   Corrosion continues to be a major industrial threat necessitating costly preventive measures [622][623][624][625][626][627][628][629][630][631][632][633][634]. In general, reported works with SHPC surfaces showed superior corrosion resistance than the corresponding bare surfaces attributed to the robust interface air layer that could effectively withstand the ingression of water/aggressive ions. The long-term durability of such surfaces in aggressive chloride and acidic solutions remains an area to be further advanced. As far as CNSs are considered, the superior mechanical/chemical durability is advantageous; however, graphite's more noble position in the galvanic series could intensify the corrosion once local damages initiated. On the other hand, CNMs are economical and effective fillers to polymeric resins to enhance mechanical/chemical durability, hydrophobicity, barrier protection, and substrate adhesion.
Only a few works explored CNT-only SHPC anti-corrosion coatings. A recent work showed that a sprayed SAPC coating (on glass) of fluorinated MWCNTs and a spray adhesive greatly promoted the chemical robustness [156]. and significantly reduced corrosion current density (i corr ). The pitting corrosion resistance was much improved [118]. Most of the reports in this area explored CNT-incorporated polymer coatings.
Several studies investigated CNT/EP coatings [153][154][155]157]. Zhang et al. explored an one-step spray-coated SHPC nanocomposite coating of EP resin and MWCNTs on CS and showed that suitable optimization of the filler could improve the corrosion resistance considerably. The highest CA (∼154°) and corrosion resistance was noted for a 30 wt% CNT-incorporated coating. The property enhancement was correlated with the surface roughness ( Figure 13). Their EIS results with different CNT contents showed that the lowfrequency modulus in the Bode plot first decreased with the increase of CNT content (due to the conductivity enhancement); however, after reaching SHPY at 30 wt%, the impedance raised to noble values. A similar trend was observed in Nyquist plots where the capacitive arcs' diameter reduced first and then augmented ( Figure 13). The trend confirmed that the trapped air layer considerably resisted the interface electron transfer. The surface displayed extraordinary mechanical durability and retained SHPY even after 100 cycles of tape-peeling [155]. Potentiodynamic polarization (PDP) studies of spray-coated SAPC EP/PVDF/FEP/SiO 2 /CNT-coated Al in 3.5 wt% NaCl revealed highly competent barrier protection to aggressive ions. The brittleness and meagre abrasion resistance of EP resin were considerably enhanced [153]. The authors also reported spray-coated SAPC EP/PANI/FEP/SiO 2 /CNT coating for Al. Electrochemical studies demonstrated significantly enhanced anti-corrosion performance ascribed to the better barrier effect and entrenched PANI's redox catalytic ability. The CA of the coating retained high values even after 10,000 times of abrasion and ∼3 months of immersion studies in pH 14 or pH 1 solutions [154]. Li et al. also showed muchenhanced corrosion resistance for an SHPC composite EP coating for Q235 steel. The E corr of the bare, EP-coated, and SHPC samples were −0.462, −0.714, and −0.876 V (vs SCE), respectively. The corresponding i corr were 6.46 × 10 −4 , 3.47 × 10 −5 , and 3.43 × 10 −6 A/cm 2 [157].
PDP studies on spray-coated and cured (300°C) SAPC PANI/CNTs/SiO 2 /ETFE/PFOS coating displayed exceptional corrosion resistance for Al. The E corr of bare Al was −0.797 V (vs SCE), whereas the SHPC sample exhibited a positive E corr of 0.129 V and a low i corr of 6.1 × 10 −11 A/cm 2 after 1 day of immersion in 3.5% NaCl. After 90 days, the E corr decreased to 0.051 V. The corresponding value for a composite coating without PANI/CNT was −0.792 V. The i corr of the SHPC sample after 90 days was 3.5 × 10 −10 A/cm 2 , which was ∼3 and 5 orders lesser than the compositecoated (without PANI/CNTs) and the bare samples, respectively. The SAPY remained unchanged during continuous immersion in 3.5 wt% NaCl and 1 mol/L HCl for 90 and 60 days, respectively [128].
