Micro/nano manufacturing aircraft surface with anti-icing and deicing performances: An overview

3.1 Surface energy on anti-/deicing

The anti-/deicing surface performance is mainly affected by the surface morphology and surface wettability. Surface morphology includes surface roughness and surface structure shape. The delayed freezing time and ice adhesion strength is related to these characteristics. The abilities to both resist icing before icing and remove ice easily after icing are influenced by surface morphology. The surface wettability is often described by Young’s equation. The larger contact angles between the water droplet and the solid surface will cause the worse wettability and stronger hydrophobicity. The hydrophobicity of solid surface is related to its surface energy. The lower solid surface energy results in larger static contact angle and stronger hydrophobicity. It is noted that the surface morphology and the surface wettability are influenced by each other. It is necessary to apply the interaction to design the anti-/deicing surface with low surface energy. The typical theoretical models of surface energy description are shown in Table 2.

In Table 2, it can be seen that the Wenzel’s model, due to the addition of a roughness factor r, can more accurately describe the relationship between surface tension and contact angle. Especially in this review, most of the preparation methods are achieved by obtaining surface structures with special morphologies, thereby achieving anti-/deicing performance. Cassie’s model proposes a broader equation that uses f to describe the solid–liquid surface fraction, while considering the characteristics of solids and liquids, as well as the influence of roughness, to accurately describe the adhesion of droplets on the surface.

3.2 Surface topography on anti-/deicing

The effect of surface roughness on the delayed freezing time can be characterized by the surface roughness with critical nucleation radius. When the surface structure size is smaller than the ice crystal nucleation radius, the functional formula f(m, x) is larger and the heterogeneous nucleation barrier is close to the homogeneous one, which significantly increases the delayed freezing time. On the contrary, when the surface structure size is larger than the ice crystal nucleation radius, the heterogeneous nucleation barrier decreases and the delayed freezing time is decreased.

The shape of surface structure also affects the delayed freezing time [28]. The different surface structure shapes will lead to different solid–liquid contact area and heat transfer resistance. The surface structure shape with small solid–liquid contact area, large heat transfer and thermal resistance has a longer delayed freezing time. Shen et al. [53] showed that the layered micro/nano SH surface has a longer delayed freezing time than the single-layer micro/nano SH surface. Due to the double-scale effect of layered micro/nano structure, it can induce more cavitation and increase the thermal resistance of liquid-solid heat transfer, which has a longer delayed freezing time.

The ice adhesion strength on the surface is affected by the surface roughness. Under the same wettability conditions, the ice adhesion strength tends to decrease with the decrease in surface roughness value [48]. Both Wenzel state and Cassie state are common wettability states as shown in Figure 7. The surface roughness has different effects on Wenzel and Cassie wettability states. According to the contact angle equation in Table 2, the apparent contact angle in Wenzel state (θ _a) has a cosine linear correlation with surface roughness factor (r). However, the apparent contact angle in Cassie state is related to the proportion of liquid contacting solid surface [54]. According to the formula from a previous study [51], the increase in roughness will not change the surface hydrophilicity in Wenzel state, while the hydrophobic surface can be more hydrophobic, and the hydrophilic surface more hydrophilic. The apparent contact angle is significantly improved in Cassie state, which makes it easier to achieve SH state. For SH surfaces, the influence of surface structural parameters on the adhesion strength of ice layer cannot be considered alone, and the droplets should be kept in Cassie state as much as possible. Thus, the droplets being in Cassie state is the key to reduce the adhesion strength of ice layer [55].

Figure 7

(a) In Cassie state, the liquid and solid are not in complete contact that there is a gas chamber and (b) in the completely wet Wenzel state, the liquid and solid are in close contact, and the interface does not contain gas. The existence of cavitation under the liquid drop in Cassie state leads to a higher backward water contact angle (WCA). In Wenzel state, the increase in solid–liquid interaction area leads to lower WCA and stronger water droplet adhesion [48].

3.3 Surface wettability on anti-/deicing

The surface wettability is determined by the cohesion between liquid molecules as well as the adhesion generated by the molecular interaction between liquid and solid. It is usually measured by the contact angle. It is shown that the surface wettability plays an important role in the ice deposition mechanism [56]. The hydrophilic surfaces are more likely to freeze than the hydrophobic surfaces. The static contact angle of hydrophilic surface is less than 90° as shown in Figure 8(a). The static contact angle of hydrophobic surface is greater than 90°, and the water droplets tend to be bead. For SH surfaces with contact angles greater than 135°, the water droplets are more spherical as shown in Figure 8(b).

Figure 8

(a) Hydrophilic surface (contact angle θ less than 90°) and (b) SH surface (contact angle θ more than 135° and low CAH) [15].

The delayed freezing time is also influenced by the surface wettability influence. The low surface wettability can significantly improve the delayed freezing time [20]. The decrease in surface wettability causing the increase in contact angle is due to the reduction in solid–liquid contact area, the delayed heat transfer process, the delayed supercooling droplets, and the prolonged freezing time. In addition, the reduction in solid–liquid contact area reduces the proportion of droplet nucleation rate at the solid–liquid interface in the droplet assembly nucleation rate. The nucleation process is closer to the mean nucleation. The ice crystal nucleation barrier is then improved and the delayed freezing time is increased.

Forward and backward contact angles represent the maximum and minimum possible contact angles for given liquid/solid combination, respectively. They can be measured using the tilt base option or using the addition/removal volume method. In the case of inclined base method, when the solid tilts from 0° to 90°, the uphill angle (or backward angle) decreases, while the downhill angle (or forward angle) increases. The difference between advancing contact angle and receding contact angle is called CAH. Figure 9(a) shows the static contact angle, and Figure 9(b) shows the movement of droplets on an inclined surface.

