Ministry of Education Key Laboratory for Cross-Scale Micro and Nano Manufacturing, Changchun University of Science and Technology, Changchun 130022, China
Metal is an essential material foundation on which human society depends on survival and development. It is a supporting material that constitutes a modern civilized society. Increasing the unique properties of metal materials and expanding the scope of application of metal materials has become a hotspot in the cross-study of natural sciences. Inspired by nature, the researchers have investigated the ultra-slip behaviour of the pitcher plant of nepenthes, injecting low surface energy lubricating liquid into the micro/nano structure substrate, forming a solid-liquid composite structure, and prepared a slippery liquid-infused porous surface(SLIPS) with special wettability. SLIPS has excellent self-healing, anti-icing, anti-fouling, anti-corrosion, anti-bioadhesion and self-cleaning properties. The method designs and constructs a SLIPS on a metal substrate and realizes its surface versatility, thereby providing the possibility for it to achieve a wider range of applications in the fields of marine anti-fouling, biomedical, aerospace, refrigeration, and industrial production. This article is divided into four parts to review the research progress of metal-based SLIPS manufacturing and its application, and summarize the design principle, preparation process, application of metal-based SLIPS, and future development trends and challenges.
Metal materials play an important role in the history of human civilization, and their discovery and use have significantly improved the productivity of human society. They are one of the most widely used engineering materials with high cost performance in various industries, and have a wide range of applications in aerospace, industrial production, marine engineering, medical treatment, construction and other fields[1⇓~3][4⇓⇓~7][8⇓⇓⇓~12][13⇓~15][16⇓~18][19⇓~21]. Due to the characteristics of metal materials, most of the metal materials are not corrosion resistant, not anti-icing, easy to pollute and other deficiencies in the use process, which has a major security risk and is easy to cause significant economic losses, limiting the application of metal materials in the above environment[22⇓⇓~25][26⇓⇓~29][30,31][32⇓⇓⇓⇓~37]. Therefore, it has become an urgent problem to increase the special properties of metal materials and broaden the scope of application of metal materials[38⇓~40].
At present, the main technologies used to improve the corrosion resistance and prolong the service life of metal materials are micro-alloying, microstructure control, electrochemical protection, surface treatment and preparation of functional coatings[41⇓~43][44,45][46,47][48⇓~50][51⇓~53]. Compared with other technologies, coating technology has the advantages of simple process, low cost and easy to combine with other methods, so it has become a hot research topic[54,55]. Based on the characteristics of lotus leaf, researchers have prepared bionic super-hydrophobic surfaces on metal substrates, which are widely used in self-cleaning, anti-fouling, anti-icing and corrosion resistance[56⇓~58][59⇓~61][62⇓~64][65⇓~67][68,69]. However, in practical applications, it is found that the hydrophobicity of superhydrophobic surfaces is unstable, which will lead to the failure of their hydrophobicity in high temperature and high pressure environments or in the case of surface damage[70].
The concept of "Slippery Liquid-infused Porous Surface (SLIPS)" was first proposed by Aizenberg et al of Harvard University in 2011. The essence of SLIPS is that the air layer in the micro-nano structure is replaced by the lubricant by injecting the low Surface energy lubricant into the micro-nano structure to form a more stable solid-Liquid composite layer[71][72⇓~74]. As a derivative of super-hydrophobic surface, biomimetic super-smooth surface has more advantages, such as hydrophobic, excellent lyophobic effect on hydrocarbons, crude oil and blood, anti-biological adhesion, special surface stability under high temperature and high pressure, and self-healing after physical damage[75,76][77,78][79,80][81,82]. These excellent properties make the bionic super-smooth surface have application prospects in the fields of marine anti-fouling, coating protection, biomedicine, corrosion resistance, self-cleaning and liquid transportation[83⇓~85][86][87,88][89,90][91,92]. In this paper, the design principle, preparation process and application of bionic super-smooth surface on metal substrate are summarized, and the future development trend and challenges are discussed.
2 Design Principle of Bionic Super-smooth Surface
Fig. 1 shows a bionic super-smooth surface prepared on the inner surface of a pitcher plant trap, and the structure is mainly composed of two parts: a solid phase substrate with a special microstructure and lubricating oil filled in the solid phase structure[93⇓⇓⇓~97]. Aizenberg et al. Presented three criteria for designing a bionic super-smooth surface: first, the lubricating oil can penetrate into the rough structure of the solid phase substrate and wet the solid phase substrate to achieve a firm bond between the two; Second, in order not to be displaced by other liquids, the solid phase substrate should be preferentially wetted by the lubricating oil; Third: The lubricant used must be immiscible with the fluid being tested.
