Fabricating bionic ultra-slippery surface on titanium alloys with

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Fabricating bionic ultra-slippery surface on titanium alloys with excellent fouling-resistant performance Yanjun Wang, Wenjie Zhao, wenting Wu, chunting Wang, Xuedong Wu, and Qunji Xue ACS Appl. Bio Mater., Just Accepted Manuscript • DOI: 10.1021/acsabm.8b00503 • Publication Date (Web): 03 Dec 2018 Downloaded from http://pubs.acs.org on December 3, 2018

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ACS Applied Bio Materials

Fabricating bionic ultra-slippery surface on titanium alloys with excellent fouling-resistant performance Yanjun Wang1,2, Wenjie Zhao1,*, Wenting Wu1,2, Chunting Wang1, Xuedong Wu1, Qunji Xue1,2 1. Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, China 2. University of Chinese Academy of Sciences 19 A Yuquan Rd, Shijingshan District, Beijing, P.R.China 100049 ABSTRACT: Due to the high strength-to-weight ratio and excellent corrosion resistant, titanium alloys are widely applied in marine industries. However, titanium alloys suffer from serious biofouling problems for their good biocompatibility. Slippery lubricant-infused porous surface (SLIPS) exerts a positive effect on inhibiting the attachment of marine microorganism. In this work, SLIPS was fabricated

on

TC4

(Ti-6Al-4V)

alloys

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

through (POTS)

anodic

oxidation,

modification

and

polyperfluoromethyl isopropyl ether (PFPE) infusion to improve the anti-fouling property of TC4 alloys. The antifouling effect of all coatings was investigated and compared to obtain the anti-fouling mechanism of SLIPS through recording the settlement of E. coli and Navicula exigua. The results showed that three different kinds of nanostructures were fabricated on TC4 alloys by anodic oxidation. The nanotubes with the largest internal volumes were conducive to storing more lubricant and firmly holding the lubricant for a long time. The SLIPS could effectively suppress

*

Corresponding authors. E-mail addresses: [email protected] (W. Zhao) 1

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the attachment of E. coli and Navicula exigua on account of the ultra-slippery behavior of lubricant layer. In particular, the SLIPS-30V fabricated by the largest nanotubes displayed a long term anti-adhesion behavior. Besides, the SLIPS could improve the corrosion resistance of TC4 alloys. Hence, such SLIPS with excellent fouling-resistant and anti-corrosion performance could be applied to titanium alloys or other alloys to expand their application in marine area. KEYWORDS: titanium alloys, anodic oxidation, nanostructures, SLIPS, anti-fouling, corrosion resistance 1. INTRODUCTION The adhesion and deposition of marine organisms on devices cause serious biofouling problems and great economic losses every year, e.g. blockage of pipelines, increase of ship gravity and friction resistance and corrosion of metals1-4. Titanium alloys are widely used in marine because of their excellent mechanical property and corrosion resistance ability5,6. But every coin has two sides, titanium alloys are also faced with serious problem of biocontamination for good biocompatibility which has limited the application in marine industry6-9. At present, there are many kinds of antifouling technologies such as physical antifouling including mechanical cleaning and acoustics method10,11, chemical antifouling and bionic antifouling2,12. Among them, chemical antifouling is the most effective and widely used antifouling method. The antifouling mechanism is killing microorganisms by biocides (tributyltin and Cu2O and etc.9,13) contained in paints to achieve a long-term antifouling performance. However, the biocides do harm to the marine environment, cause marine organisms genetic variation and death and ultimately threaten the health of human beings14-16. Bionic antifouling is inspired by the skins of animals and flowers or leaves of plants to circumvent, interfere or block the attachment of marine organisms which is environment friendly and has a wide application prospect17,18. In nature, the bottleneck of the pitcher plant can secrete lubricant liquid, making the surface display a particularly slippery behavior so that the insects can easily slide 2


