Recent advances in preparation of metallic superhydrophobic surface by chemical etching and its applications

Shitong Zhu,Wenyi Deng,Yaxin Su

College of Environmental Science and Engineering, Donghua University, Shanghai 201620, China

Keywords:Metal Superhydrophobic surface Chemical etching Low adhesion Self-cleaning

ABSTRACT In the past few decades,inspired by the superhydrophobic surfaces(SHPS)of animals and plants such as lotus leaves,rose petals,legs of water striders,and wings of butterflies,preparing metal materials with metallic SHPS(MSHPS)have attracted great research interest,due to the great prospect in practical applications.To obtain SHPS on conventional metal materials,it is necessary to construct rough surface,followed by modification with low surface energy substances.In this paper,the action mechanism and the current research status of MSHPS were reviewed through the following aspects.Firstly,the model of wetting theory was presented,and then the progress in MSHPS preparation through chemical etching method was discussed.Secondly,the applications of MSHPS in self-cleaning,anti-icing,corrosion resistance,drag reduction,oil-water separation,and other aspects were introduced.Finally,the challenges encountered in the present application of MSHPS were summarized,and the future research interests were discussed.

1.Introduction

Wettability is one of the most important properties of solid surfaces.It has many important applications in industry,agricultural production,and people’s daily life.To study the wettability of a solid surface,two different aspects must be considered,namely the static behavior and dynamic behavior of water droplets on the solid surface.The static behavior is characterized by the static contact angle (CA) θ of a water droplet resting on a solid surface,and the dynamic behavior is characterized by various dynamic parameters such as sliding angle θSAand CA hysteresis θCAH[1,2].The sliding angle θSAis the minimum inclination angle that enables the droplet to roll down on an inclined plate,whereas the CA hysteresis is due to the fact that the actual solid surface is rough and heterogeneous.On rough heterogeneous solid surfaces,the CA of a water droplet at the front of the surface(advancing CA θadv)is usually larger than the CA of the liquid at the rear of the surface (receding CA θrec),The difference between the two is called CA hysteresis (θCAH) or sliding angle [3].Therefore,solid surfaces can be classified according to static and dynamic characterization parameters.If the CA θ satisfies 0° ＜θ ≤90°,then the solid surface is called a hydrophilic surface;conversely,it is called a hydrophobic surface.Generally,a surface with CA less than 10° is called a superhydrophilic surface.A surface with CA greater than 150° and θCAHless than 10° is called a superhydrophobic surface(SHPS).If the θCAHof a surface is lower than 10°,the surface adhesion will be low and the surface pollutants could be easily taken away by water droplets rolling off from the surface,which is known as self-cleaning [4].

CA is an interfacial angle at solid,liquid,and gas three-phase interface,and the magnitude of CA usually depends on the level of surface energy and geometry[4-7].The concept of CA and wettability was first proposed by Thomas Young[8]in 1804,and Wenzel [9] proposed the effect of roughness on superhydrophobicity(SHP) based on Thomas Young’s work in 1936.Subsequently,Cassie and Baxter[10]in 1944 proposed the effect of gas-liquid-solid three-phase composite interface on SHP.Since 1990s,various SHP phenomena in nature have drawn increasing research attention.The ‘‘lotus effect”,in which water droplets roll down and carry away contaminants from the surface of a lotus leaf,was first discovered by Barthlott and Neimhuis [11] in 1997,who found that the micro-convex structure on lotus leaves and the waxy component composed of long-chain hydrocarbons are critical for the formation of SHPS.Based on Barthlott’s work,Fenget al.[1]discovered that there are also nanoscale convex latex structures on the lotus leaf surface,which are important components for constructing SHPS.Subsequently,Gaoet al.[12] experimentally confirmed the importance of this dual-scale micro-/nanomorphology for the formation of SHP similar to the ‘‘lotus effect”.In contrast to the ‘‘lotus effect” of low adhesion of lotus leaves,there is the ‘‘petal effect” of high adhesion of rose petals.Fenget al.[13] found that there are hierarchical micron and nanoscale structures on the petal surface of red roses,which provide sufficient roughness for SHP but at the same time have strong adhesion to water.The water droplets on the petal surface were spherical and could not roll off even if the petal was turned upside down,which is called the ‘‘petal effect”.By studying water striders,Gaoet al.[14] found that the legs of water striders are SHP and waterproof due to the existence of a large number of fine bristles with fine nano grooves covered by cuticle wax,which enables them to stand steadily and walk quickly on the water surface.In addition,there are also wide reports about the SHP phenomena in organisms such as butterfly wings [15],rice leaves [1],desert beetles [16],geckos[17],and sharks[18].The discoveries of these SHPS in biology are important for the subsequent development of SHP materials,which promoted the development of SHPS from natural creation to artificial SHPS.Inspired by various SHPS in nature,many researchers have prepared biomimetic SHPS on various metal surfaces using various methods [19-24],and these studies have broadened the range of applications of various metals,which are beneficial for the development of production in industry,agriculture,and everyday life.

The early report on preparing MSHPS using chemical etching can be traced to Ref.[25],followed by numerous studies in metals such as copper (Cu),aluminum (Al),magnesium (Mn),iron,and alloys [26-31].Liet al.[32] prepared SHPS on Al foil by etching in HCl solution,followed by hydrothermal treatment and surface modification with stearic acid (SA) and ethanol solution.The multi-step process is time-consuming and expensive for largescale preparation.Thus,Chenet al.[33] proposed a simple onestep method to prepare the SHPS of Al alloy (AlA) by simply dipping the alloy into the mixed solution of hydrochloric acid (HCl)and SA.The AlA with rough surface and low surface energy could be simultaneously constructed.Yanget al.[34] constructed SHPS with excellent anti-icing properties on AlA by HCl etching and surface modification with 1H,1H,2H,2H-perfluorodecyltriethoxysi lane (FAS-17).Wanget al.[35] prepared SHP steel surfaces with excellent mechanical stability,UV durability,and anti-icing properties by chemical etching in the mixed solution of hydrogen peroxide (H2O2),HCl,and nitric acid (HNO3),followed by treatment with FAS-17.Chenet al.[36]developed a one-step method to prepare SHPS on stainless steel (STS) by simply immersing the STS sheets in a mixture of H2O2,hydrofluoric acid (HF),and perfluorooctyl trichlorosilane (PFOS),a process that simultaneously performs chemical etching and surface modification.The prepared SHPS exhibit excellent wear resistance,corrosion resistance,and durability.The preparation of MSHPS has evolved from a complicated multi-step method to a simple one-step method,and from the synthesis of single-functional SHPS to multifunctional SHPS.