PDP and EIS studies of PPS/SiR/CNT/RGO spraycoated and heat-treated (320°C) Al in 3.5 wt% NaCl disclosed that the E corr was shifted from −0.889 V of bare Al to −0.810 V for PPS/SiR/CNT-coated sample and further to −0.726 V for PPS/SiR/RGO/CNT-coated sample. The i corr of the SHPC sample was three orders of higher magnitude (1.47 × 10 −8 A/cm 2 ) than the bare (3.5 × 10 −5 A/cm 2 ). The high protection efficiency (>99%) was retained even after 28 days of continuous immersion in 3.5 wt% NaCl [142]. Composite spray coating of Al 2 O 3 /CNT/PDA/ PTFE (calcined at 500°C) on steel plate demonstrated a significant shift of i corr from 10 −2.2 to 10 −3.8 μA/cm 2 and E corr from −0.672 to −0.388 V (Figure 14a). The CA of the coating demonstrated only a slight decrease even after 15 days of immersion in pH 14 and pH 1 solutions (Figure 14b). The CNT's addition significantly enhanced the wear resistance. The coating also displayed excellent hot water repellency (99°C) and withstood water jet (20 m/s) test. A few other reports also have the potential for anti-corrosion application [132,134].
A few works explored metal NP-incorporated coatings [176,177]. Zhou et al. reported Ni NPs and MWCNTs doped DLC film by one-step ED. As the CNT concentration in the coating increased, the i corr regularly decreased and reached the best protection (i corr ∼ 3.124 × 10 −10 A/cm 2 ) at an optimum concentration of 0.07 mg/mL. Higher concentrations of CNTs, however, increased the i corr further [176]. The authors also reported a similar study with Co NPs [177].

Oil separation
Compared to traditional separation methods with low efficiency and selectivity, the unique wettability materials (filtration materials such as meshes/membranes and absorption materials such as sponges/aerogels) are deemed promising for oil/water separation. Significant research efforts are dedicated to fabricating CNM-based SHPC-SOPL or SHPL-SOPC materials in this line [198,277].
CNT-based materials are attractive due to their unique high porosity and interconnected nanopores. Several reports investigated CNT-only cases. Fard et al. employed highquality CVD-made CNT packs for oil removal. The sample could hold oil up to 17 times its weight. Higher removal efficiency of ∼97% was recorded compared to a commercial counterpart (87%) [198]. Hang et al. employed CVD CNT-coated anodized Al template. High oil flux (∼300 L/m 2 /h/bar) with >99% purity was sustained even after 20 cycles of separation [94]. A 3D porous SiC cellular skeleton with long and aligned CVD-grown MWCNTs was used [69]. A MnO 2 NW/MWCNT hybrid membrane made up by vacuum filtration displayed excellent separation performance (4,900 L/m 2 /h/bar with >99.7% efficiency) for surfactant-free and surfactant-stabilized oil/water emulsions. The high separation capacity was maintained even after ten cycles. The filtrate's oil contents were <33 ppm. A simple EtOH cleaning was enough to clean a fouled membrane [222].
SHPC polymer/CNT membrane fabricated via covalent linking of PS onto CNT network ( Figure 15) displayed a sorption capacity of ∼270 times of its weight and that was retained even after ten repetitions. Excellent separation efficiency (∼99.9%) and high flux (5,000 L/m 2 /h/bar) were recorded for surfactant-stabilized emulsions also. After the sorption process, alcohol rinsing removed the absorbed oil, and the dried membrane at 40°C displayed excellent SHPY [163].
Li et al. employed SHPC/SOPL PC/MWCNT monolith with 90.1% porosity, fabricated via non-solvent-induced phase separation (Figure 16a-c). The monolith had excellent mechanical properties and can withstand 400 times its weight without any deformation. The saturated adsorption capacity and equilibrium adsorption time recorded with soybean oil were 12.62 g/g and 20 s, respectively (Figure 16d and e). The adsorption capacity maintained at 4.57-6.24 g/g even after ten cycles [214].