Figure 9

(a) Static contact angle θ, and (b) the movement of a droplet on an inclined surface (θ _adv is the forward contact angle, θ _rec is the backward contact angle, and the lagging contact angle is the difference between the forward contact angle and the lagging contact angle) [57].

The design of ice repellent surface should not only have high static contact angle, but also maintain non wetting SH state and low hysteresis contact angle under the hydrodynamic pressure of impacting droplets [57]. The delay time of the surface with low hysteresis contact angle is longer. The lower CAH reduces the adhesion and dynamic friction between the droplet and the surface, which resulted in a higher possibility of hitting the supercooled droplet escape (i.e., rebound) from the surface.

The effects of surface wettability on the anti-icing and deicing performance of the surface before icing are relatively consistent. However, the effect of surface wettability on the adhesion strength of ice layer is controversial. Some studies [58,59,60] claimed that the ice adhesion strength decreased with the surface wettability. However, other studies reported that there was no correlation between surface wettability and ice adhesion strength [61]. Although the influence of surface wettability on ice adhesion strength has not been researched clearly, the large surface contact angle and small contact angle lag can reduce the solid–liquid surface contact area. The solid–liquid surface adhesion is then decreased, which is helpful to reduce the ice adhesion strength [62]. Thus, the surfaces with SH properties may have weak ice adhesion [63].

4.1 Electrochemical, chemical, laser, and wire-cut etching

Etching process is one micro/nano manufacturing method mainly through physical or chemical modification to improve the roughness of the substrate and to obtain low surface energy substances as to obtain super-hydrophobic surface and anti-/deicing performance.

Sun et al. [66] etched the Al substrate electrochemically to obtain the hierarchical porous structures which were composed of AlOOH and Al₂O₃. After injecting silicone oil, the obtained slippery liquid infused porous surface (SLIP) presented a water roll-off angle of 3°. The freezing rain experiment was carried out on them. The freezing adhesion of SLIP was reduced from 36.8 to 3.025 N, which was 91.8% lower than that with bare aluminum surface. The SLIP had excellent anti-/deicing performance and corrosion resistance in extreme environments. Song et al. [67] prepared a layer of micro/nano binary rough structure composed of uneven platform structure on the die steel substrate by electrochemical etching (Figure 10). After modified by fluoroalkylsilane (FAS), the WCA of the sample surface was 167.2°, while the water sliding angle (SA) was 4.3°.

Figure 10

(a) Schematic diagram of etching device (a) (I–II) image after etching of Al substrate, (a) (III–IV) image after FAS processing [67], and (b) Ni substrate [74].

Jin et al. [68] fabricated the layered structure on pure aluminum plate by the combination of electrochemical corrosion and hydrothermal treatment (as shown in Figure 11(a)). The hydrothermal growth of nano wool spheres significantly increased the specific surface area, which was conducive to the complete modification of the surface with low free energy materials. Then, the fluorination modification was carried out to obtain SH surface with WCA of 164° and SA of 1.5°. Compared with the prepared micron structure and nanostructure, the ice extension time of micro/nano hierarchical structure was 1,697 s, while the ice extension time of micron structure and nanostructure were 431.2 and 708.3 s, respectively. The ice adhesion strength of the sample was 35.7 kPa. The strength was much lower than that of 720.5  kPa on the untreated surface, which was presented with good anti-icing performance. However, it was shown that the SH anti-/deicing performance fabricated by only electrochemical etching was extremely limited, and the micro/nano structure was required to be further modified. Xu et al. [69] carried out electrochemical machining on the magnesium alloy plates to fabricate the excellent performance by controlling the processing current density and processing time. The fabricated magnesium alloy was immersed in FAS ethanol solution. The super hydrophilic magnesium alloy surface was then successfully transformed into SH one. The WCA was 165.2° and the water tilt angle was about 2° for the magnesium alloy SH surface. Though the preparation process with electrochemical etching is simple, it can only process and be applied to the conductive materials, and it is hard to process metal materials with weak activity.

Figure 11

(a) Process equipment, (a) (1) vertical view (a) (2) bottom view and (b) (I–IV) SEM images with different intensities [66].

The chemical etching process is simple and the product cost is low. The etching liquid is mostly strong corrosive liquid. It has been widely applied in the processing of SH surfaces. Zhang et al. [70] took the concentration of NaAlO₂ and reaction time as factors, prepared the superhydrophobic surface by chemically etching the aluminum substrate, covered with a large number of smaller conical particles, and the micro bone array was formed by highly crystalline Al(OH)₃ on the aluminum sample surface. The adhesion of ice was measured with a dynamometer. The adhesion of the original aluminum plate and the prepared aluminum plate were 168 and 33 kPa, respectively. The prepared SH aluminum plate had better anti-icing adhesion performance. Lu et al. [71] applied the electrochemical etching method to construct coral like micro/nanoscale rough structure on aluminum substrates. After being modified by 1H,1H,2H,2H-perfluorodecyltriethoxysilane, the waterdrop slip probability value reached 96.7% when the inclination angle was within the range of 25∼30°. The WCA was about 169°. Barthwal et al. [72] fabricated a rough structure through chemical etching and anodic oxidation, and then used polydimethylsiloxane (PDMS) and injected silicone oil for surface modification to prepare a SH surface. The ice adhesion strength of the modified slippery oil-infused PDMS coating was low, which was 35 ± 15 kPa. Li et al. [73] etched the surface of magnesium alloy with a mixed solution of NaCl and NaNO₃ to manufacture a micro rough structure. After fluorination modification, the SH magnesium alloy surface with a contact angle of about 162° and the SA of only 3° was fabricated.