2.Theoretical Models of Wetting

2.1.Young model

The Young model assumes that the solid surface is ideal,i.e.,the surface is smooth,flat,and chemically homogeneous.Thomas Young [8] derived a mathematical equation for the relationship between CA and surface tension through thermodynamic equilibrium,as shown in Fig.1(a),considering the following relationship for the gas-solid-liquid three-phase interface when the liquid is on an ideal smooth solid surface:

Fig.1.Typical contact states between a liquid droplet and solid surface:(a)Young contact state,(b)Wenzel contact state,and(c)Cassie contact state(reproduced from Ref.[24] with permission of Elsevier).

where θYis the intrinsic CA(or equilibrium CA because it is derived from the thermodynamic equilibrium);γSG,γSLand γLGare the surface tension of solid-gas,solid-liquid,and liquid-gas interface,respectively,indicating that the intrinsic CA of a solid surface is determined by the surface tension on the different interface.

2.2.Wenzel model

Since the solid surfaces are usually rough,Wenzel [9] introduced a roughness factor (r) to modify the Young equation,and replaced the CA (θY) of a smooth surface by the apparent CA (θw)of a rough surface.The droplet-interface CA equation is derived by comparing adjacent state droplets in metastable equilibrium as:

whereris the roughness factor,which is the ratio of the actual surface area to the projected area.The actual surface area will be larger than the projected surface area (r＞1) when the surface is rough.When the contact surface is ideally smooth,Eq.(2) is simplified to Eq.(1).According to Eq.(2),the roughness factor shows the effect of amplifying wettability.At γSG＞γSL,θw＜90°and surface is hydrophilic,and the introduction ofrmakes the surface more hydrophilic;At γLG＜γSL,θw＞90° and surface is hydrophobic,and the introduction ofrmakes the surface more hydrophobic.As shown the Wenzel model in Fig.1(b),the droplet can easily penetrate the rough cavity structure,and the surface roughness increases the solid-liquid contact area,and shows a pinning effect on the liquid droplet,which makes it unable to slide on the surface.

2.3.Cassie-Baxter model

For surface with high roughness or high porosity (r?1),the absolute value on the right side of the Wenzel equation may be higher than 1.In this case,the Wenzel model is invalid.Thus,Cassie and Baxter [10] extended Wenzel equation to heterogeneous interface,and derived Eq.(3) through energy analysis for CA calculation.

where θcis the apparent CA under the Cassie-Baxter state;f1andf2represent the fractions of area occupied by component 1 and 2,respectively;θ1and θ2correspond to the intrinsic CA of component 1 and 2,respectively.Given component 1 and 2 are solid and air,respectively,then liquid drop is in a composite contact with solid and air as shown in Fig.1(c).The air fraction isf2=1 -f1,and θ2=180° for air,thus Eq.(3) can be simplified as:

Eq.(3) indicates that the equilibrium CA on the heterogeneous surface is the weighted average CA of their respective phases[37].As shown in Fig.1(c),the droplet under this model is suspended on the rough structure,and only the tip part at the top of the rough cavity is in contact with the droplet.The synergistic effect of low surface energy and the rough structure results in the formation of an air layer in the cavity on the solid surface.According to this model,hydrophobicity is enhanced by the presence of air layer in the rough cavity under the droplet,which reduces the solid-liquid contact area,and water droplets can roll off easily due to low adhesion.However,SHPS with high CA obtained using this heterogeneous wetting model is metastable.From a thermodynamic energy point of view,the lower CA will be more stable,because the Gibbs free energy is a monotonically increasing function of CA.Thus,the high CA must be metastable [38].Patankaret al.[39] found that the transition from the high-energy Cassie-Baxter state to the low-energy Wenzel state needs to overcome a high energy barrier,and work must be applied to achieve the transition.Bormashenkoet al.[40] demonstrated the Cassie-Baxter model with textured metal-hydrophobic interfaces at the microscale,where they found that the hydrophobicity of metals could be explained by air trapping in the patterned pores.These studies promote the understanding of the wettability of heterogeneous surfaces and provide theoretical guidance for the preparation of SHPS.

3.Preparation Method

With the successful preparation of artificial SHPS in the field of bionics,various methods were developed to artificially fabricate metal SHPS,like electrochemical method [41-43],chemical deposition [44-46],vapor deposition [47],hydrothermal method [48-50],laser etching[51,52],chemical etching[53,54].All these methods have advantages and disadvantages in terms of preparation and the scope of application.The electrochemical method usually has certain requirements on the shape of the electrodes and can only prepare limited rough geometries.It usually needs to combine with other methods to construct SHP surfaces with micro-/nanostructures.e.g.,micron structures are usually first constructed electrochemically on the substrate and then combined with hydrothermal methods to generate nanostructures [55,56].The adhesion of deposition particles to the substrate is a key problem to be solved for chemical deposition and vapor phase deposition,and the types of substrates are usually limited to those with good adhesion to the deposition particles and low impact on the strength of the substrate.The hydrothermal method is a method of using crystal growth to control the surface microstructure.However,this crystal growth is difficult to control and often requires water bath environment with elevated temperature.Expensive equipment and high cost limit the large-scale application of laser etching.The chemical etching method is also difficult to control the surface microstructure,but it shows obvious advantages like easy operation and low cost.It can be applied to various substrates and is one of the most suitable methods for large-scale promotion.In the following sections,we mainly focused on the progress of metallic SHPS (MSHPS) preparation by chemical etching.