In order to meet the first principle, nanotechnology is used to prepare a solid phase substrate with a microscopic rough structure to increase the specific surface area and the storage capacity of lubricating oil. The process of modifying the substrate to make it more lipophilic and capable of stably storing lubricating oil is shown in fig. 2a. In addition, Wu Dequan et al. Found that the optimal roughness of the bionic super-smooth surface is nanometer, which can provide capillary force to lock the lubricating oil without affecting the sliding of droplets[98]. The cloak effect is easy to occur on the surface with too little lubricating oil, and the sliding droplets will carry the oil film away. Song et al prepared a dense Co3O4 layer on the surface of pure aluminum and anodized aluminum to obtain a bionic super-smooth surface[99]. The results show that, compared with the pure aluminum bionic super-smooth surface, the anodized aluminum bionic super-smooth surface has better durability, and its surface microporous structure can store more lubricating oil. The porosity of the substrate surface is the key factor to maintain the bionic super-slip performance. The larger the specific surface area and the more oil storage, the better the application performance of the bionic super-slip surface.
In order to satisfy the second principle, the lubricating oil and the solid substrate must have matching physical and chemical properties to form a stable working system, as shown in Figure 2b. The researchers set the surface energy of the solid-liquid interface as the EA when the test liquid completely wets the solid substrate; When the test liquid floats on the top and the lubricating oil completely wets the solid substrate, the surface energy of the solid-liquid interface is E1; When no test liquid floats on top, the surface energy of the solid-liquid interface is E2 when the lubricant completely wets the solid substrate. In order to ensure that the solid phase substrate is preferentially wetted by the lubricating oil and that the lubricating oil stored in the microtexture is not displaced by the test liquid, it is necessary to guarantee the :ΔE1=EA-E1>0 and the ΔE2=EA-E2>0, which can be expressed as:
Where γA and γB are the surface tensions of the test liquid and lubricant, respectively, γAB is the interfacial tension of the liquid-liquid interface, θA and θB are the equilibrium contact angles of the test liquid and lubricant on a flat solid surface, and R is the roughness coefficient. The lubricant with the bionic super-smooth surface can effectively isolate the contact between the test liquid and the solid surface, especially in an aqueous solution, the lubricant and the aqueous solution are not mutually soluble, and can be stored for a long time, thereby protecting a substrate for a long time.
Smith et al. described the above criteria based on thermodynamic principles and identified 12 different thermodynamic conditions for the four-phase system of gas, lubricant, test liquid and solid substrate[100]. It is assumed that the test liquid is an aqueous solution, the lubricating fluid is a fluorinated oil, and the other two phases are air and a solid substrate. The "o", "w" and "a" in the diffusion coefficient represent oil, water and air, and the formula is defined as:
Where the γwa is the water-air surface tension, the γow is the oil-water surface tension, and the γoa is the oil-air surface tension. When the diffusion coefficient is , the water droplets will be completely enveloped by the fluorinated oil, or when , the water droplets will float on top, as shown in Figure 3A. Similarly, the diffusion coefficient of oil on a solid substrate when in contact with air or water can be derived from Equations 4 and 5.
From the analysis of the four-phase system, it can be seen that the lubricating oil will completely wet, partially wet, or not wet the solid phase substrate. Furthermore, the wetting condition is related to the roughness of the substrate surface according to the contact angle formula cosθ = (φ-r)/ (r-1), where R is the surface roughness factor defined by the Wenzel equation and φ is the projected area fraction occupied by the liquid. According to the three criteria of bionic super-smooth surface, when the γow(r-1)/(r-φ)< is less than 0, the solid substrate is more hydrophilic to the oil on the outside of the water droplet; ≥ 0, the lubricant will completely wet the solid phase substrate. The following phenomena can be observed: (1) if there is no lubricant below the droplet, the droplet will produce a Wenzel state and be pinned to the substrate; (2) if there is a part of lubricant, the droplet will slide and friction force will be generated between the droplet and the solid substrate; (3) The lubricant does not completely wet the substrate and the droplets slide to the dry surface and pin.