SHPC and SOPL CNT-based sponges/aerogels were employed [203,215]. Several works used commercial PU sponges incorporated with CNTs. The monolith's superior mechanical elasticity facilitated easy reuse after multiple absorbing/squeezing cycles [206,208]. Ge et al. showed that CNT/SiO 2 -coated PU sponge displayed excellent mechanical stability and separation efficiency. No apparent change in saturated adsorption capacity was observed even after five cycles [204]. A PU/MWCNT monolith exhibited an efficiency of 98.4% even after ten cycles of separation. High absorption capacities at the range of 6.9-42.3 g/g were recorded for different oils and solvents [206]. A few studies employed ML sponges [208,209]. MWCNT-coated ML sponge using polyphenol-Fe 3+ adhesive exhibited absorption capacities at the range of 38-127 times of its weight and a separation efficiency of ∼97% [208]. LbL covalent grafted CNT/GO on ML foam presented absorption capacity ∼113 times its weight and a notable flux of 32.6 L/m 2 /s. Instead, PDA grafting resulted in SHPL/  underwater SOPC foam with a flux of 19.3 L/m 2 /s [506]. CL sponges with incorporated CNMs were also used [197,262]. Lu et al. employed SHPC/SOPL-reinforced-Et-cellulose sponge prepared by cross-linking Et-cellulose with epichlorohydrin and complexing with silanized CNTs (Si-CNTs) (freeze-drying), followed by SiO 2 NP coating and HDTMS modification (Figure 17). The sponge displayed high porosity (>98%), low density (<20 mg/cm 3 ), excellent chemical durability (over wider pH), good mechanical strength (withstand 28.6 kPa), and absorption capacity of ∼64 times its weight. Even after 50 separation cycles, the absorption capacity was 86.4% [197].

Anti-icing
Ice accumulation could significantly decrease the performance of ships, wind turbines, and various other systems. SHPC surfaces are known for their anti-icing properties that can prevent surface ice nucleation. The conductivity and photothermal effect of CNTs are added advantages.
Reports are available on SHPC CNT-only [99,115,199] and CNT-based composite [135,155,174,185,191,218] coatings in anti-icing applications. Rajiv et al. showed excellent anti-icing properties for MWCNT/CNF-coated FRP sheet. Ice formation was absent when supercooled water (−20°C) was poured onto the coated surface, whereas rapid ice formation observed on the uncoated plate [115]. Su et al. investigated electrothermal and photothermal performances of SHPC ODA/(carboxylated-MWCNTs/aminated-MWCNTs)n film (Figure 18a). Under 30 V, the SHPC film quickly (within 60 s) gets heated to 60°C and the ice slides of within 34 s. The ice on the unheated SHPC film was not removed even after 2 min (Figure 18f and g). The film's temperature variation was dependent on the number of bilayers of MWCNTs (Figure 18b). The film maintained SHPY and thermal effect even after several heating/cooling cycles (Figure 18c). [99]. SHPC anti-icing coating was fabricated from peeled MWCNT agglomerates from a milled xerogel [199].