Xu et al. [74] prepared nickel templates by HNO₃ chemical etching, and fabricated large-area micro/nano structured PDMS films with SH properties by thermal curing process (Figure 10(b)). First, the different surface morphologies of templates were manufactured by changing the concentration of chemical solution and etching time. Then, the PDMS films with different wettability were prepared, in which the contact angle >160° and SA <10° could be achieved. Finally, the large-area PDMS film was obtained by roll to roll (R2R) thermal curing process. After contact angle of PDMS film is 164.9°, abrasion treatments, the average and the average SA is 6.2°, indicating that it still has SH characteristics. Wenxuan et al. [75] applied the method of ultrasonic chemical etching combing with hydrothermal treatment to manufacture the SH surface. The cleaned aluminum plate in hydrogen fluoride solution was immersed first. Then, it was mixed with acid solution and hot water in turn. Finally, the FAS-17 solution for etching plate was modified. The SH surface with Hydrangea like micro/nano structure was manufactured. It was with the contact angle 161.4° and the SA was less than 1°. Compared with the bare surface, the freezing time of the prepared surface was delayed by about 4∼5 times on average.

In recent years, the laser ablation is becoming one faster and less polluting choice, especially for some alloys with strong corrosion resistance in the field of micro/nano manufactured surface with anti-icing and deicing performance. The laser ablation is characterized with high machining precision and simple machining process. It can efficiently construct complex micro/nano rough structures, but it is often expensive. Yang et al. [76] employed ultra-short pulse femtosecond laser system to prepare TC4 alloy surface with a spacing of 50 μm trench microstructure (Figure 12). The groove depth of TC4 alloy surface increased with the laser scanning speed. The contact angle first increased and then decreased with the increase in micro/nano structure size. When the scanning speed was 2,000 mm/s, the maximum contact angle was 103.26°, and the maximum droplet freezing time was 60 s. Liu et al. [77] fabricated the micro/nano surface on TC4 substrate by laser ablation technology. The TiO₂ nanostructure was formatted by hydrothermal treatment. The sample was modified with the FAS-17 solution at room temperature and fully dried to obtain the sample (Figure 10(b)). The freezing time was delayed from 34 min to more than 90 min relative to the control sample whose surface was treated only by the laser processing. The high precision of laser ablation compared with other etching methods enables it to prepare more precise and regular periodic texture patterns. The high precision of laser ablation compared with other etching methods enables it to prepare more precise and regular periodic texture patterns. Milles et al. [78] used direct laser interference patterning, direct laser writing, and combination methods to prepare five different periodic texture patterns on the aluminum cantilever. All structures show superhydrophobic performance, with static contact angle exceeding 166° and SA below 10°. Different textures have different performance for specific icing conditions, and surface patterns with larger feature sizes often have higher interfacial shear stress.

Figure 12

(a) White light interference microscope image of TC4 alloy surface at different scanning speeds: (a₁) V = 5,000 mm/s; (a₂) V = 2,000 mm/s; (a₃) V = 400 mm/s, and (b) (b₁–b₃) surface frost crystal morphology at different scanning speeds [76].

Gaddam et al. [79] reported that two-tier multiscale (MS) nanofibers had the best anti-icing performance compared with the laser induced single-tier nanofibers on the stainless steel substrate. Within 5 min after the beginning of the freezing process, the surface of LIPS was covered by frost. For the surface with double-layer MS structure, there was only 60% of the MS surface area was covered by frost after 50 min, which indicated that the anti-icing performance was significantly improved. In addition, the double-layer MS had excellent wear resistance and maintained good anti-/deicing performance even if the surface encountered severely repeated wear. Hou et al. [80] etched micro column arrays with different distances between columns on the surface of Si substrate by plasma etching, and the SH surface was obtained after modification. The icing delay time can reach 1,295 s when the distance between columns was 70 μm. When the distance between columns is 30 μm, the deicing effect on the surface was the best, and the ice adhesion strength was 16 kPa.

Moreover, the electrochemical surface processing can increase the service life of composite materials to a certain extent, and can ensure the relative integrity of surface microstructure after long-term use. Femtosecond laser-induced methods also have an impact on the service life of the surface. Compared to untreated surfaces, the wear resistance of electrochemical treated surfaces has also been improved, and their anti-icing performance can still be maintained after multiple wear cycles [79].

The etching method can fabricate the SH surface on the substrate interface with good stability and durability, but it often needs other low surface energy materials for modification. The processing takes a long time, and the mechanical properties of the etched surface are affected to some extent. There are high requirements for the substrate material.

4.2 Chemical, electrochemical, vapor, and plasma deposition

The deposition is a micro/nano manufacturing method in which the deposition agent matrix is pre- prepared in the deposition solution, or the film with special micro/nano structure is formed on the substrate through atomic and intermolecular chemical reaction through gas transfer reactants. The super hydrophobic micro/nano structure can be fabricated by deposition method to realize the anti-icing and deicing performance.

Chemical deposition and electrochemical deposition are the two most widely applied manufacturing methods. Huang et al. [81] deposited a spherical structure with a diameter of hundreds of nanometers in the mixed solution of nickel sulfate and potassium persulfate on the stainless steel substrate. After being modified by perfluorooctanoic acid, the surface of SH stainless steel sheet with a contact angle of about 158° was obtained. Jia et al. [82] soaked the magnesium alloy sheet in silver nitrate solution and prepared a SH membrane with network and uniform microsphere structure composed of irregularly arranged nanostructures on the surface (Figure 13(a)). The surface of the treated magnesium alloy has high hydrophobicity characteristics with the WCA of 153° and water SA of 4°. Zhu et al. [83] soaked the Cu sheet in silver nitrate solution to deposit coral like rough structure on the surface of the copper sheet. After being modified with perfluorooctanoic acid, the SH copper surface with a contact angle of about 163° was fabricated. The chemical deposition method can replace the products with less active metals (e.g., expensive nickel and silver).

Figure 13

(a) (I-IV) SEM images of chemically deposited surfaces at different magnifications [82], (b) (I–IV) SEM images of surfaces after 0-120s of Cr deposition [84], (c) (I–II) SEM images of untreated surfaces, (c) (III–IV) SEM images of electrochemical deposition [85], (d) (I–II) TEM images of plasma deposition of Cr, and (d) (III–IV) TEM images of plasma deposition of chromium carbide [89].