Semprebon et al. used an analytical model to analyze the contact angle and contact angle hysteresis on a bionic super-smooth surface (Figure 3B), and found that the contact angle on a bionic super-smooth surface is not a constant material property, but an energy balance among three fluids (air, lubricating oil, and test liquid)[101]. In 2015, Schellenberger et al. Used laser confocal microscopy to observe the state of aqueous solution on bionic super-smooth surface, which proved the correctness of Smith et al.The wetting ridges between different lubricating fluids and aqueous solutions are also significantly different (Figure 3C), and the advancing and receding contact angles of aqueous solutions on the bionic super-smooth surface under dynamic conditions are tested (Figure 3D). It is found that it is very important to select the appropriate surface microstructure and test fluid in practical applications[102].
3 Process for preparing bionic super-smooth surface of metal substrates
The essence of bionic super-smooth surface is to store lubricant by using the micro-nano structure with rough surface, and to form a smooth surface with solid-liquid composite structure, so that the test liquid can slide on the lubricant surface without directly contacting the [103~105] of the substrate material. Therefore, the preparation process of bionic super-smooth surface is mainly divided into: micro-nano structure processing and lubricant filling [106~108]. There are many processing methods for micro-nano structure of metal substrate, such as electrodeposition method [109~111], chemical etching method [112~114], anodic oxidation method [115~117], chemical vapor deposition method [118~120], spraying method, hydrothermal method, etc., as shown in Figure 4[121,122][123,124]. Table 1 summarizes the innovative technologies applied to the surface processing of metallic materials.
3.1 Hydrothermal method
Hydrothermal treatment is a method of putting the precursor in an autoclave and reacting under high temperature and high pressure. Hydrothermal reaction can be divided into hydrothermal oxidation, hydrothermal reduction, hydrothermal precipitation, hydrothermal synthesis, hydrothermal hydrolysis, hydrothermal crystallization and so on according to different reaction types.Hydrothermal method is one of the most commonly used [138~140] for the preparation of micro-nano structures for bionic super-smooth surface. Song et al prepared a super-hydrophobic surface with hierarchical structure on the surface of AZ31 magnesium alloy by hydrothermal method, and immersed it in polypropylene ether to obtain a bionic super-smooth surface with a static water contact angle of (114 ± 9) °[141]. Compared with the magnesium alloy substrate and the super-hydrophobic coating, the bionic super-smooth surface did not corrode after immersion in 3. 5 wt% NaCl solution for 30 d, and the liquid lubricant effectively isolated the invasion of corrosive medium and prevented the surface electron transfer.The bionic super-smooth surface showed good corrosion resistance and durability. Jiang et al. generated a layered double hydroxide film on the surface of the oxidation film of AZ91D magnesium alloy by a hydrothermal method, and prepared a bionic super-smooth surface through chemical modification and injection of lubricating oil. The schematic diagram of the preparation process is shown in Figure 5, and the static water contact angle is 121 °[142]. Immersion test and electrochemical test show that the bionic super-smooth surface has durable water repellency and self-repair ability of surface damage, which significantly improves the corrosion resistance of AZ91D magnesium alloy. The coating prepared by hydrothermal method has the advantages of good dispersibility and easy control of crystal form, but the preparation process requires high temperature and high pressure environment, which restricts the application and development of hydrothermal method, and the preparation conditions are poor and the technology is difficult, which is not suitable for large-scale production[143,144].