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off the surface under a small incline angle. Inspired by the pitcher plant, slippery lubricant-infused porous surface (SLIPS) obtained by adsorption and storage of lubricant by surface micro-nanostructures was firstly introduced by Aizenberg et al.19. It has attracted wide attention in applying to anti-fouling materials because some mud, proteins or microorganisms can rapidly slide off the SLIPS20-26. Zouaghi et al.27 reported that the SLIPS prepared by infusing Krytox 103 perfluorinated oil into textured stainless steel displayed outstanding fouling-release as absolutely no trace of dairy deposit was found after 90 min of pasteurization test in pilot-scale equipment. Xiao et al.28 showed that microporous butyl methacrylate-ethylene dimethacrylate surfaces infused fluorocarbon lubricants exhibited remarkable inhibition of settlement of both zoospores of the alga Ulva linza and cypris larvae of the barnacle Balanus amphitrite. Wang et al.29,30 revealed that aluminum anodic films infused with perfluoropolyether could inhibit the SRB and C. vularis settlement in both static and dynamic marine conditions. Amini et al.31 suggested that 3 D polydimethylsiloxane polymer network infused with silicone oil showed the strongest antifouling behavior by deceiving the mechanosensing ability of mussels, deterring secretion of adhesive threads, and decreasing the molecular work of adhesion. Based on the previous research, it could be obtained that the antifouling behaviors of SLIPS are determined by two critical factors including micro/nano-structures and infused lubricant. It is well known that much attention is paid to prepare complicate micro/nano-structures on materials via several technologies including hydro-thermal method32, layer-by-layer self-assembly method33 and chemical etching34 and etc.35-37. However, to the best of our knowledge, fabricating a series of homogeneous and ordered structures with different size in nano-scale on TC4 alloys showing excellent lubricant storage performance is rarely presented. Therefore, the effects of nanopores and nanotubes on the storage of lubricant should be systematically investigated. The nanostructures have the larger capillary force to lock the lubricant firmly, which is helpful to prolong the effectiveness of lubricant layer. This work will be very important for deeply understanding the anti-fouling mechanism of SLIPS. 3



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In our work, SLIPS was prepared on the titanium alloys through anodic oxidation method, modification with fluorosilane and lubricant infusion. The nanostructures (nanopores and nanotubes) with different diameter and depth were in-situ grown on titanium alloys surface by anodic oxidation which displayed an important effect on the stability of lubricant layer38,39. Fluorosilane was used to reduce the surface energy to ensure that the lubricant was easily immersed into the nanostructures. The fouling resistance performance of titanium alloys with different nanostructures, wettability and lubricant layer was investigated and compared through inhibiting the settlement of E. coli and Navicula exigua to illuminate the anti-fouling mechanism of SLIPS. As a typical rod-like gram-negative bacteria, E. coli is widely distributed in marine environment and causes serious biofouling, so it is often used to evaluate the contamination resistance of materials26,40. Navicula exigua belongs to diatoms which has a strong adaptability and can attach on metal surfaces before macrofouling organisms. So E. coli and Navicula exigua are generally thought to be involved in the early stages of biofouling and play a dominant role in biofouling41,42. In addition, the electrochemical response of the SLIPS was investigated to clarify the influence of SLIPS on the corrosion resistance of titanium alloys. 2. EXPERIMENTAL 2.1. Materials and Reagents. The TC4 plates (Ti-6Al-4V, 6.2% Al, 3.98% V, 1.02% O, 0.16% Fe, 0.04% C, 0.03% N, 0.006% H, 0.3% others and Ti as the remainder) were purchased from Baoji Changzheng metal material company, China. The phosphoric acid, sodium fluoride, sodium chloride and ethanol used were analytical

grade.

Tryptone,

yeast

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

(POTS)

extract and

power,

polyperfluoromethyl

isopropyl ether (PFPE, Fomblin Y-1800) were purchased from Aladdin. Nutrient agar was purchased from Hangzhou microbial Reagent Co., Ltd. 2.2. Preparation of Nanostructures. The TC4 plates with the dimension of 30 mm × 20 mm ×3 mm were polished by SiC papers up to 2000 grade. Then the 4


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nanostructures were obtained on TC4 alloys by anodization under different voltages (10 V, 15 V, 30 V) for 30 min at 25 ℃. A mixture solution of 0.5 M phosphoric acid and 0.14 M sodium fluoride was used as electrolyte. In the two-electrode cell, the polished TC4 plate and platinum foil were set as anode and cathode respectively. After anodization, the anodized TC4 alloys were cleaned by deionized water to remove the residual electrolyte. 2.3. Hydrophobic Modification and Lubricant Infusion. The anodized TC4 alloys

were

immersed

in

ethanol

solution

with

0.25

vol.%

1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS) for 3 h at 25 ℃ and then dried at 120 ℃ for 1 h. After modification, 50 μL PFPE was poured onto the modified TC4 alloys to obtain the SLIPS. Then the TC4 alloys infused with lubricant were put into vacuum drying oven to facilitate the removal of air in nanostructures. Finally, they were vertically placed for 4 h to drain off the excess lubricant. The schematic illustration of SLIPS fabrication is showed in Figure 1.