Wu et al. employed flexible SHPC PDMS/MWCNT membrane fabricated by vacuum filtration. An ice layer (4 mm) dyed by methylene blue was then prepared on the membrane at −15°C, and a potential of 15 V was applied (vertically fixed, 70% RH, −5°C). Due to the SHPY and the electrothermal conversion effect, the covered ice layer was totally detached from the membrane in 120 s itself [218]. Studies with a composite coating of MWCNTs and silicone rubber showed that the sample retained SHPY under 0°C and the droplet freezes with a CA of ∼158°. On shifting the plate to room temperature, the ice melted, and water rolls off rapidly. The SHPY was retained even after 12 times freezing/melting steps. Their ice-accumulation studies showed that the gathered ice on the SHPC Figure 17: (Top) Schematic of the sponge preparation. (Bottom) Photographs/CAs corresponding to (a) cross-linked Et-cellulose sponge, (b) CNT-reinforced sponge, (c) load-bearing, and oil-absorption tests [197]. Reproduced with permission from ref. [197]; © 2017 American Chemical Society. surface (∼0.213 g) was significantly lesser when compared to the ordinary plate (∼0.478 g) [174]. Anti-icing studies on spray-coated SiC/CNT coating showed that the water droplet was spherical even after freezing. The ice adhesion strength of the uncoated EVA was 25.65 kPa, while that of the SHPC surface was only 2.65 kPa. With the SHPC surface, the droplet's freezing time increased ∼340% to that of the bare (from 15 to 66 s) [185]. Zhu et al. studied icing time of a spray-coated ZnO/MWCNT/PDMS film on various substrates at −6°C. The freezing time of the droplet on uncoated and coated SiR substrates were 150 and 300 s, respectively. The superior effect was explained because of the reduced droplet/surface contact area and the hindered heat transfer due to the air layer [191]. Remarkable anti-icing properties were also reported with EP/MWCNTs [155], flame-synthesized amorphous carbon NP-based [308], and CNT/adhesive polymer/ SiO 2 [135] coatings.

Anti-biofouling/biomedical
SHPC surfaces are highly desirable for anti-bacterial/ anti-biofouling applications for marine equipment, biomedical implants etc. [275]. Both SHPC and SHPL surfaces are shown to be effective in minimizing microbial adhesion. Yoon et al. compared the extent of bacteria adherence on SHPC and SHPL coatings (fabricated via annealing SS plates coated with CNT-PTFE and TiO 2 , respectively) under different fluid flow conditions. Fluorescence intensities of adhered E. coli on SHPC and SHPL surfaces were ∼80% and 65% lower than that of the bare sample. For the SHPL surface, the readily spread out water could form a tightly bound layer. It could act as a lubricating film or water shield, lessening the foulant/ surface electrostatic attraction. On the other hand, SHPY and self-cleaning attributes were decisive in determining the property enhancement of the SHPC surface. The results were further confirmed by aerobic plate count data ( Figure 19). The bacterial attachment was considerably affected by the wall shear rate as well ( Figure 19). The upsurge of flow rates resulted in reduced bacteria adherence [124].
Mittal et al. have shown that SHPC mesoporous carbon-nanocapsule/PVDF coating rarely contained E. coli bacterial colonies. More than 90% decline of bacterial attachment was observed [275]. Song et al. revealed excellent anti-microbial property for a fluorosilane-modified nanohybrid membrane based on chitosan matrix, cationic chitosan, MWCNTs, and silane coupling additive [144]. A nanocomposite film made up using CNT/Ag/ PTFE composite target also showed similar results where the surface SHPY and the Ag NPs's anti-microbial effect provided positive effect and suppressed the bacterial growth/proliferation [642]. Ionic liquid-functionalized MWCNT coatings were studied as antibacterial coatings by Bains et al. [173].
SHPC surfaces with immobilized CNFs are shown to be excellent hemostatic materials. The CNFs encouraged rapid fibrin growth and clotting. The SHPC surface severely limited blood wetting and radically diminished bacteria adhesion. The clot detachment was also easy [258]. Marcon et al. achieved selective cell patterning on SHPC-SHPL patterns on diamond NW surfaces [285]. A few reports addressed SHPC CNS-based coatings for microfluidics applications [643]. Several works explored CNS-based anti-biofouling desalination membranes [231,232,341,644,645].

Anti-corrosion
GR-only and GR-based composite SHPC anti-corrosion coatings are widely investigated. As discussed in Section 2.1, GR-based composite coatings could provide a more effective barrier effect and reduced coating cracking. However, the graphite's more noble position in the electrochemical series can harm the base metal at localized defects. More details of anti-corrosion GR coatings are described elsewhere [649][650][651][652][653]. As discussed earlier, SHPC surfaces always presented better corrosion resistance than the bare surface. This section briefs reported works on SHPC GR-based anti-corrosion coatings based on the coating deposition method used.