3.2 Chemical Vapor Deposition
Chemical vapor deposition (CVD) is a preparation method that uses one or more vapor-phase compounds or simple substances containing film elements to chemically react on the surface of the substrate to form a film. CVD reactions generally include thermal decomposition reaction, chemical synthesis reaction and chemical transport reaction[145,146]. Shi et al. Generated a porous Cu2O coating on the surface of CuZn alloy by combining galvanic corrosion and chemical vapor deposition, and dodecanethiol was used for super-hydrophobic modification. Lubricating oil was injected into the coating by spin coating to obtain a bionic super-slippery surface, as shown in Figure 6[147]. Electrochemical tests show that the corrosion current density of the bionic super-smooth surface is only 1/500 of that of the CuZn alloy substrate, and the corrosion resistance of the bionic super-smooth surface is greatly improved.After cutting, erosion and other damage tests in seawater, the bionic super smooth surface shows good self-healing and tolerance. Qiu et Al. Used chemical vapor deposition method to produce carbon fiber layer on the surface of dendritic micro-texture of Cu, Cu/Al alloy, Al/Mg alloy and low alloy steel, and injected lubricating oil on the surface to prepare bionic super-smooth surface[148]. Compared with the substrate, the electrochemical test results show that the bionic super-smooth surface can maintain good stability after being immersed in 3. 5% NaCl corrosion solution for one month.The bionic super-smooth surface can form a solid barrier, which makes it difficult for the corrosion medium to penetrate the surface and corrode the substrate.In addition, the bionic super smooth surface also shows excellent anti-fouling performance. The chemical vapor deposition method is easy to control the processing process, can generate alloy films with various components according to requirements, has good diffraction capability, short preparation period and uniform film layer, but impurities in the film layer are difficult to remove, and volatile substances used in the preparation process are harmful to human bodies and pollute the environment[149,150].
3.3 Lectrochemical corrosion
Electrochemical corrosion means that metal and electrolyte form two electrodes to form a corrosion galvanic cell, the metal is oxidized, the outer layer will form many small bubbles with different diameters, and the secondary layer is black powdery ulcer corrosion pits, which are conducive to the storage of lubricating oil and the preparation of bionic super-smooth surface[151]. Luo et al. generated a microstructure on the zinc alloy surface by electrochemical corrosion (as shown in Figure 7a-c), modified the surface with fluoroalkyl silane, and injected silicone oil lubricant to prepare a bionic super-smooth surface with a static water contact angle of 102 °, as shown in Figure 7d[152]. Through the experiments of liquid sliding, bouncing and water jet, it is found that the bionic super-smooth surface has stable water repellency. The simulated ice rain and deicing force tests show that the deicing force of the bionic super-slip surface is reduced by 50% ~ 67% compared with that of the zinc alloy substrate after the sample is placed at (-15 ± 2) ℃ for 1 H, which indicates that the bionic super-slip surface has good anti-icing and anti-condensation properties. Sun et al. Used electrochemical corrosion and anodic oxidation to form a layered porous coating on the surface of aluminum, and injected silicone oil into the surface to prepare a bionic super-smooth surface, which has repellency to water, diethylene glycol, glycerol, formamide and milk[153]. The electrochemical test and the cold rain test show that the bionic super-smooth surface has good stability, and has good cold rain icing resistance in the freezing environment compared with the aluminum substrate; and the bionic super-smooth surface has lower corrosion current density and higher corrosion potential in the simulated seawater, so that the corrosion resistance is greatly improved. In addition, the bionic super-smooth surface also shows excellent self-cleaning property. Electrochemical corrosion has the advantages of short cycle, low cost, high efficiency, easy storage of lubricant, large-scale preparation and easy control of coating, but the process will produce chemical waste liquid, which is easy to pollute the environment and difficult to post-treat[154,155].
3.4 Electrodeposition method
Electrodeposition refers to the process of electrochemical deposition of metals or alloys from aqueous solutions, non-aqueous solutions or molten salts of their compounds, which is mainly used to prepare thin film materials including metals, alloys, semiconductors and conductive polymers, so as to change the surface properties of substrates[156]. Ouyang et al. Deposited Ag dendritic structure on titanium surface by electrodeposition method, modified the surface by dodecanethiol, and the surface showed super-hydrophobic/super-lipophilic properties, and then injected lubricant to obtain a bionic super-smooth surface with high flatness and smoothness[157]. The biofouling test showed that the number of diatoms and green algae on the untreated titanium substrate was up to 1.60×1011cells/cm2 and 1.57×1011cells/cm2 after the sample was immersed for 14 d, while the number of diatoms and green algae on the bionic super-smooth surface was only 6.84×107cells/cm2 and 5.05×107cells/cm2, which decreased by four orders of magnitude.The bionic super-smooth surface effectively improved the biofouling performance of titanium substrate and provided an effective means for inhibiting biological adhesion in seawater environment. Qiu et al. generated a Cu(OH)2 cluster structure on the CuZn metal surface by a method combining electrodeposition and chemical vapor deposition, modified it with dodecyl mercaptan to achieve super-hydrophobic and super-lipophilic properties, and prepared a bionic super-slippery surface after injecting lubricating oil into the surface, as shown in Figure 8[158]. It is found that the bionic super-smooth surface has good corrosion resistance in seawater environment. The electrodeposition method has the advantages of simple process, easy control, low energy consumption, low cost, high deposition rate, large-scale generation, no addition of other chemical reagents and the like, but the prepared coating has low bonding force with the substrate, and heavy metal pollution is generated in the processing process[159,160].