Generally,a large number of dislocation defects exist in ordinary crystalline metals.These dislocation sites are easily destroyed due to their relatively high energy.Therefore,dislocation sites will be firstly dissolved when the metal surface is attacked by chemical etchants.This feature is critical to construct rough micro/nanostructure on the surface of various metal substrates for chemical etching method.Qianet al.[25]found that the propriate use of dislocation etchant is critical for chemical etching,which selectively and preferentially dissolves high-energy dislocation sites in the crystal lattice,resulting in the formation of rough structures.They successfully fabricated SHPS on common Al,Cu,and zinc(Zn)substrates by chemical etching using various dislocation etchants and fluoroalkyl-silanes modification.Chu et al.[26] also used a similar dislocation etching technique to construct moss-like and lotuslike rough structures on Cu and Al foils and then prepared SHPS by hydrophobic modification with fluoroalkyl-silanes.Experiments on the condensation properties of droplets using the two prepared SHPS revealed that the growth rate of condensation nucleation on the SHPS became slower and could be used for many applications in the field of anti-icing.Kumaret al.[57]synthesized a SHP coating on the Al surface by etching a multilevel hierarchical structure on the AlA surface with a mixture of HCl and HNO3as etching agent,followed by treatment with hexadecyltrimethoxysilane (HDTMS),and the prepared SHP coating showed good heat resistance.After annealing at 200°C for 1 h,the SHP of the coating remained stable,suggesting a wide potential for industrial application.

4.Mechanism of Chemical Etching

4.1.Construction of the rough structure

The first step in fabricating MSHPS by chemical etching is to construct rough surface structures.Polycrystalline metal surfaces have many grain boundaries or dislocations which have higher energy compared to other parts of the metal and a weak binding force with the metal matrix.The etching rate at the grain boundary or dislocations will be much faster than metal grains when they are in contact with chemical etchants,due to defects in the grain boundary structure or changes in the alloy composition.The difference in the etching rate between the grain and its boundaries will remove the weaker bonded parts of the metal and protrude the grain,leading to the creation of rough surface structures[58].Most metals are multiphase and the phases of metals have an important effect on chemical etching.e.g.,the second phase of Mn alloy (βphase) with more positive potentials acts as the cathodic region in solution,while the pure Mn phase(α-phase)with more negative potentials acts as the anodic region,resulting in microelectrochemical corrosion [59],which affects the chemical etching rate.As mentioned above,the wettability of solid surfaces is determined by the surface chemical composition and geometric roughness.It was found that the water CA of smooth surface can only reach about 120°,even if the surface energy of the material is minimized[60],indicating that the construction of the rough surface structure is critical to achieve surface SHP (CA ≥150°).As shown in Fig.2,Bryuzginet al.[61] prepared SHPS of AlA by constructing rough micro-/nanostructures on the AlA surface using HCl etching,followed by modification with fluoroalkyl methacrylate copolymers for lowering surface energy.The unetched AlA modified with the copolymer could only achieve a CA of 107°,whereas the etched and modified AlA could achieve SHP state with a CA of 169°.Forooshaniet al.[62] constructed rough structure on the AlA surface by chemical etching combined with mechanical shot-blasting.They found that the CA of AlA sheet without mechanical and chemical treatment was close to 40°,and that the roughness created by shot-blasting alone was not sufficient to produce hydrophobicity.After the synergistic effect of mechanical shot-blasting and chemical etching,the hydrophilic surface was converted to hydrophobic surface.Varshneyet al.[63]also constructed a SHPS of AlA through chemical etching with HCl etching and surface modification with lauric acid (LA).The SEM images in Fig.3 showed the original and the modified AlA surface after chemical etching.Compared with the untreated AlA in Fig.3(a),the formation of high-density rectangular pits in the rough structure after 15 min of HCl etching was clearly shown in Fig.3(b).These studies show that the increase in roughness enhances hydrophobicity,which is in accordance with Cassie-Baxter’s theory.The hydrophobicity is enhanced for two reasons.Firstly,the increase in surface roughness results in the formation of more cavities in which air can be trapped,which greatly reduces the contact area between the metal surface and liquid droplets.Secondly,roughness increases solid surface area,as well as the grafting sites for low surface energy species [64,65].The above studies all show that the formation of a suitable rough structure is very important to realize surface SHP.

Fig.2.SEM images of the AlA surface: (a,b) original (×6000,×50000);after etching in HCl with a concentration of 3 mol·L-1 (c,d) (×5000,×60000) and 5 mol·L-1 (e,f)(×6000,×50000) (reproduced from Ref.[61] with permission of Elsevier).

Fig.3.SEM images of AlA surfaces: (a) original;chemically etched AlA surface by HCl solution for 15 min (b) and 5 min (c);(d) chemical etching for 15 min followed by surface modification with LA.Insert shows a water drop image on the corresponding AlA surface (reproduced from Ref.[63] with permission of Elsevier).

4.2.Modification with agents of low surface energy

Another key point to achieve SHPS of metals is modification with low surface energy materials.Many studies have shown that metal surface is usually hydrophilic in nature,and the hydrophilicity increases with roughness by chemical etching[66-69],which is consistent with Wenzel theory that the introduction of roughness can amplify the wettability and make the hydrophilic surface more hydrophilic.It is difficult to achieve SHP on metal surfaces without modification with low surface energy substances.The commonly used modifiers for metal substrates mainly include organosilane and long-chain fatty acids,and this paper mainly focuses on these two types of modifiers.

The chemical formula for the most of common organosilane modifiers is RSiX3,where R is a long-chain alkyl group consisting of—CF3,—CF2—,—CH3,—CH2—,etc.,and X is a leaving group,such as Cl,OCH3,OCH2CH3,etc.[70],since the surface energy of the functional groups is in descending order: —CH2— ＞—CH3＞—CF2—＞—CF2H ＞—CF3,the tension of a metal surface terminated with —CF3groups usually shows the lowest surface energy,much lower than that terminated with—CF2—groups[71-73].The more—CF3groups contained in the side chain of the organic fluoropolymer,the lower surface energy of the polymer[74].Perfluorosilanes like 1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS)[25,53,75-86],PFOS [83-85,87],FAS-17 [26,34,35,54,88-93],1H,1H,2H,2H-perfluorodecyltrimethoxysilane [21,27,69],and 1H,1H,2H,2H-perfluorodecyltrichlorosilane [94-96] can minimize the surface energy of metals,so they are one of the most widely used modifiers for preparing SHPS of metals.However,due to the high cost,toxic,and hazardous nature of perfluorosilanes,more attention has been paid to non-fluorosilanes,such as octyltriethoxysilane [97],cetyltrimethoxysilane [57,98-101],and octadecyltrichlorosilane (OTS) [102,103].It was found that the modification of the metal surface can only occur in the oxidized region,due to the presence of hydroxyl (—OH) groups which is a prerequisite for the formation of covalent linkages with hydrolyzed silanes [35].After chemical etching,most metals will form oxide film which could form surface-bound —OH groups after exposure to humid air or activation treatment.Metal oxide surfaces are usually terminated by oxide ions,which are considered to be exposed to solutions as lattice oxide ions rather than lattice metal ions due to their large size and low polarizing power [104,105].The metal oxide surface tends to strongly adsorb water molecules in aqueous solutions [106].As shown in reaction (R.1),the adsorbed water exists in the form of—OH groups,two types of surface—OH groups formed by donating protons from the adsorbed water to the oxide ions,while the water molecules become —OH groups of another type by losing protons.The two types of —OH groups (acid type and base type) formed are the conjugated acid (a) of the lattice oxygen ion and the conjugated base(b)of the original water molecule,with the acid type—OH group(a)becoming a cation exchange site and the base type—OH group(b)becoming an anion exchange site [105,107].In agreement with the results of Boehmet al.[104,105].The—OH groups on the metal oxide surface are amphiphilic,with half of the—OH groups being acidic and the other half being mainly basic.