The electrochemical deposition method has lower manufacturing cost, more stable structure and improved wear resistance. Xiang et al. [84] deposited Ni on the low carbon steel substrate and then deposited Cr coating on the Ni (Figure 13(b)). The maximum WCA of the coating modified by myristic acid was 167.9 ± 2.4°, and it still maintained good hydrophobicity after 100 abrasion cycles. Tan et al. [85] deposited micro/nano rough structure dominated by micro/nano laminated crystals on the surface of iron sheet by electrochemical deposition method (Figure 13(c)). After modification with stearic acid, the SH surface with contact angle of about 154° and SA of about 2° was fabricated. The SH surface had good anti-corrosion performance. Liu et al. [86] used the ethanol mixed solution of cerium nitrate hexahydrate and myristic acid as the sedimentation solution to deposit the coral like rough structure on the magnesium alloy substrate. Due to the low surface energy characteristics of myristic acid, the surface of SH magnesium alloy sheet with a contact angle of about 160° was obtained. Su and Yao [87] deposited Ni nanoparticles (NPs) on the Cu surface by electrochemical deposition method. After being modified by 1H,1H,2H,2H-perfluorodecyltriethoxysilane, the surface of pineal layered micro/nano structure and low surface energy fluorinated components was manufactured. The SH Cu surface with contact angle of 162° and SA of about 3° was obtained.

The general deposition method usually consumes a long time. The efficiency can be greatly improved by using the vapor deposition method, plasma deposition method, and the coating thickness and uniformity can be guaranteed easily. Deng et al. [88] deposited a silica shell on the soot layer using chemical vapor deposition (CVD) and coated the hydrophilic silica shell with hemifluorosilane. The super hydrophobic coating, which was easy to manufacture, is transparent, and can rebound oil, was prepared. The WCA of the coating was 165 ± 1°, and the SA was less than 1°. Liu et al. [89] prepared parallel microstructure on stainless steel substrate sample by laser ablation, and then the sample was inserted into the vacuum chamber for sequential plasma deposition of Cr/chromium carbide/diamond-like carbon (DLC)/F-doped diamond-like carbon (F-DLC) to prepare SH coating (Figure 13(d)). The content of F element in the coating was 34.6%, which can significantly reduce the surface energy of F-DLC coating and improve its hydrophobicity. The supercooled water droplets are easy to roll off the coating surface with a WCA of 152° and a water SA of about 8°, which presented that the coating had good anti-icing ability. Niu et al. [90] took the advantage of the layer-by-layer (LbL) assembly technique that was compatible with different substrates, the positively charged poly(diallyldimethylammonium)/MXene) _n multilayers with varied numbers of deposited bilayers (n) can be fabricated on different substrates. Its photothermal effect can be effectively employed in anti-/deicing field.

The deposition method has the characteristics of wide processing ranges and simple preparation process. It can process the complex surfaces. However, the SH surface prepared by the electrochemical deposition method is apt to wear and has poor mechanical strength, which makes it hard to meet the performance requirements under harsh conditions. The SH surface performance fabricated by deposition method needs to be strengthened through repeated depositions and additional manufacturing processes.

4.3 Dip coating

The whole coated objects are immersed in the tank containing the coating. These objects are then taken out of the tank after a short time, and the excess coating liquid flows back into the tank. This manufacturing technique is called dip coating method. The treated substrate is immersed in low surface energy material to obtain low surface energy coating. The dip coating thickness fabricated by this micro/nano manufacturing method can be controlled in the range of 20∼30 μm.

As shown in Figure 14, Lo et al. [91] chemically etched the Al substrate in HCl solution to obtain graded micro/nanostructures, and then the sample was immersed in toluene solution with different FD-TMS/PDMS-TES weight ratio to obtain the coating. The WCA of the coating was about 165°. The surface presented relatively low ice adhesion strength of 25.3 kPa when the PDMS was 2.9 wt%, and the strength was only 47.2 kPa in 100 freezing/melting cycles, which showed excellent durability and anti-/deicing performance. Peng et al. [92] dispersed NH₄HCO₃ powder in the mixed solution of poly(vinylidene fluoride) (PVDF) with N,N-dimethylformamide and coated it on wind turbine blades. The WCA and SA of SH PVDF coating were 156 ± 1.9° and 2°, respectively (Figure 14(b)). Tan et al. [93] fabricated PVDF/SiO₂ coating by dip coating method as shown in Figure 14(c). The WCA was 159°, and the SA was less than 3°, which presented superhydrophobicity and remarkable anti-/deicing performance. The coating can reduce the adhesion of ice layer on the substrate by 40%. Yang et al. [94] manufactured SiO₂/ poly (vinylalcoho1) (PVA)/polyacrylic acid (PAA)/fluoropolymer hybrid SH coating with biomimetic peg function on wood samples by dip coating method (Figure 15(d)). The fabricated bionic wood was characterized not only with superhydrophobicity (the WCA was 159°) but also with excellent stability and water resistance.

Figure 14

Manufacturing process by dip coating method [91].

Figure 15

(a) SEM images of the Al surface prepared by (I–II) HCl etching and (III–IV) that treated in boiling water after etching [91], (b) (I) pure PVDF coating, (II) SH PVDF coating surface [92], (c) (Ⅰ and Ⅱ) PVDF coating, (Ⅲ and Ⅳ) PVDF/SiO₂ coating [93], and (d) (I–IV) HRSEM morphology of SH-treated glass surface at different magnification [94].