3.5 Spraying method
The spraying method is to use a special spraying machine to spray the raw material made of loose fibers and inorganic binders together with water on the solid surface through gas power[161]. Geraldi et al. sprayed hydrophobic nanoparticles on stainless steel meshes with different meshes by spraying method, and obtained hydrophobic/lipophilic porous nanoparticle coatings after five times of continuous spraying, and then immersed in silicone oil to prepare a bionic super-smooth surface with high droplet fluidity and double-sided droplet control (roll angle of 1.2 ± 0.2 °)[162]. The results show that after 180 min laminar flow and 90 min turbulent flow, the rolling angle of the surface of the bionic super-smooth stainless steel mesh is still less than 2 °. Zhang et al. sprayed the SiO2 suspension solution onto the surface of the aluminum substrate to obtain a wrinkled and porous hollow tubular silica coating, treated it with trimethoxysilane to prepare a super-hydrophobic coating, and then soaked it in Krytox 100 lubricating oil for 1 H to prepare a new type of porous hollow tubular silica biomimetic super-slippery surface, as shown in Figure 9[163]. After 24 cycles of icing/deicing, the ice adhesion strength was almost maintained at 18 kPa, and the icing time was delayed by 40 times compared with the untreated aluminum substrate. Spraying is a simple and environmentally friendly coating preparation method, which is easy to operate, low cost, suitable for a variety of substrates, and can be prepared in large areas, but the coating is not uniform, and the preparation process is potentially harmful to the health of operators[164,165].
3.6 Chemical etching method
Chemical etching is a method to obtain micro and nano rough structure by putting metal in strong acid, strong base or concentrated salt solution and using the solution to corrode the surface of metal. The process is simple and easy to operate, and the requirements for equipment are low, which provides a possibility and [166~169] for the realization of large-scale, low-cost and rapid preparation of bionic super-smooth surface of metal substrate. Nguyen et al. Used chemical etching method to generate micro/nano structures on the surface of aluminum alloy, used perfluoropolyether to modify the surface superhydrophobicity, and then used spin-coating method to coat different types of lubricating oil on the surface of superhydrophobic aluminum alloy to prepare different types of bionic superslippery surfaces[170]. It is found that the combination of superhydrophobicity and lubricant makes the ice adhesion strength of the bionic super-slippery surface lower. Zhang et al. Generated double-layer zinc oxide on the aluminum surface by etching, sol-gel and hydrothermal methods, with the bottom layer being flower-like and the upper layer being rod-like, and then modified the surface with trimethoxysilane for super-hydrophobic modification, and then immersed in lubricating oil to prepare a bionic super-slippery surface, as shown in Figure 10[171]. It is found that the bionic super-smooth surface has hydrophobicity, oleophobicity and good anti-fouling effect. The corrosion resistance of the bionic super-smooth surface is improved by four orders of magnitude compared with that of an aluminum substrate, and the corrosion resistance of the bionic super-smooth surface is not obviously changed after being placed in the air for three months, so that excellent stability is shown. The surface prepared by the chemical etching method has the advantages of good controllability of the structure, easy storage of lubricating oil, simple equipment, no need of applying current and voltage, easy operation and the like, but waste liquid is generated in the preparation process and pollutes the environment[172,173].
In general, the development trend of preparation methods of bionic super-smooth surface of metal substrate is green, low cost and fast. With the development of simple chemical/electrochemical methods, hydrothermal methods and spraying methods, it is no longer a problem to prepare bionic super-smooth surfaces on metal substrates on a large scale. So far, the research on bionic super-smooth surface is gradually increasing, which lays a good foundation for further development of preparation methods of bionic super-smooth surface of metal substrate to meet more needs.