These—OH sites can serve as anchor points for the formation of covalent chains between —OH groups and the active silanols hydrolyzed from silane[70,108,109],and thereby the hydrophobic alkane chains are attached to the metal surface,enhancing the surface hydrophobicity.Yuet al.[69] prepared a SHPS by modifying the surface of pipeline steel with (heptadecafluoro-1,1,2,2-tetrade cyl) trimethoxysilane (HFTTMS) after sandblasting and HCl etching.Firstly,HFTTMS underwent a hydrolysis reaction (R.2) in isopropyl alcohol solution,and the ethoxy groups in HFTTMS were replaced by—OH groups to become active silanols.After wet etching treatment,the surface of the pipeline steel sample was covered with—OH groups from surface-adsorbed water and exhibited good hydrophilicity.As shown in Fig.4,when the steel sample was in contact with the hydrolysis product of HFTTMS,the vertical dehydration condensation reaction would occur between the Si—OH groups in HFTTMS and the —OH groups on the steel surface,and the horizontal condensation reaction would occur between the active silanols,forming a low surface energy molecular layer on the steel surface.The rough structure formed by chemical etching and the low surface energy molecular layer formed by surface modification contributes to the transition of the substrate from hydrophilic to SHP.As shown in Fig.5,Liet al.[19]also used a similar method to prepare SHPS on heat-resistant steel.

Fig.4.Schematic diagram of the formation process of fluorinated silane monolayer on a solid surface (reproduced from Ref.[69] with permission of Elsevier).

Fig.5.Schematical preparation procedure of the PFDS (FAS-17) coating (reproduced from Ref.[19] with permission of Elsevier).

Long-chain fatty acids such as LA,myristic acid,and SA are also commonly used metal surface modifiers,and the hydrophobic mechanism is similar to that of fluorosilanes.Huanget al.[110]prepared a SHP AlA surface by etching with NaOH solution,followed by modification with an ethanol solution of SA.The hydrophobic mechanism is shown in Fig.6.After NaOH etching,a layer of Al hydroxide (Al(OH)3) was formed on the surface of the AlA.Al(OH)3reacted with SA to form Al stearate (AlSA),and the hydrophobic long-chain alkane was grafted on the surface of the AlA,which effectively reduced the surface energy to achieve SHP.Chenet al.[33] proposed a simplein situone-step method to prepare a SHP AlA surface by simply immersing the Al plate in a mixture of HCl and SA,and reactions (R.3) and (R.4) occurred on the metal surface.

Fig.7 shows the schematic illustration of preparing SHPS on the AlA.Firstly,HCl provided acidic conditions to remove the oxide layer and attack the relatively high-energy location on the AlA surface.Next,in the presence of oxygen and acid,the metal surface was completely oxidized,and the released Al ions could be successively captured by SA anions to form AlSA,thereby the long carbon chain hydrophobic tails could be attached to the metallic Al oxide surface to obtain a SHPS.

Fig.7.Schematic illustration of preparing SHPS on AlA (reproduced from Ref.[33]with permission of Elsevier).

Table 1 shows the summary of recent research on the preparation of SHPS through different wet chemical methods.

Table 1 A summary of SHPS preparation through wet chemical methods

5.Application of MSHPS Prepared from Wet Chemical Method

Metals such as Mg,Al,Cu,and Fe and their alloys have a wide range of industrial applications due to their excellent electrical and thermal conductivity and castability,and the introduction of SHP is important to further expand their applications.MSHPS with high water repellency and low adhesion have promising applications in self-cleaning [88,120,121],anti-icing [122-124],anticorrosion[81,91,93,125],drag reduction[126-128],and oil-water separation [129-131].

5.1.Self-cleaning

The self-cleaning phenomenon was first discovered on lotus leaf which allows water droplets to remain spherical shape on its surface,due to its strong water repellency and low adhesion.The air pockets between the rough structures on the SHPS of lotus leaf can significantly reduce the adhesion between water droplets and the surface,and the dust particles on the surface can be easily carried away when the water droplets roll down from the surface,achieving the so-called self-cleaning process.In recent years,studies on the self-cleaning ability of SHPS have been extended to metal materials,and some reports on the self-cleaning behavior of MSHPS can be found.Lattheet al.[132] prepared SHP steel by chemical etching with H2SO4for 8 h,followed by surface modification with methyltrichlorosilane.As shown in Fig.8,rolling water droplets could easily collect dust particles from the surface,suggesting its strong self-cleaning ability.Kimet al.[133] prepared SHPS on STS by HF etching and PFOS modification.The selfcleaning tests were carried out by rolling water droplets on the inclined surface covered with solid powders (in Fig.9).Compared with the poor self-cleaning performance of bare STS,most of powders could be removed by the rolling water droplets on the surface of SHP STS,indicating its excellent self-cleaning properties.Cremaldiet al.[134] prepared both SHP and superliquiphobic (SLP)surfaces on STS 304 through HCl etching followed by passivation with HNO3and then surface modification through a methylchlorosilane dip coating.The self-cleaning properties of hydrophilic,SHP and SLP surfaces were investigated using 10-15 μm SiC as contaminants as shown in Fig.10.Both the SHP and SLP surfaces demonstrate strong self-cleaning properties,and water droplets could roll easily and carry away almost all contaminant particles.The results of these studies revealed a promising prospect on the application of MSHPS in self-cleaning.