Electrodeposited coatings
ED is a simple approach in fabricating protective coatings [622,654]. GR-based SHPC and self-cleaning anti-corrosion coatings fabricated by ED displayed superior corrosion resistance that includes reduced GR coating on Cu [407], carbon-based film doped with GO and Co on Si [460], MA-modified Ni/GR hybrid film on MS [456], Ni-RGO-MA coating on CS [457], and RGO/Ni coating on SS [458]. Yan et al. showed that GO and Co were well embedded in the amorphous carbon matrix and the SHPC film displayed better protection at a GO doping concentration of 0.007 mg/mL [460]. Jena et al. achieved good adhesion and mechanical stability of the fabricated SHPC coating by a phosphate pre-treatment. The SHPC sample presented ∼2 orders reduced i corr and ∼3 orders higher EIS impedance when compared to the bare [457]. Bai et al. showed that ED RGO/Ni composite coating on SS showed more than 99.9% protection in 3.5 wt% NaCl [458]. A novel SHPC GR/amorphous carbon/Ni film fabricated by high-voltage non-aqueous ED also displayed superior corrosion resistance [459].

Solution/spin coatings
A facile design of ternary nanocomposite of PDMS/GO nanosheets with ZnO NRs was studied for steel. GO-ZnO hybrid nanofiller was synthesized using one-step chemical bath deposition, and the nanocomposite coating was fabricated by solution casting. The dried and cured coating with 1 wt% of GO-ZnO addition provided the best protection [445]. Liu et al. reported a spin-coated SHPC GR Figure 19: Fluorescence intensities and aerobic plate counts. The significant differences corresponding to fluorescence intensity and aerobic plate count measurements are indicated by lower and uppercase letters [124]. Reproduced with permission from ref. [124]; © 2013 Elsevier Ltd.
film with excellent corrosion protection for Al alloys [382]. Abbas et al. described a drop-coated fluorinated GR coating on Cu with an insignificant decrease of CA in 3.5 wt% NaCl [411]. A homogeneous SHPC coating was made on Zn using EtOH-xylene solution of HDPE containing 1-5 wt% of GO via drop coating. A 5 wt% GR-incorporated SHPC film provided adequate corrosion protection during continuous immersion in 3 wt% NaCl for up to 29 days [486]. Liquid-phase exfoliated fluoro-GR nanosheets were spatially trapped on the surface of EP resin coating. The fabrication process consisted of two steps, dip coating in EP resin/curing agent and then surface fixing the fluoro-GR nanosheets (fluoro-GR powder dispersed on the EP coating' surface and pressurized using ∼10 g/cm 2 weight). The coating displayed superior protection for Cu [655]. PFDTS-modified GO/Cu silicate coating also showed excellent anti-corrosion property [444].

Spraying/painting
POSS-modified GO's proper blending could significantly enhance the anti-corrosion capability of EP-based coatings [484]. A 0.5 wt% POSS-GO-incorporated EP coating presented ∼2 orders of greater impedance than pure EP coating after 150 days of immersion in 3.5 wt% NaCl, and that was ascribed to the combined effect of barrier protection, self-lubrication effect, and improved mechanical properties [481]. Zhang et al. studied electrostatic-sprayed EP-PTFE/GP-SiO 2 -PFOS coating on steel. The GR-PDA (GP)/SiO 2 interfacial strength was enhanced by in situ growth of SiO 2 on the GP surface by utilizing dopamine self-polymerization and sol-gel methods. The surface was further modified by PFOS. The resultant SHPC coating survived more than 105 abrasion cycles with significantly less weight loss (54.4 mg). The SHPY was maintained even after 60 days of immersion in 3.5 wt% NaCl [483]. A simple painted SHPC surface with a mixture of electrochemically exfoliated GR and PDMS showed 8,868-fold reduced i corr than the bare Al sample [420]. Nine et al. presented a method to make GR-based SHPC composite anti-corrosion coating with a formulation of RGO, PDMS-modified diatomaceous earth, and TiO 2 NPs [440]. A fluorosilane-modified-GR incorporated siloxane-acrylic coating also displayed excellent corrosion resistance [494].