4 Application of Bionic Super-smooth Surface on Metal Matrix
The essence of the bionic super-smooth surface is that the air layer in the micro-nano structure is replaced by the filled lubricating oil, which makes the low surface energy liquid slide on the surface and can not directly contact the substrate, and endows the bionic super-smooth surface with excellent low adhesion [174~178]. In addition, the lubricating oil on the bionic super-smooth surface has fluidity under capillary action, can fill scratches and repair damage, and has self-healing performance. Bionic super-smooth surface has shown great advantages and application prospects in biomedical, industrial production and marine anti-fouling fields with its excellent self-healing, anti-icing, anti-fouling, corrosion resistance, anti-biological adhesion, self-cleaning and other characteristics (as shown in Figure 11), which has attracted great attention and [179~181] from researchers at home and abroad.
4.1 Self-healing
The self-healing property of the bionic super-slip surface can still maintain good super-slip characteristics under the condition of substrate damage, thereby avoiding the loss and [182~184] of anti-icing, anti-fouling, anti-corrosion, anti-biological adhesion, self-cleaning and other characteristics. Xiang et al prepared a super-hydrophobic coating with porous structure on the surface of low-carbon steel by electroplating and chemical replacement, and injected lubricating oil into the surface to obtain a bionic super-smooth surface with a static water contact angle of (117.5 ± 1) °[185]. It was found that the scratch on the bionic super-smooth surface damaged by the knife became narrow and almost disappeared after 2 H, which indicated that the lubricant injected into the porous structure could penetrate into the damaged area to fill the scratch and showed good self-healing property. In addition, the electrochemical test and the 3.5 wt% NaCl solution immersion test also show that the scratched bionic super-smooth surface is self-healing, as shown in Figure 12.
4.2 Anti-icing performance
Bionic super-slippery surface has great application prospects and [186~188] in the field of anti-icing and de-icing because of its excellent super-slippery characteristics. The principle is that when water vapor is cooled, it forms droplets and condenses on the bionic super-smooth surface, and when the diameter of the droplets reaches a critical value, they will slide off the surface. The formula for judging the DC of the critical diameter of the liquid drop is as follows:[189]
Where the θ1 is the advancing contact angle of the droplet, the θ2 is the receding contact angle of the droplet, γ is the surface tension of the droplets, α is the roll angle, Δθ is the contact angle hysteresis, G is the gravitational acceleration, and ρ is the density of water. It can be seen that the smaller the contact angle hysteresis is, the smaller the sliding diameter of the droplet is, and the droplet has fallen on the bionic super-smooth surface before freezing.
Juuti et al. Prepared a titanium dioxide nanoparticle coating on titanium surface by flame aerosol synthesis method, and injected silicone oil into the coating to obtain a bionic super-smooth surface[190]. It is found that the bionic super-slip surface has better anti-icing performance compared with the polyethylene film, and the ice adhesion strength decreases from 110 kPa to 12 kPa, and the bionic super-slip surface can still maintain good anti-icing performance even after many cycles of icing tests. Kim et al prepared polypyrrole coating on the surface of aluminum plate by electrodeposition method, modified the coating with super-hydrophobic property, and injected Krytox 100 lubricant into the surface of the sample to prepare a bionic super-smooth surface with a contact angle hysteresis of (2 ± 1) °[189]. It was found that the water droplets condensed on the bionic super-slippery surface could slide down before icing, thus reducing the amount of icing, while the accumulated ice and melted water on the surface could be easily removed by gravity at low inclination (as shown in Figure 13). Compared with the matrix material and the super-hydrophobic surface, the ice adhesion force of the bionic super-slippery surface is 15 kPa, which is reduced by 1 to 2 orders of magnitude. Bionic super-smooth surface provides a promising method for anti-icing and de-icing of metal materials in refrigeration and aviation industries and other high-humidity environments.
4.3 Corrosion resistance
The lubricant on the bionic super-smooth surface can effectively isolate the corrosion medium from the corrosion of the substrate, especially in solution, the lubricant and the aqueous solution are not soluble, can be stored for a long time, and has the application potential of protecting and slowing down the corrosion of the substrate[191,192]. Inoue et al used chemical etching method and anodic oxidation method to generate nano-porous anodic aluminum oxide film on the surface of aluminum sheet, then modified the aluminum oxide film with super-hydrophobic, and injected lubricant to obtain bionic super-smooth surface[193]. It was found that the bionic super-smooth surface exhibited better corrosion resistance in both NaCl and HCl solutions compared with the aluminum substrate and the super-hydrophobic aluminum sheet, and the corrosion current density was reduced to a 10-10A/cm2. Ouyang et al. Prepared stainless steel bionic super-smooth surface by three steps of electrodeposition, vapor deposition and oil injection[194]. It was found that the corrosion current density of the surface was as low as 2.52×10-10A/cm2 after immersion in abiotic seawater for 28 days, which was 5 orders of magnitude lower than that of the stainless steel substrate. After immersion in the SRB suspension for 10 days, the corrosion current density of the bionic super-smooth surface is only 1.06×10-8A/cm2, which is lower than that of the super-hydrophobic coating, as shown in Figure 14.