Cohen et al. [95] synthesized SiO₂ NPs with SH coating with better durability by dip coating method. The contact angle of fabricated SH surface was more than 155° and the SA was less than 5°. The ice wind tunnel experiment showed that the SH coating could reduce the ice adhesion and achieve the effect of anti-/deicing. Gwak et al. [96] coated Al substrate with antifreeze proteins (AFPs) from the Chaetoceros neogracile (Cn-AFP). The Al was fused with Cn-AFP to make Cn-AFP wrap Al. Compared with bare Al substrate and traditional hydrophilic Al coating, the AFPs significantly hindered the freezing of aluminum surface. This effect was due to the high thermal hysteresis value of Cn-AFP and its ability to reduce the freezing point. Zhang et al. [97] fixed the reinforced coating through the adhesive expansion and bonding process to prepare the SH TiO₂ coating, respectively. The reinforced coating formed by this coating in continuous roughness improved the adhesion between the coating and the substrate, and could repeatedly produce super wettability after bearing mechanical stress.

As shown in Table 3, the dip coating method has the advantage characteristics of saving labor and materials, high production efficiency, simple equipment, and operation. It can adopt mechanization or automation for continuous production. The dip coating technique is most suitable for mass production with single variety. However, the fabricated surface should not have concave pattern of paint accumulation which could produce the defects including thin as well as uneven coating, and other disadvantages.

4.4 Mixing spray and combine spraying

Spraying is one of micro/nano manufacturing methods. Spraying technique is depositing micron or nano particles with low surface energy on the substrate surface to form a rough structure. The sprayed surface has superhydrophobic properties. The spraying method has the advantages of simple operation, wide application range, low equipment cost, and large-scale production (Figure 16). It has been widely applied in the preparation of SH anti-icing/deicing coatings.

Figure 16

(a) Schematic of cluster ZnO coating preparation process [109], and (b) preparation of EP coating, SH ZIF-8/POTS coating, and ZIF-8/POTS/EP coating [113].

The solution in the spray prepared coating can be prepared by simple mixing, and the preparation operation of coating is relatively simple. Zeng et al. [98] mixed perfluoropolyether (PFPE) and ZnO and sprayed them on the glass (Figure 17(a)). The addition of PFPE reduced the coated surface energy from 18.12 mJ/m² at 0% to 13.22 mJ/m² at 15%. There was a linear relationship between the surface free energy and ice binding force. The coating with the highest contact angle of 158° and SA of 2° could be obtained, the freezing delay time could reach 107.1 s, and the freezing adhesion was reduced to 0.59 N which presented good anti-/deicing performance. Pan et al. [99] mixed dichloropentafluoropropane solvent (Asahiklin 225), 1H, 1H, 2H, 2H-hperfluorohexyltrichlorosilane (PFTS), and n-butyl cyanoacrylate (n-BCA) onto PDMS substrate to fabricate the coating with high contact angle of more than 150° and SA of 0°.

Figure 17

(a) (I–VI) Micro morphology of coatings with different ZnO content (0, 2, 4, 6, 8, 10%) [98], (b) SEM images of FP/FSEP coating: (I) FP-100%, (II) FP-200%, and (III) FP-300% [108], (c) SEM images of (I) bare magnesium alloy, (II) etched magnesium alloy (III and IV) clustered ZnO/EP surface [112], and (d) SEM images of (Ⅰ) Q235 steel, (Ⅱ) EP coating, (Ⅲ and Ⅳ) ZIF-8/POTS coating, and (Ⅴ and Ⅵ) ZIF-8/POTS/EP coating [116].

Jiang et al. [100] sprayed PVDF/CNTs mixed solution on the surface of Se cured strontium, and heated the coating to increase the bonding strength among surface particles. After treatment with FAS-17, the WCA of the coating was 163°, the icing delay time was increased to 584 s, and the ice adhesion was as low as 13.2 kPa. Due to the continuous network formed by PVDF melt during heating, it could maintain excellent anti-icing performance even after severe mechanical wear and chemical corrosion. Yin et al. [101] mixed SiO₂ NPs, ethanol, and polytetrafluoroethylene (PTFE), and loaded the obtained suspension onto the copper mesh/Cu composite samples by spraying. The prepared coating samples had a WCA of more than 150°.

Liu et al. [102] used azodiisobutyronitrile to initiate the polymerization of acrylate monomer and perfluorooctyl methacrylate to prepare fluoro-modified polyacrylate (FPA). The FPA polymer was fully mixed with ECNTs in mortar and the mixture was dispersed in solvent. After spraying, the micro/nano structure composed of FPA and ECNTs was formatted, which gave PESC stable superhydrophobicity, and the WCA was increased to 150°. Jiang et al. [103] applied the combination of inorganic particles and carbon nanotubes (CNTs) to spray onto the substrate to prepare an anti-icing coating with micro/nano composite materials. The WCA of the coating was 161°, and the ice adhesion was as low as 2.65 kPa. Xie et al. [104] coated the attapulgite nanorods with polypyrrole through the oxidative polymerization of pyrrole, and then coated the attapulgite nanorods with cetylpolysiloxane through hydrolysis condensation. A photothermal superhydrophobic coating was manufactured by spraying the mixture of modified attapulgite suspension and silicone resin on aluminum plate. The WCA of the coating was 162.7°, and the SA was 2.7°.

The SH coatings with different properties can be fabricated through controlling the spraying pressure and spraying sequence. Xue et al. [105] prepared the superhydrophobic photothermal coating with anti-icing performance by sequentially spraying cuttlefish juice melanin and SiO₂ NPs. Due to the black pigment in cuttlefish juice, the coating showed high photothermal conversion performance. The freezing time of water droplets on the surface was delayed to 144 s, which was five times that of ordinary glass. The adhesion strength of ice was reduced to 25.65 kPa.