Conclusions and perspectives
Here, we have provided a comprehensive review of CNSbased SHPC surfaces and coatings. SHPC composites/ hybrids of CNSs with metals, ceramics, and polymers are detailed. SOPC and SAPC surfaces are also included. Works reported in specific applications areas are presented separately. All the information available in this area is systematically classified and presented.
Among the different SHPC CNSs, the most investigated is GR, and the second most is CNTs. The present review mainly focuses on CNT-and CNF-based SHPC surfaces. Both aligned and non-aligned CNT arrays/coatings and their various composites/hybrids are presented. Works reported on sponge, foam, aerogel, fabric, mesh, and membrane-based systems are discussed separately. Due to the availability of a few reviews, the section on GR is presented concisely. The idea was to provide an overall outlook on the research trends in the entire domain of CNS-based SHPC surfaces.
The higher surface roughness with nano and micro (aggregates) scale hierarchical surface structuring is highly beneficial for SHPY. Surface reduction processes (removal of hydrophilic surface groups) and low SE component/treatment enhance it. CNM-based polymer composite coatings are well known for their multi-functionalities such as conductivity, photothermal effect, superior mechanical/thermal/chemical durability, barrier protection, and several others. A durable SHPC surface could enhance the device performance in several applications, as discussed in this review.
Earlier studies, before 2005, were focused on fabricating SHPC VACNT arrays. Although the high-temperature reduction associated with the CVD process and the enhanced surface roughness of the CNT arrays could make an SHPC surface, the durability gets compromised with time as the droplets can eventually seep into the inter-spaces. Later studies categorically proved that durable SHPC VACNTs could be realized via an additional low SE polymer modification to alleviate the water ingression. VACNTs fabricated by Si or SiC template-assisted methods also displayed excellent durability against water ingression, which were closely dependent on the template structural parameters. Later studies proved that a simple vacuum annealing treatment could be enough to regain the SHPY of ACNT arrays. Several studies, however, showed that the as-grown ACNTs had high SA/CAH values leading to water pinning. An optimized high-temperature treatment or a low SE modification could reduce the SA to the desired limit of self-cleaning surfaces. Several attempts were made via nanocomposite/hybrid formation with other CNMs, ceramic oxides, and polymers to enhance the durability further. All these studies support that such an additional modification is mandatory to achieve long-term durable SHPC VACNT arrays. The most used method for the CNT's fabrication is indeed CVD. A few works attempted EPD.
Since 2006, researchers started exploring non-aligned SHPC CNT coatings with superior durability. Other than CVD, several fabrication methods, including vacuum filtering, spray coating, drop/dip coating, laser-assisted approaches, sintering approaches, and pressing, were employed in making CNT-based SHPC surfaces. Quite a few studies demonstrated SHPY without added low SE component or high-temperature annealing. However, most works employed a low SE component in the formulation or a post-low SE treatment. Some studies used a combined vacuum filtration-LbL assembly to fabricate SHPC membranes/papers. Many works were dedicated to nanocomposite polymer coatings where CNTs were used as fillers to the polymeric matrix. The aspect ratio and the concentration of the CNTs were decisive for the property enhancement. One of the widely studied systems is CNT-PDMS. A significant number of reports are available on PU, EP, and PS-based nanocomposite coatings. Spray coating is perhaps the best method in making highly robust CNT/polymer-based coatings. Several studies showed that superior durability was achieved only after a high-temperature curing. A few studies used CNTs as templates. There exist further scopes for the fabrication of nanocomposites of CNTs with several other polymers and ceramic oxides. More studies can be focused on flexible SHPC surfaces. For practical applications of SHPC CNT-based coatings, it is desirable to further enhance the durability against severe mechanical abrasion and longer-term water immersion.