4.4 Antifouling performance
There is a lubricating oil film on the bionic super-smooth surface, which has low surface tension and is immiscible with other liquids. Droplets floating on the lubricating oil film are easy to slide off, which can effectively prevent dirt from precipitating and [195~197]. Wang et al. described the antifouling mechanism of antifouling materials from the thermodynamic point of view. The free energy of the fouling adhesion process consists of two parts: one is the enthalpy of the interaction between the fouling, the solvent and the surface, and the other is the entropy change between the fouling and the surface[81]. Adsorption occurs only when there is a ΔGads=ΔHads-TΔSads<0. When the antifouling material has a strong interaction with the environmental solution, the lubricant layer will reduce the enthalpy increase of the interaction between the surface and the fouling, so that the ΔHads in this process is small or even positive.In this case, the change in fouling entropy (ΔSads) dominates the adhesion process, causing the surface-bound fouling to diffuse back into solution, resulting in effective antifouling properties (Figure 15).
Charpentier et al. Used electrodeposition method to produce microporous polypyrrole coating on the surface of 316 L stainless steel, and injected low surface energy lubricating oil into the surface of microporous coating to prepare bionic super-smooth surface[198]. It was found that the bionic super-smooth surface had excellent anti-fouling performance compared with the 316 L stainless steel substrate, and the amount of calcium carbonate deposition on the surface was significantly reduced, as shown in Figure 16. It can be seen that the bionic super-smooth surface has a broad application prospect in the treatment of mineral scale and dirt.
4.5 Antibioadhesion
The lubricating oil film of the bionic super-smooth surface effectively isolates the contact between the microorganism and the substrate, thereby increasing the difficulty of the attachment of the microorganism on the surface and achieving the effect of anti-bioadhesion[199,200]. Ouyang et al. Prepared a super-hydrophobic cobalt dendrite coating on copper substrate by electrodeposition, oxidation and chemical modification, and injected silicone oil into the coating surface to obtain a bionic super-smooth surface[201].
It was found that the adhesion density of SRB on the bionic super-smooth surface decreased by 3 and 2 orders of magnitude, respectively, compared with the untreated copper substrate after immersion in seawater for 3 and 6 days.It is mainly attributed to the inhibition of the adhesion of sulfate reducing bacteria by the silicone oil layer with bionic super-smooth surface, and it is found that the higher the viscosity of silicone oil is, the better the effect of inhibiting biofouling is. He et al prepared SiO2 super-hydrophobic coating on the surface of stainless steel by layer-by-layer assembly method, and injected DuPont Krytox 100 lubricating oil into the surface to obtain a bionic super-smooth surface containing SiO2[202]. It was found that the adhesion rate of Chlorella pyrenoidosa and Phaeodactylum tricornutum with the bionic super-smooth surface was reduced by 93% and 71%, respectively, compared with the stainless steel substrate, as shown in Figure 17.
The anti-biological adhesion of the bionic super-smooth surface has good application prospect in the aspects of seawater application, human body anti-bacteria and the like[203]. Lee et al. Used anodic oxidation method to generate nano-porous anodic aluminum oxide on the surface of aluminum foil, and prepared bionic super-smooth surface by hydrophobic treatment and immersion in lubricants[204]. It was found that compared with the untreated aluminum substrate, the adhesion and adhesion of Escherichia coli on the bionic super-smooth surface were reduced by 0.7 log and 99.2%, respectively (as shown in Figure 18e), which was mainly attributed to the fact that the lubricating oil film of the bionic super-smooth surface inhibited the contact between Escherichia coli and solid surfaces, making it difficult for Escherichia coli to adhere to the surface (as shown in Figure 18a-d), thus achieving antibacterial effect.