Fig.8.Optical photograph of self-cleaning behavior of the SHP steel (reproduced from Ref.[132] with permission of Elsevier).

Fig.9.Self-cleaning test: (a) substrates (b) before the self-cleaning test (c) after the self-cleaning test (reproduced from Ref.[133] with permission of Elsevier).

Fig.10.Images showing the self-cleaning behavior of surfaces varying in wettability (reproduced from Ref.[134] with permission of Elsevier).

5.2.Anti-icing field

Metals are usually exposed to outdoor environments in wide applications like high-speed rail,aircraft,ships,power and telecommunication equipment,etc.Surface freezing in cold weather could lead to potential hazards.Currently,physical or chemical treatment are the main methods for surface anti-icing,which are inefficient,energy consumptive,and of high operation cost [135-140].In recent years,turning metal surface into MSHPS is found to be one of the most promising approaches to solve the above problems,and MSHPS offer significant advantages in delaying and reducing the accumulation and adhesion of snow,ice and frost due to the excellent water repellency.

Tonget al.[121]prepared SHPS of AlA through chemical etching in the mixed solution of HCl and H2O2and subsequent modification with SA.In the anti-icing test,the freezing time of droplets increased from 855 s on the bare Al plate to 2365 s on the SHPS,indicating the excellent icing-delay performance of the SHPS.The rough structure and the air layer in the hierarchical structure reduced the contact area between the surface and the droplet,which is an important reason for the anti-icing of the MSHPS.Similarly,Guoet al.[141]fabricated SHPS on Cu substrates by etching with FeCl3solution and surface modification with octadecanethiol(ODT),and the prepared MSHPS showed good SHP,mechanical durability,and chemical stability.The anti-icing properties were investigated by over-icing delay tests,as shown in Fig.11,where both the SHP Cu mesh and the neat Cu mesh were placed in a refrigerator (-18 °C) and three drops of water (10 μl per drop)were placed on each surface.The water droplet icing time on the SHP mesh averaged 1354 s,152% longer than that on the neat mesh.The icing on the metal surface is a heterogeneous nucleation process,and a larger contact area will increase the possibility of nucleation and increase the icing rate.Due to the decrease of the contact area between water droplets and the metal surface,the icing process on the MSHPS can be considered as homogeneous nucleation,which requires longer time and lower temperature to freeze [142].Zhanget al.[143] fabricated MSHPS on Cu foils by chemical etching with HCl and H2O2,and applied the fabricated surface to a novel anti-icing system.In the anti-icing test,the temperature was lowered to -10 °C,and a small number of unfrozen water droplets were still observed on the SHPS after 2 h,indicating an outstanding anti-icing performance.During the mechanical thermal deicing process,the ice melted to form water droplets.Due to the low adhesion of the MSHPS,the water droplets quickly rolled off the surface,which greatly inhibited secondary icing,shortened deicing time,and reduced energy consumption.The above studies indicate that metal SHP materials have broad application prospect in the field of anti-icing.

Fig.11.(a) A photograph of ice-over delay test.(b) A bar chart of the ice-over time of droplets on neat and superhydrophobic mesh (reproduced from Ref.[141] with permission of Elsevier).

5.3.Anti-corrosion field

Metal corrosion has become one of the most serious problems in socio-economic development,causing tremendous lost in the economy every year,thus it is of great significance to develop methods for the prevention of metal corrosion.One of the most commonly used prevention methods is surface treatment with chromium-containing compounds.However,this method poses serious adverse effects on human health and environment.In recent years,SHP coatings have been widely used on various metal surfaces,such as Al,Cu,Fe,Ti,Zn,and various alloys,to improve their corrosion resistance.Direct manufacturing SHP coating on the metal surface should be a very promising solution to improve its corrosion resistance.

The corrosion resistance mechanism of MSHPS can be attributed to the existence of an air layer between the substrate and the solution,which provides an effective barrier to prevent the migration of corrosive ions.Liet al.[95] successfully prepared SHP AlA substrates with excellent corrosion resistance by combining HCl treatment,potassium permanganate passivation,and fluoroalkylsilane modification.The combination of multi-level concaveconvex micro/nanostructure and flocculent structure constitutes a SHPS morphology,and the refinement on the AlA surface resulted in significant corrosion resistance.Compared with the method of chemical etching followed by fluorosilane modification,introducing a process of potassium permanganate treatment for 180 min between the etching and modification increased the corrosion resistance efficiency by 68%.Zhanget al.[144] obtained SHPS of steel by immersing the steel in a mixture of HF and H2O2solution at room temperature,followed by modification in an ethanol solution of SA,and the corrosion resistance of different samples were analyzed by potentiodynamic polarization method.The corrosion potential(Ecorr)of the SHP steel increased to-472 mV,and the corrosion current density(icorr)decreased to 1.56×10-6A·cm-2compared with the bare steel (Ecorr=-859 mV,icorr=4.99 × 10-5A·cm-2),indicating its excellent anti-corrosion properties.Because the lowericorrvalue and the higherEcorrvalue indicate the lower corrosion rate and the lower corrosion trend,respectively[145,146].Guoet al.[76] fabricated SHPS on AA7055 AlA by electrolytic polishing and etching with MgCl2solution,followed by modification with an ethanol solution of POTS.The electrochemical test results show that theicorrof the SHPS was reduced by more than 2 orders of magnitude compared with the polished AA7055 surface,and the corrosion inhibition efficiency of the SHPS was 99.67%.Parket al.[147] constructed micro-nanostructures on STS 304 by chemical etching with FeCl3solution,oxidation with H2O2solution,and surface coating with a self-assembled monolayer hydrophobic coating prepared with heptadecafluoro-1,1,2,2-tetra hydrodecyl trichlorosilane.Theicorrdecreased to 1.08 × 10-7A·cm-2compared with the raw sample of 3.35×10-7A·cm-2,indicating a lower corrosion rate.

5.4.Drag reduction

Motion resistance is a common phenomenon in various facilities like aircraft,ships,pipelines,submarines,and microfluidic control equipment.It is of great importance for reducing energy consumption and operating cost to develop materials with low resistance.Inspired by various natural SHP materials,the development of artificial SHP materials with low resistance has become a research hotspot in the field of drag reduction.