A number of reports are available on SHPC hybrids and composites of CNTs with different ceramic oxides and metals such as SiO 2 , SiC, Al 2 O 3 , Co 3 O 4 , WO 3 , ZnO, Ni, and Co. The most used is SiO 2 . A significant extent of works are available on SHPC and SOPL or SHPL and SOPC CNTbased foams, sponges, aerogels, meshes, and membranes. Several studies focused on developing CNT-incorporated commercial sponges with improved mechanical properties and durability. More works can be focused on composites of eco-friendly and abundant materials such as CL sponges. MOF-based systems can be further explored.
SHPC CNF-based systems displayed exceptional chemical and mechanical durability. The research trend is similar to CNTs; however, the quantum of reported works is significantly less. A few earlier reports, before 2010, addressed SHPC vertically aligned CNFs. Most of the works explored CNFs as fillers in organic coatings. Several studies presented CNF-added foams and aerogels. A preferred combination is CNF/PDMS/PU. The superb durability of such systems was attributed to the robust interfacial adhesion, where the PDMS could act as an effective interfacial adhesion agent and waterproof protective layer. Different hybrids of CNFs with ceramic oxides and metals were also explored. A few studies addressed SHPC surfaces based on carbon nanospheres. HT-synthesized or soot-based carbon nanospheres were mainly employed. Like CNTs, an optimized mixed structure of CNFs with carbon spheres could yield enhanced hydrophobicity. A few works are available on SHPC carbon nanothorn array, tree-like carbon nanospheres, CMFs, and onion-like carbon microspheres. Excellent durability during long-term water immersion up to 800 days was noted. All these areas, however, demand more works to arrive at precise decisions and products.
A few works are available on nanodiamond. The typically used method of fabrication is reactive ion etching followed by fluorination. Several works addressed SHPC fullerenes. The most employed fabrication strategy utilized with fullerenes is molecular self-assembly. The review also provided a concise description of SPHC surfaces of nanostructured carbon soots, graphitic carbons, CB, carbon aerogels, and their various composites.
A detailed discussion of SHPC graphene is out of the scope of this review. However, all the information available in this area have been comprehensively collected, systematically classified, and briefly presented. A significantly higher number of reports are available on SHPC GR and their various composite coatings with silanes/ fluorosilanes, PU, and EP. More works can be focused on PS, polyethylene, and polypropylene systems. Among the silanes/fluorosilanes, the trend was similar to that of CNTs; the most investigated was GR-PDMS, followed by GR-PVDF. Several works explored SHPC composites of GR with other CNSs, ceramic oxides and metals. Many reports are available on GR-based composite sponges, aerogels, and monoliths. Eco-friendly and naturally abundant materials can be explored with GR in this line. GR Qdots could be explored.
CNSs, in particular GR-based coatings, were widely investigated for anti-corrosion applications. Due to the potential galvanic effect, more studies are desirable in this direction to arrive at precise conclusions. The longterm durability of such surfaces in aggressive chloride and acidic solutions needs to be further improved. Significant data are available on CNS-based materials for oil separation applications. This is an area explored to a great extent. A few reports are dedicated to biomedical, anti-icing, sensors, and several other applications. There exists high scope for further research in these areas.
Despite the extensive information available, as discussed in this review, further in-depth studies and pilot plant experiments are required to widespread practical usage of SHPC CNMs in different applications. Areas to be further improved are the mechanical and chemical durability of such SHPC surfaces in extreme exposure conditions for longer durations. Systematic studies are demanding the development of protocols and standards for commercial products. Author contribution: The author has accepted responsibility for the entire content of this manuscript and approved its submission.

Conflict of interest:
The author states no conflict of interest.