Figure 1. Schematic illustration of preparation process of SLIPS on TC4 alloys. 2.4. Characterization of Surface Morphology, Composition and Wettability. The Field emission scanning electron microscopy (Hitachi S4800, Tokyo, Japan) was used to observe the surface morphologies of anodized TC4 alloys. The crystal structure was characterized by Raman spectroscope (Renishaw inVia Reflex, Renishaw, UK). The water static contact angles and sliding angles on treated TC4 alloys were measured by a contact angle meter (OCA20, Germany) with a water droplet volume of 5 μL. The sliding speed of the water droplet (5 μL) was obtained by a high-speed video camera (Photron, Mini UX100, Japan) with a declined angle of 20°. The obtained data was the average value of five different locations per sample. 2.5. Evaluation of the Stability of Lubricant layer. To evaluate the long term 5



stability of the lubricant layer in seawater, the SLIPS were immersed in simulated seawater with continuous agitation to accelerate the failure process of lubricant layer, the detailed steps were as follows: the SLIPS were vertically attached to the inner surface of beaker which contained 3.5 wt.% NaCl solution and was stirred by agitator with a stirring speed of 1000 rpm to simulate the movement speed of seawater over SLIPS and the speed was calculated to be about 439.6 m·min-1. After stirring for 1 day, 5days, 10 days and 15 days, the stability of the lubricant layer of SLIPS was evaluated through comparing the contact angle, sliding angle and sliding speed. 2.6. E. coli Settlement Analysis. Gram-negative E. coli cells were inoculated in LB broth for 24 h at 37 ℃ in a constant temperature incubator shaker (THZ-98C, Shanghai) with a shaking speed of 120 rpm. The LB broth included tryptone, sodium chloride, yeast extract power and deionized water in proportion to 2:2:1:200. A microplate reader (MD SpectraMax 190, USA) was employed to determine the concentration of E. coli at the wave length of 540 nm. Then, the concentration of E. coli cells was diluted to 108 cell/mL. The polished TC4 alloys, anodized TC4 alloys, modified TC4 anodic films and PFPE infused TC4 alloys were sterilized by ultraviolet radiation for 30 min and then immersed vertically in as-prepared bacterial solution and cultivated for 1 day, 5 days and 10 days in constant temperature incubator shaker. After cultivation, the samples were rinsed with 0.9 wt.% NaCl solution three times to remove the non-adherent bacteria. After that, every sample was put into 10 mL 0.9 wt.% NaCl solution and ultrasonically treated for 30 min to release the adherent bacteria cells. Next, 100 μL of ultrasonic bacteria solution was taken out and diluted to 10 times, 100 times and 1000 times respectively. Finally, 100 μL diluted bacteria solution was spread on agar plate and cultured at 37 ℃ for 24 h. The agar solution contained nutrient agar and deionized water in proportion to 8:25. The photographs of bacterial colonies on agar plates were recorded by camera. The toxicity of POTS and PFPE was evaluated by their effect on the growth of E. coli cells. 0.1 vol.% POTS and 0.1 vol.% PFPE were added into two groups of 50 mL E. coli solution with a concentration of 107 cell/mL, respectively. The other group of 6


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50 mL E. coli solution was as control group. Then three groups of solution were cultivated in constant temperature incubator shaker. When the cultivation time was 1 h, 6 h, 12 h, 24 h, the concentration of E. coli solution was measured by microplate reader. 2.7. Navicula exigua Settlement Analysis. The Navicula exigua solution with concentration of 1.5×105 cells·mL-1 was provided by the 725 Institute of China Shipbuilding heavy Industry Corporation. All the samples were immersed vertically in Navicula exigua solution and cultivated for 7 days and 14 days in biochemical incubator. The growth condition of Navicula exigua was keeping the lamp for 12 h of light and 12 h of darkness at 22 ℃. During cultivation process, 50 mL Navicula exigua solution was replaced by the fresh culture medium every 5 days to maintain the activity of Navicula exigua solution. After cultivation, the samples were rinsed with deionized water once to remove the seawater and unattached Navicula exigua. The quantity of Navicula exigua on all samples was counted by the Field emission scanning electron microscopy. Each sample was randomly selected 10 places with an area of 420 μm × 300 μm to observe the amount of algal adhesion and then calculated the average value. The antifouling efficiency of each sample was evaluated by the reduction ratio (R) of Navicula exigua compared to bare TC4 alloys. R