The main reason for the ability of SHPS to reduce fluid resistance is the water repellency of the surface,which produces an air film with a lubrication-like effect that greatly reduces the interaction between the substrate and water.In 1996,Watanabeet al.[148]first discovered drag reduction for laminar Newtonian fluids flowing in square and rectangular pipes with highly hydrophobic walls,which uncovered the great potential of MSHPS for its application in drag reduction and attracted much research interest.Yinet al.[149] prepared a SHP steel plate through chemical etching with a mixed hot solution of HCl and potassium chloride under oxygen-enriched conditions,followed by surface modification with fluorinated silane,and the friction resistance of the SHP plate was 61.3%lower than that of the untreated steel surface at the flow rate of 1.66 m·s-1.The large but discontinuous liquid-air contact layer on the SHPS could reduce the interaction between liquid and solid surfaces.Liet al.[114] successfully fabricated lotus-like hierarchical structures on Al foil by a simple three-step solution immersion method.The Al foil was first etched by HCl solution,then soaked in hot water to obtain a rough surface containing interconnected convex/concave microstructures and uniformly distributed nanosheets,and finally immersed in a SA solution to reduce its surface energy.The SHP Al foil exhibited a friction coefficient of 0.15 in the whole sliding cycle (1200 s),much lower than the friction coefficient (around 0.7) of the untreated Al foil.

5.5.Oil-water separation

The separation of oil-water mixtures is considered a global challenge due to a large number of offshore oil spills every year,which leads to serious environmental pollution and significant economic loss.Currently,the research on oil extraction from water mainly focuses on various materials which can be divided into oil absorbents and separation materials.The latest and most promising oil absorbent is the modified polyurethane sponge,which has the advantages of high adsorption capacity and excellent oil/water selective absorption.However,the application of oil absorbents in practice often requires repeated absorption-extrusion processes,which are time-consuming and inefficient compared with continuous separation processes.In recent years,functional surfaces with SHP and superoleophilicity (SOP) have proven to be very effective in separating oil-water mixtures[150-154].The use of SHP materials for oil-water separation is increasingly becoming a research hotspot in this field due to its advantages of environmental friendliness and free of secondary pollution.

Varshneyet al.[155] prepared SHP and SOP surfaces on STS by using dilute aqua regia etching and functionalized modification with LA.The prepared surfaces were used for oil-water separation experiments by preparing 20 ml of petroleum ether-water and benzene-water mixtures,in which the oil-water ratios were varied as 1:4,1:2,1:1,2:1,and 4:1.It was found that the separation efficiency of all oil-water mixtures exceeded 99%.Nandaet al.[101]prepared SHP and SOP coatings on the surface of STS mesh by chemical etching with FeCl3and HCl solutions,followed by treatment with HDTMS,with the water static CA of 167° ± 3° and the oil static CA of about 0°.Gravity oil-water separation experiments were performed using the prepared SHP STS mesh as in Fig.12.The average separation efficiencies of the hexane-water mixture and the kerosene-water mixture were 99.6% and 98%,respectively.Salehet al.[103]fabricated SHPS on STS by a simple two-step process.Firstly,a rough surface and oxide functional groups were generated on the STS surface by sulfuric acid treatment.Secondly,it was functionalized with OTS.The functionalized STS surface exhibits SHP and SOP behaviors and has excellent oil-water separation ability,with the separation efficiency of over 99% for separating various non-polar organic components from water.

Fig.12.The separation efficiency of oil removal with respect to the number of cycles for equal volume ratio (1:1) in the two different systems of kerosene-water and hexane-water (reproduced from Ref.[101] with permission of Elsevier).

Presently,most of the literatures focused on the separation efficiency of metallic SHP meshes for single oil-water mixtures,and more attention should be paid to their separation performance for the more complicate mixtures.The main drawbacks of the metallic SHP meshes are the poor reusability,poor endurability(easily lost SHP once contaminated with oil,leading to a sharp decrease in oil-water separation efficiency),and poor anti-wear and anti-corrosion properties,which greatly inhibit their practical applications.Therefore,further research on promoting the reusability,endurability,mechanical and chemical stability of metallic SHP meshes is of great significance.

5.6.Other applications

In addition to the aforementioned applications,MSHPS also shows great potential of applications in microfluidic,medical devices,triboelectric nanogenerator (TENG),condensation,heat transfer,etc.Mummet al.[113]prepared a droplet-based SHP chip on a polycrystalline Cu surface by combining chemical etching,thermal oxidation,and electrodeposition processes and applied it to a microfluidic control system for controlling the movement of liquids.Donget al.[156]fabricated robust micro/nanoscale hierarchical structures on titanium substrates through electrical discharge machining and electrochemical etching,followed by modification with fluorinated silanes to obtain a SHPS.They investigated the transport mechanism of water droplets on the SHPS and found that the droplets moved along the designed trajectory,which can be applied in the field of droplet microfluidics and lab-on-a-chip systems.Qiet al.[44] made porous silver micro/-nanostructures by immersing STS needles in silver nitrate solution and then prepared SHPS by modification with ODT.The silvercoated SHP STS exhibited strong antibacterial activity against the Gram-negative bacteriumEscherichia coli.Agbeet al.[157] prepared Al surfaces with SHP and antimicrobial properties using a simple two-step chemical etching method.The bacterial adhesion reduction factors of the prepared SHPS were 99.9%forStaphylococcus aureus,99%forPseudomonas aeruginosa,and 99%forEscherichia coli,indicating the excellent anti-biocontamination properties.This SHP material with antibacterial properties has a promising application in modern medical devices.Zhaoet al.[96] prepared SHPS on Al plates by HCl etching followed by modification with 1H,1H,2H,2H-perfluorodecyltrichlorosilane/toluene solution.And the SHP Al plate was fabricated into an electrode to design a TENG as shown in Fig.13.Fig.13(a) shows a schematic diagram of the fabricated TENG,which consists mainly of a polyamide (PA) membrane and two Al electrodes covered with a SHP membrane.Fig.13(c) illustrates the working mechanism of the TENG,which is based on the wind-induced vibration of the PA film to achieve periodic contact and separation between two different triboelectric materials.When wind flows into the device,the PA film can vibrate up and down,causing the SHP film on the surface of Al electrode to contact and rub against the PA film,and electrons can be transferred from the PA film into the SHP film of the Al electrode,resulting in the output of voltage/current signals.A constant output voltage of 3.3 V and a pulsed output current of about 100 mA can be provided to the wireless smart temperature sensor node,and the output voltage of TENG does not decrease when the wind speed is 12 m·s-1for 14 h of continuous operation.Guet al.[75]prepared Al SHPS by etching in a mixture of HCl and HF solution and subsequent modification with fluorosilanes.The prepared SHPS of Al was applied to the fins of heat exchanger and its frost suppression properties were investigated.Compared with the bare Al surface,the condensation of droplets on the SHPS was delayed and the number of droplets was decreased due to the large nucleation energy barrier and the coalescence-bounce behavior of droplets.The droplets on the SHPS showed high thermal conductivity and low freezing rate.The amount of frost on the SHPS and the time for complete melting of the frost on the SHPS were,respectively,73.29%and 64%of that on the bare Al surface.After defrosting,76.56% of the water remained on the bare Al surface compared to 28.70% on the SHPS.All these advantages demonstrate that MSHPS holds great promise for enhanced heat transfer applications.