Li et al. [106] sprayed the SH suspension prepared from fluorinated epoxy resin and PTFE particles on the glass substrate with a spraying pressure of 0.25∼2 bar to control the surface morphology of tunable adhesion superhydrophobic coating (TASC). The anti-icing properties of TASCs with different surface morphologies and roughness were investigated by freezing time and ice adhesion strength. Compared with TASC-0.25, the TASC-2.0 showed excellent anti-icing performance with freezing time of 392 s and ice adhesion strength of 51 kPa. Xie et al. [107] proposed waterborne polyurethane acute solution and hexadecyl polysiloxane modified diatomaceous earth acute suspension were successively sprayed on glass, Al alloy plate, Mg alloy plate, and other substrates, respectively. The coating had SH properties with contact angle of 168.1° and SA of about 4.9°. Compared with the bare substrate, it significantly increased the icing time to 361 s, and the icing strength at −15℃ was decreased to 73.9 kPa. Zeng et al. [108] combined hydrophobically modified epoxy resin matrix with micro/nano fluoropolymer particles (Figure 16(b)). The superhydrophobic coating was prepared by multi-layer spraying with EP as adhesive. The maximum contact angle and minimum SA of the coating were 160° and 2°, respectively, and its SH properties were maintained even after 240 wear cycles and 360 s sandblasting. The coating had good corrosion resistance and self-cleaning performance, too.

Zheng et al. [109] prepared a magnetically responsive flexible SH photothermal film (PFe-PCS) composed of PDMS, iron powder, and candle smoke by spraying technique. The coating had good superhydrophobicity with a WCA of up to 154.5° and a SA as low as 3.13°. The PFe-PCS membrane could be folded repeatedly from 0° to 180°. No delamination or cracking was observed after 500 folding cycles, which showed that the sample had good flexibility and fatigue resistance. Yin et al. [110] fabricated the micro/nano polyhedral oligomeric silsesquioxane/PDMS (POSS/PDMS) composite coating with a similar spraying technique. When the POSS content exceeded 5%, a spherical/cylindrical micro/nano structure was formatted, which made the POSS increase concentration and spray cycle times by fourfold, and could significantly reduce the distance between the particles. This spatial geometry embedded in the interphase hollow structure combined an effective heat transfer barrier, which made the surface wettability and ice adhesion greatly reduce. It caused the water freezing temperature to reduce to −26.3°C.

In the preparation of SH coatings by spraying, other manufacturing methods such as chemical etching and grafting can be combined to obtain more excellent hydrophobic properties. Shen et al. [111] chemically etched the aluminum plate in HCl solution. They sprayed the fluorinated SiO₂ NPs on the PDMS coating substrate after curing to obtain an SH surface with a contact angle of 155.3°. The results indicated that the delayed icing performance was extended from 4.8 to 276.2 s of bare aluminum. Zhou et al. [112] etched Mg alloy in HCl, sprayed ZnO seeds and placed them in ZnO growth solution to prepare cluster ZnO coating composed of interdigital ZnO rods (Figure 16(c)). After modification with stearic acid, the coating showed SH properties, and the contact angle was 163°. The coating showed excellent wear resistance and corrosion resistance. Wu et al. [113] sprayed SiO₂ NPs onto glass fiber reinforced epoxy (GFRE) substrate to obtain superhydrophobic coating. The ice adhesion value of SiO₂ NPs coating was about 64.7 ± 5.4 kPa. After ultraviolet weathering and sand erosion, the ice adhesion strength was still less than 100 kPa.

Brown et al. [114] prepared a SH double-layer coating composed of thick TiO₂ coating deposited by plasma spraying and thin coating deposited by CVD. The coating presented a contact angle of 159° and a CAH of 3.8°. Zhu et al. [115] sprayed PDMS/TiO₂ NPs onto the surface of pre-deposited PDMS micron particles to fabricate a micro/nano hierarchical structure, and prepared a SH anti-icing coating with reversible wettability and durability. The WCA of SH anti-icing coated surface was up to 168.6°, and the adhesion was as low as 4 µN. The freezing time of droplets was extended to 895 s.

Chen et al. [116] sprayed EP coating on the surface of substrate, and then sprayed it after curing. The ZiF-8 NPs modified by 1H,1H,2H-2H perfluorooctyltriethoxysilane (POTS) were used to prepare SH coating with contact angle of about 168.2° and SA of about 2° (Figure 16(d)). Xie [117] prepared a SH coating bydirectly spraying APTES treated silica/ethanol suspension onto a steel substrate. Xie [117] prepared a SH coating by directly spraying APTES treated silica/ethanol suspension onto a steel substrate. Its WCA exceeded 150°. Ma et al. [118] dispersed PFEPx system and FCNTs in acetone, and the mass score of PFEP and FCNTs was named as PFEPx FCNTsy (y referred to as the content of FCNTs in PFEP). The surface roughness could be adjusted by grafting CNTs with perfluorinated segments and flexible spacer groups (FCNTs). The mixed solution was sprayed on the substrate to prepare SH coating. Among them, the PFEP30/FCNTs40 had the best water repellent performance with a contact angle of about 152° and SA of about 5°. Wang et al. [119] designed a perfluorododecylated graphene nanoribbon film, which was applied by spraying method. Due to both the low polarizability of perfluorinated carbons and the intrinsic conductive nature of graphene nanoribbons, the film had SH properties and deicing performance by applied voltage heating. Guo et al. [120] prepared a photothermal effect anti-/deicing coating by combining photothermal carbon nanofibers with PDMS and polyvinylpyrrolidone. Compared with the films, there was 34-fold increase in freezing delay time, and the ice adhesion strength was reduced by 18 kPa.

The spraying technique has been widely applied. However, it usually has poor adhesion between coating and substrate and low durability of SH surface (Table 4). The manufacturing chains are complex, and the change in process parameters has great impact on the performance of finished products. It is often required to carry out various treatments on the surface to be sprayed to ensure the adhesion of spraying medium.