4.6 Self-cleaning property
The lubricating oil stored on the bionic super-smooth surface can increase the surface lubricity and reduce the friction resistance, so that the liquid on the surface can not stay for a long time, thus taking away pollutants and achieving the purpose of self-cleaning[205,206]. Ma et al prepared a microstructure on the surface of S45C carbon steel plate by nanosecond laser processing technology, modified the microstructure surface with perfluorodecyl triethoxysilane, and then injected DuPont Krytox 105 perfluoropolyether lubricating oil to obtain a bionic super-smooth surface[207]. The results show that when water drops fall on the surface of untreated S45C carbon steel, Fe3O4 solid contaminants and aqueous solution remain on the surface. When the water drops fall on the bionic super-smooth surface, the water drops immediately slide off and take away Fe3O4 solid contaminant particles from the surface, thus restoring the surface to a clean state, as shown in Figure 19. Liu et al. Prepared polytetrafluoroethylene coating on the surface of titanium plate, copper plate and other materials by spin coating method, and dropped perfluoro lubricant on the coating surface to obtain bionic super-smooth surface[208]. It was found that, compared with the substrate, the sand and water-soluble methyl blue powder on the bionic super-smooth surface were easily carried away by water, and the surface contaminated by oil-soluble dye Sudan red could be easily cleaned by n-decane, showing excellent self-cleaning performance.
The bionic super-smooth surface constructed on the metal substrate can effectively solve the problems of poor corrosion resistance and easy icing of metal materials in the use process, and endow them with special functions such as water repellency, self-repair, anti-bioadhesion, anti-bacterial and anti-fouling.The multifunctional bionic super-smooth surface of the metal matrix is constructed, the application prospect of the metal material is further widened, the continuously improved market application demand is met, and the method has extremely high application value and market prospect.
5 Conclusion and prospect
Metal is an important material basis for the survival and development of human society and a supporting material for modern civilized society. It has become an urgent problem to increase the special properties of metal materials and broaden the scope of application of metal materials. The preparation of the bionic super-smooth surface on the metal substrate realizes the multifunctional characteristics of the metal material, such as self-healing, anti-icing, anti-fouling, corrosion resistance, bioadhesion resistance, self-cleaning and the like, and lays a foundation for wider application of the metal material in various fields. Nevertheless, there are still many problems to be solved in the preparation and application of bionic super-smooth surfaces on metal substrates:
(1) The actual application environment of bionic super-smooth surface is complex and changeable, and the lubricant will be lost after a long time of use, resulting in the failure of surface super-smooth performance. In order to solve these problems, at present, we mainly optimize the processing and preparation methods of micro-nano structures, increase the complexity of surface microstructure, and prepare nano-structures with regular shapes. A lubricant which is difficult to volatilize, high in viscosity and stable in performance is adopted; The application of covalently linked flexible groups in the bionic super-smooth surface and the substitution of solid lubricants for liquid lubricants can effectively improve the service life of the bionic super-smooth surface of the metal substrate[209][210]. However, the use of toxic fluorides is potentially harmful to the environment and the performance of lubricants decreases over time. Therefore, there is an urgent need to develop a lubricant which is difficult to volatilize, stable in performance, economical and environmentally friendly, so that it has greater use value in practical applications.
(2) At present, bionic super-smooth surfaces are mainly applied to traditional metal materials, including aluminum, copper, stainless steel, titanium and their alloys, while new metal materials, such as super steel, impact-resistant and fire-resistant steel, aluminum-lithium alloy, medical alloy, shape memory alloy, metal porous material, hydrogen storage alloy, shock absorption alloy, metal glass and metal matrix composites, are less studied. Therefore, the preparation of biomimetic super-smooth surface of new metal materials will become the next promising research field, which needs to be further promoted.
(3) There are still some problems to be solved urgently in the various processing technologies of bionic super-smooth surface of metal substrate. Hydrothermal method is not suitable for large-scale production because of its poor preparation conditions. The spraying method uses volatile organic solvents, which are harmful to human body, and the coating is not uniform. The film prepared by the electrodeposition method has weak binding force with the substrate and generates heavy metal pollution; The chemical etching method pollutes the environment and the cost of waste liquid treatment is high; Volatile substances used in chemical vapor deposition are harmful to human body and pollute the environment. The film produced by anodic oxidation is brittle and difficult to process complex workpieces. Therefore, it is of great significance to find a preparation method with simple process, stable performance, universality and large-scale production for the promotion of bionic super-smooth surface.
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