Fig.13.The device structure of the fabricated TENG and its operation principle.(a)Schematic diagram of the TENG.(b)Photograph of the TENG.(c)Schematic illustration of the operating principle for the TENG (reproduced from Ref.[96] with permission of ACS publications).

6.Challenges and Future Prospects

Over the past decades,research on SHPS has greatly increased.Nowadays,the preparation of SHPS on various substrates such as metal,silicon,glass,and fiber materials has become a hot research topic,and its potential application prospects have stimulated much research interest.Many challenges have emerged with respect to the further development and industrialization of wet chemical method for MSHPS preparation,including the synthesis of lowcost,environmentally friendly,corrosion-resistant,and wearresistant SHP materials,the development of easy methods for large-scale preparation,the development of standard test methods for the resistance of MSHPS to various harsh environments,and the realization of multifunctionality.

Fine control of the hierarchical rough structure of metal surface is one of the most difficult aspects of successfully obtaining a SHPS,and it remains particularly important to explore an efficient,simple,and scalable process to obtain micro/nano hierarchical structures.Although many methods have been reported to routinely prepare MSHPS,most of them are not only complex and costly,but also use reagents that are toxic and less suitable for largescale production and applications.e.g.,fluorinated organosilanes are still one of the most commonly used modifiers for MSHPS,but they will degrade in the natural oxidative environment to form harmful substances which will pollute the environment and pose a potential threat to human health.As illustrated in Introduction,there are many SHP cases in nature environment which can realize SHP without the assistance of fluorinated substances.It is promising to find more efficient and environmental-friendly reagents and to explore increasingly simple and efficient methods to develop MSHPS.Zhaoet al.[158]constructed SHPS on Mn alloys by simply immersing them in a mixture of solutions containing FeCl3,deionized water,tetradecanoic acid,and ethanol.Wanget al.[159] prepared SHPS on Cu alloys by a simple one-step solution immersion process,where the applied solution consisted of sodium hydroxide,ammonium persulphate and FAS-17.Zhuet al.[160] prepared SHPS on Mn with excellent corrosion resistance by a simple onestep solve-thermal reaction in which the Mn alloy was placed in an autoclave containing ultrapure water,ethanol,and SA at 130 °C for 4 h.The CA of the prepared SHPS achieved 158.5°.

The wide application of MSHPS in various areas (self-cleaning,anti-icing,anti-fog,anti-fouling,anti-bacteria,drag reduction,oil-water separation,etc.) suggests the multifunction of MSHPS materials.However,most of the present reports only focused on the single function of specifically developed MSHPS material,and there are only a few reports on multifunctional MSHPS.Zhenget al.[161] prepared SHPS of AlA with self-cleaning and anticorrosion properties using HCl etching and molten myristic acid modification.Rodicet al.[162] constructed rough structures by immersing Al sheets in HCl/H2O2solution and then used 1H,1H,2H,2H-perfluorooctyltrimethoxysilane for grafting and obtained SHPS with anti-corrosion,anti-icing,and self-cleaning properties.Single-function MSHPS cannot meet the rapid development of industrial production,thus it will be helpful to further broaden the application field of MSHPS materials by coupling more functions into a single MSHPS material.

It has been found that corrosion of metals inevitably occurs once they are exposed to corrosive environments.Although the corrosion rate can currently be slowed down by preparing SHPS,its long-term stability in corrosive solutions has rarely been investigated.Therefore,the durability of MSHPS remains a great challenge for their practical applications,and it will be a key area of focus for future research efforts.Many studies have found that most MSHPS have excellent stability in alkaline and salt solutions,but easily lose hydrophobicity in strong acid solutions.Therefore,it is important to test their chemical stability over a wide range of pH values.Zhanget al.[78] prepared SHP AlA surfaces using sonoassisted HCl etching,boiling H2O treatment,and POTS grafting.Aqueous solutions in the pH range of 1-14 were prepared with HCl and NaOH,and the SHP AlA maintained good chemical stability in aqueous solutions of different pH values.However,most of published articles only presented the short-term chemical stability of SHPS.It is necessary to investigate the long-term chemical stability of SHPS in various corrosive solutions.