4.5 Chemical grafting and coupling grafting

Grafting is a micro/nano manufacturing method that reacts with the surface components of polymer through chemical reagents to produce active centers, so as to trigger the polymerization of monomers. The grafting usually introduces the energy substance on the bottom surface into the substrate surface or forms a special micro/nano structure to obtain SH and anti-/deicing performance (Figure 18).

Figure 18

(a) Grafted raw glass [122], and (b) epoxy matrix [124].

Fenero et al. [121] prepared layered micro/nanostructures by chemical etching to induce SH. To reduce its surface free energy, the PFPE was grafted onto corrugated Al substrate and the system was flooded. The surface presented SH characteristics, and the WCA was 160°. In addition, the floodlit surface significantly delayed the freezing time of water droplets to 5,100 s, which was about 20 times that of original aluminum (260 s). Yu et al. [122] grafted polyaniline nanofibers onto glass plates by oxidative polymerization, and used perfluoroalkyl mercaptan modification and PFPE lubricant penetration to fabricate the slippery lubricant-infused porous surfaces (as shown in Figure 18(a)). After treatment, the surface WCA was about 153° and the water SA was about 5° (Figure 19(a)). Li et al. [123] grafted SI-eATRP of methacryl-POSS (MA-POSS) on cotton and reacted with Michael addition of 1H,1H,2H,2H-perfluorododecyl-1-thiol (PFDT) through amine catalyzed thiol- MA-POSS to prepare durable MA-POSS-PFDT coating. After fluorination, the WCA was increased to about 160° and the water SA was about 10°.

Figure 19

(a) SEM images of glass surface (I–IV) after different time grafting treatments [122], and (b) (I–II) SEM images of CNTs-SiO₂ grafted surface [124].

Zhang et al. [124] prepared CNTs-SiO₂ hybrid by grafting silica NPs onto CNTs to manufacture epoxy matrix based SH coating (Figure 19(b)). The surface morphology, roughness, and wettability of CNTs-SiO₂/epoxy coating could be adjusted by controlling the ratio of CNTs to SiO₂. The CNTs-SiO₂/epoxy coating had SH characteristics and significant photothermal conversion ability of CNTs (Figure 19(b)). Under laser irradiation, it could significantly delay the freezing time of water and effectively melt the ice in a few seconds. Gao et al. [125] prepared micro/nanostructured metal organic framework (MOF) coating (LIMNSMC) by fixing lubricating fluid with porous micro/nano structure coated with MOF. The prepared coating exhibits high ice resistance, with a freezing temperature of approximately −39°C for condensed water on the coating and an ice adhesion strength of approximately 10 kPa. The high ice resistance of LIMNSMC could be maintained in ten freezing/melting cycles and six anti-icing/deicing cycles, indicating that the coating has good durability.

Zhan et al. [126] linked fluoropolymers to silica NPs through free radical polymerization. It had controllable molecular design, excellent thermal stability, and high SH. The WCA up to 170.3° and the CAH less than 3° could effectively promote the removal of droplets. Zhang et al. [127] synthesized nanoscale thickness MOF coating (NTMC) on polypropylene (PP) substrate, and grafted silane coupling agent KH560 onto NTMC to form KH560-NTMC. The hydrogen terminated silicone fluid was grafted onto KH560-NTMC to prepare self-lubricating MOF coating (SLNTMC). The WCA of KH560-NTMC coating was 84.6°, and that of SLNTMC was 133.2°. Because the silicone layer SLNTMC covered the ice nucleation sites of ice and reduced the binding force of water/ice, the SLNTMC could reduce the binding force of ice to 9.4 kPa and maintain low icing adhesion after 20 anti-/deicing cycles, which indicated that the SLNTMC had good durability.

It is shown that the grafted product is stable, has good surface properties, wide processing range, and good wear resistance. However, the grafting process is not suitable in mass production due to its long reaction time, low efficiency, and high production cost.

4.6 Organic material template and metal template

Template method is to take the material with micro/nano rough structure or low surface energy as the template, and to deposit the relevant materials into the holes or surfaces of the template through physical or chemical processing (Figure 20). The template is then removed to obtain the nano materials with the standard morphology and size of the template. This micro/nano manufacturing method can be applied to fabricate material surface with SH properties.

Figure 20

(a) Fabrication procedures of a model surface. (b) Ice formation and the adhesion characteristics: Lubricant (SF, Ⅰ), fluorinated porous film (FF, Ⅱ)，porous film (PF, Ⅲ), and the average ice adhesion strength on different films (Ⅳ) [128]. (c) SEM images of (Ⅰ and Ⅱ) SH plane. (Ⅲ and Ⅳ) SH column arrays with d = 0.3 mm, s = 0.2 mm, and h = 1 mm [129]. (d) SEM images of duplicate surfaces with different column spacings: (I) 80 μm, (II) 100 μm, and (III) 120 μm [130].

Yin et al. [128] assembled polystyrene (PS) microspheres into a compact opal crystal template, and then poured the mixed solution of Fe₃O₄ NPs silicone predictor onto the template. After solidifying and removing the template, the coating was fluorinated to form an inverse opal porous PDMS film with evenly distributed Fe₃O₄ NPs. The ice adhesion strength of fluorinated porous film infiltrated with perfluoro polyether (Krytox from Dupont) lubricant (LF) was about 25 kPa.

Song et al. [129] fabricated a super hydrophobic column array with a diameter of 1.05 mm, a height of 0.8 mm, and a spacing of 0.25 mm. They observed typical pancake bounce and reduced the contact time with the surface by 57.8%. The pillar arrays with millimeter diameter and height-to-diameter ratio of <1 can be easily manufactured on a large area. Shao et al. [130] prepared shape memory polyurethane surface with shape memory effect through aluminum template. Due to the special hierarchical structure, the surface showed typical lotus effect and presented SH properties (contact angle 154 ± 2°, SA 3 ± 1°). After the vertical microstructure was crushed by external pressure, the contact angle of the material surface was 151°, and the SA was greater than 180°. It showed high viscosity. After heating at 50℃ for 5 min, the original microstructure was restored, and the transformation from “lotus leaf effect” to “rose petal effect” was realized.