Wear resistance is one of the key properties of MSHPS for industrial applications.Currently,the most commonly reported method for testing the wear resistance of MSHPS is sandpaper friction which could not represent most of the actual industrial application conditions.Therefore,it is urgent to establish more suitable method for testing the abrasion resistance of SHPS.In fact,the rough structure and chemical composition of SHPS are easily damaged by mechanical wear,resulting in the loss of SHP.Luet al.[77]prepared a SHPS on mild steel by combining HCl etching and fluorosilane modification,and tested the wear resistance of the SHP coating using sandpaper abrasion,and found that most of the SHP coating on the surface was destroyed after 30 abrasion cycles.Tanet al.[163] reported a method to prepare a SHPS on brass,where brass was immersed in a mixed solution of HCl,NaCl and Na2SO4for micro-etching and subsequently modified with an ethanolic solution of SA to obtain SHPS.Abrasion tests were performed by pressing the sample on 1200 grit SiC sandpaper with a weight of 50 g and pushing the sample 40 cm along the length of the sandpaper.The CA decreased from 152.4°to 137.7° after abrasion.Zhanget al.[86] achieved SHPS on STS by SiO2-assisted HF etching followed by modification with SA.The CA of the SHPS decreased from 162.45° to 150.2° when a SHP sample with a load of 100 g was dragged along sandpaper with a grit size of 2000 to a wear distance of 400 cm.Durability concerns the lifetime of SHPS,which is crucial for practical applications.Kimet al.[84] constructed SHPS of Al plates by etching in the mixed solution of HCl and HF,followed by treatment in NaCl solution at 100 °C and modification with PFOS.The durability of the SHPS was investigated both in low(-15°C)and high(150°C)temperature environments,and it was found that the MSHPS still had excellent SHP with a CA greater than 165° after 30 days of storage in two different environments.Barthwalet al.[150] prepared SHPS of Al plate by HCl etching,thermal treatment in the mixed solution of Zn(NO3)2·6H2O and hexamethylenetetramine (HMTA) at 90 °C for 4 h,and surface modification with polydimethylsiloxane (PMDS).The CA of the prepared samples decreased from 161° to 151°after exposing in the ambient environment for 10 months,indicating the excellent long-term durability in air.

Currently,there are two main strategies to improve the wear resistance and durability of SHP coatings,one of which is by constructing robust micro-and nanostructures.Wanget al.[164]fabricated microstructures on the SHPS substrates of nanostructures with materials like silicon,ceramics,metals,and glass.The microstructures act as ‘‘armor” to resist abrasion and to improve the mechanical durability of SHPS by placing the water-repellent nanostructures in protective microstructural ‘‘armor”,and the results showed that the water repellency of the resulting SHPS was maintained even after abrasion by sandpaper and sharp steel knives.Yanget al.[165] prepared three SHP Cu coatings with different microstructure sizes by pulsed electrodeposition on Cu substrates,and all three coatings exhibited excellent mechanical strength,corrosion resistance,and self-cleaning properties.Importantly,the coatings with microstructure sizes of 10-15,30-40,and 60-70 μm can,respectively,withstand 15,50,and 100 wear cycles without lost their SHP state,indicating the significant influence of microstructure size on the durability of SHPS.The second main strategy to address the problems of weak mechanical properties and poor durability is to create repairable SHPS.According to the literatures of [150,166-168],the damaged MSHPS could be restored simply by immersing the damaged surface into the eching solution and suface modification solution in sequence.Zhanet al.[168]prepared a repairable SHP Al foil by etching with CuCl2solution and subsequent modification with SA.It was found that the SHP disappeared after crumpling the SHP Al foil,and by simply dipping the damaged SHPS into CuCl2solution and ethanol solution of SA in turn,the SHPS of the Al foil was restored.This easily repairable SHPS increases its service life.Therefore,exploring the preparation of more wear-resistant MSHPS and extending its lifetime by various methods are still the focus of research in this field.

Both wear resistance and durability strongly relate with the bond strength between SHP coatings and substrates.Achieving uniform and dense coverage of the SHP coating on the substrate surface is essential to enhance the bonding strength between them.The coating treatment time,concentration,and temperature are important factors affecting the formation of a robust SHP coating.Zhuet al.[169] successfully coated an ordered integrated hydrophobic coating on the surface of STS 316L by extending the reaction time in a toluene solution of OTS during the modification process and adding an appropriate amount of alkali to the reaction solution.Extending the reaction time within a certain range is beneficial to the uniformity of the grafting of the coating to the substrate.Adding an appropriate amount of alkali is to neutralize the acid HCl produced by the hydrolysis of alkylchlorosilane to avoid the corrosion on the STS substrate,which affects the bonding strength of the hydrophobic coating.Jianget al.[170] pretreated AlA by immersing in an oxidizing solution(NaF·CrO3·H3PO4·H3BO3)to form Al2O3layer,and prepared[CH3(CH2)16COO]3Ce SHP coating by immersing the pretreated sheets in an electrolyte solution containing SA,Ce(NO3)3·6H2O,and ethanol.It was found that the Al2O3layer improved the adsorption of[CH3(CH2)16COO]3Ce SHP coating and enhanced the interfacial bonding strength between the coating and the substrate.Cyclic wear tests showed that the durability of[CH3(CH2)16COO]3Ce coating on Al2O3substrate was 41.5% higher than that on Al substrate.Yanget al.[171] prepared SHPS on mild steel by chemical etching and found that the addition of H2O2to the acidic etchant solution could enhance the adhesion of the hydrophobic coating to the substrate,and a dense interfacial layer was formed on the mild steel surface only through 4 min of etching.It was found that H2O2facilitated the formation of—OH groups on the mild steel surface,and the—OH groups acted as the grafting sites of the hydrophobic coating,thus enhanced the bonding of the hydrophobic coating to the substrate.

In a word,more attention should be paid to the abovementioned aspects,in order to promote the development of SHP material and the quicker transition of these technologies from the laboratory to practical applications.

7.Summary

Metals are essential materials for our daily life and socioeconomical development.However,the problems like surface contamination and corrosion could lower their performance,limit their applications,or even badly damage metals.The peculiar wetting phenomena of some natural SHP materials have provided many inspirations to solve these problems.This review introduced the progress of research on low-adhesion MSHPS by starting from the SHP phenomenon in nature environment,reviewed the three basic theoretical models used to explain the wetting phenomenon on SHPS,and summarized the mechanism of chemical etching on preparing SHPS on metal surfaces.The commonly used chemical etchants and surface modifiers were systematically reviewed,as well as the wide reports on the different wet chemical methods for preparing MSHPS.Subsequently,recent achievements in the application of MSHPS were presented,including self-cleaning,corrosion protection,anti-icing,drag reduction,oil-water separation,and other applications.Finally,the challenges and future prospects of MSHPS were briefly described.We hope that this review could help to deepen the understanding of the wet chemical method for preparing MSHPS,demonstrate the broad application potential of MSHPS,and provide some guidance for engineering applications and scientific research.

Data Availability

No data was used for the research described in the article.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We gratefully acknowledge the financial support of Shanghai Pujiang Program (22PJD001),the Scientific Research Project from Science and Technology Commission of Shanghai Municipality(19DZ1204903),and the Fundamental Research Funds for the Central Universities (2232021G-11).