Yuan et al. [131] used PDMS as an intermediate mold as a soft copy to transfer T-shaped overhanging microstructures on silicon and dual resist masters to curable materials to form a SH surface with a WCA greater than 150°. Su et al. [132] coated porous Al template with ethyl acetate solution containing 0.5 wt% PU precursor to prepare SH membrane. The contact angle of water droplets on the SH membrane was 166.3 ± 2.41°.

As shown in Table 5, the template method has the advantages of simple processing, low requirements for process parameters, repeatability, and reproducibility. However, the coating/surface prepared by template method has poor durability and the manufacturing cost is high. In the separation process, it may damage the surface micro/nano structure and reduce its performance.

4.7 Silicon compound sol–gel coating

In the sol-gel method, the compounds containing high chemically active components are used as precursors. These raw materials are uniformly mixed in the liquid phase, and the hydrolysis and condensation chemical reactions are carried out to form a stable transparent sol system in the solution (Figure 21). The sol is slowly polymerized between aged colloidal particles to form a three-dimensional network structure of gel. The gel network is filled with solvent that has lost fluidity to form a gel. After drying, sintering, and curing, the gel can be used to prepare molecular and nano substructure materials.

Figure 21

(a) (I–IV) FE-SEM images of SiO₂ nanoparticles with different concentrations enhancing the surface of coatings [133], (b) water and dodecane droplets drop on the uncoated (Ⅰ and Ⅲ) and coated (Ⅱ and Ⅳ) GFRE substrates, respectively [133], (c) SEM images of SiO₂ NPs (I–II) and Si(OH)₄ (III–IV) sol coatings with different loading amounts [134], and (d) images of water (I) and oleic acid (II) droplets on acoating made of 1.5:1.31 SiO₂ and Si(OH)₄ [134].

Wu et al. [133] prepared TEOS and GLYMO sols by sol–gel method. The SiO₂ NPs with particle size of 10–20 nm treated by perfluorooctyltriethoxysilane were added as reinforcements and sprayed onto GFRE substrate. The synthesized surface presented a contact angle of up to 166°. Ke et al. [134] mixed tetraethyl orthosilicate and hydrochloric acid through sol–gel technology to prepare Si(OH)₄ sol, and mixed different amounts of Si(OH)₄ sol with SiO₂ suspension. The above mixture with 1 mL was dropped on a clean glass substrate and coated with a 10 µm Mayer rod to obtain a coating with micro/nano layered roughness. When introduced 1.5 g SiO₂ NPs, the WCA of the coating was 154°, which presented superhydrophobicity.

Cai et al. [135] prepared the superhydrophobic coating of silica NPs modified by methyltrimethoxysilane, ethyltriethoxysilane, and 1H,1H,2H,2H-perfluorooctyltriethoxysilane (HFOTES) via the sol–gel process. The WCA could reach 150.2°, and the minimum water hysteresis was 3.3°. Eshaghi et al. [136] prepared composite films on glass substrates by sol–gel method using silica-silica nanotube nanocomposite, and soaked the samples in PFTS solution to modify the nanocomposite films. The silica nanotube/PFTS film increased the WCA of glass substrate from 16° to 152°. The icing test results showed that the superhydrophobic coating caused the icing time of glass surface to increase from 102 to 874 s.

The raw materials of the sol–gel method are first dispersed into the solvent to form a low viscosity solution. Therefore, the molecular level uniformity can be obtained in a very short time. Due to the solution reaction step, it can uniformly and quantitatively add some trace elements to achieve uniform doping at the molecular level, and the reaction conditions are simple. However, the sol–gel processing of raw materials is complex, takes a long time, and is expensive.

4.8 Other surface modification methods

The wide application of micro/nano manufacturing in machining [137], microelectronics [138,139], and other fields shows that the diversified process can be employed to the field of anti-icing and deicing technology. Wu et al. [140] prepared a multifunctional SH coating by compounding iron oxide NPs (Fe₃O₄ NPs) with fluorinated epoxy resin through reverse osmosis process. The coating had excellent water repellency (WCA up to 161.0°, SA as low as 1.4°), and could maintain superhydrophobicity in 260 sandpaper abrasion. Because the special micro/nano structure greatly hindered the heat transfer, the freezing of water droplets was delayed by 35 min compared with that of the uncoated substrate under the same conditions. Due to the photothermal effect of Fe₃O₄ NPs, the surface temperature of the coating could quickly rise to more than 0℃ under infrared radiation, which was conducive to the melting of ice on the cold surface.

Liao et al. [141] fabricated ZnO/SiO₂/PTFE sandwich nanostructures on the glass surface by RF magnetron sputtering. The fabricated nanostructure surface had a contact angle of 167.2° and a SA of less than 1°, which was due to the low surface energy of PTFE and micro/nano cavitation induced by rough nanostructures. After spraying in glazed ice for 120 min, only 17.9% of the prepared coating surface was frozen. In addition, due to the addition of SiO₂ intermediate layer, the corrosion resistance and insulation properties of the prepared SHP were significantly improved. Zhu et al. [142] prepared the anti-icing composite coating with phase change performance by adding phase-change microcapsules (PCMs) to room temperature vulcanized (RTV) or fluorosilicone (FS) copolymer. Because PCMs released a certain amount of latent heat during cooling, the composite coating could delay the freezing of water droplets on the coating surface. The ice shear strength on the RTV/PCM coating surface (102∼132 kPa) was lower than that on the FS/PCM coating surface (378∼489 kPa). The shape memory alloy (SMA) materials showed the ability to change shape and generate force through martensitic phase transformation [143]. After transferring appropriate energy to the material, the SMA can remove ice through unique surface bending, shearing, and other methods.