Ns  Nt Ns

Ns is the amount of algal adhesion on bare TC4 alloys, Nt is the amount of algal adhesion on SLPS. 2.8. Electrochemical Experiments. The corrosion resistance of anodized TC4 alloys, POTS modified TC4 anodic films and SLIPS was evaluated in 3.5 wt.% NaCl solution by electrochemical workstation (Chenhua, Shanghai) with the TC4 alloys as the working electrode, a platinum plate and a Ag/AgCl electrode as counter electrode and reference electrode, respectively. The electrochemical impedance spectroscopic (EIS) was carried out in the frequency range of 10-2-105 Hz when the open circuit 7



potential was at the equilibrium state. Tafel polarization curves were obtained by scanning a potential from -0.3 V to 0.3 V versus open circuit potential at a scan rate of 5 mV/s. The EIS and Tafel polarization curves were used to evaluate the corrosion resistance of the treated TC4 alloys. Each experiment was carried out in triplicates to ensure the reproducibility. 3. RESULTS AND DISCUSSION 3.1. Surface Morphology of TC4 Anodic Films. Three different kinds of nanostructures were fabricated on TC4 alloys by anodic oxidation as shown in Figure 2. The surface morphologies and size of nanostructures varied with anodizing voltage. When applied at a low voltage (10 V), the homogeneous nanopores with the diameter of around 20 nm and depth of 185 nm were generated on the surface of TC4 alloys (Figure 2 (a)(b)). With the increase of anodizing voltage, the nanopores were converted into neatly arranged nanotubes with the diameter of around 40 nm and depth of 250 nm (Figure 2 (c)(d)). The diameter and depth of nanotubes was further increased with the increasing voltage. The nanotubes with larger diameter of 100 nm and longer depth of 520 nm were obtained on the surface of TC4 alloys when the anodizing voltage was 30 V (Figure 2 (e)(f)). The internal volumes of per square micron on the three kinds of anodic films surface were calculated to be 1.533×10-2 μm3, 3.7680×10-2 μm3 and 14.287×10-2 μm3, respectively according the diameter and depth of nanostructures. It can be seen that the internal volume of nanotubes (14.287×10-2 μm3) was about 9 times as much as that of the nanopores (1.533×10-2 μm3). The nanotubes with the largest volumes could be conducive to store more lubricant.

8


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Figure 2. The FE-SEM images of the TC4 anodic films formed at different voltages:(a, b) 10 V, (c, d)15 V, (e, f)30 V. 3.2. Composition of TC4 Anodic Films. Figure 3 shows the Raman spectra of bare TC4 alloys and TC4 anodic films formed at different voltages. Compared with the bare TC4 alloys, the intense peaks of rutile TiO2 were appeared in the Raman spectra of TC4 anodic films. The peak at 141 cm-1 was assigned to the B1g vibrational mode. The peak at 443 cm-1 was responded to the Eg symmetry vibrational mode. The peak at 619 cm-1 was originated from the A1g vibrational mode. The broad peak at 233 cm-1 connected to B1g vibrational mode was also the characteristic peak of rutile TiO2 which was a compound peak. It was easy to see that when the oxidation voltage was 10 V, the characteristic peaks of rutile TiO2 were appeared but the intensity was very weak. With the increase of voltage, the intensity of each characteristic peak was increased, but no new peak was appeared. These results showed that the rutile TiO2 9



were formed on TC4 alloys through anodic oxidation. At the same time, the size of TiO2 nanotubes prepared at the anodizing voltage of 30 V was the largest on account of the intense peak corresponded to the more oxide, which was consistent with the results of Figure 2. Rutile TiO2 was reported to display no bactericidal property, so the three kinds of anodic films were not toxic to bacteria and algae43,44.

Figure 3. Raman spectra of bare TC4 alloys and TC4 anodic films formed at different voltages: 10 V, 15 V, 30 V. 3.3. Wettability Tests. It can be observed from Figure 4 (a1) that the contact angle of water droplet on the polished TC4 alloys was about 93°. After anodic oxidation, the contact angles were below 10° and the water could easily spread on the anodized TC4 alloys on account of the surface occupied by anodic films with the hydrophilic performance (Figure 4 (a2-a4)). The wettability of anodic films was changed by POTS modification to making lubricant easier to immerse into nanostructures. After modification, the anodic films revealed hydrophobicity and water droplet could stand on the porous structures known as Cassie state (Figure 4 (b)). Nevertheless, this state under water was unstable and easily transferred into Wenzel state for the reason that the gas phase between the solid and liquid was destroyed by larger water pressure. The contact angles decreased slightly with prolonging the immersion time in E. coli solution indicating that the durability of POTS modification layer was better. After PFPE infusion, the air pockets between 10


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porous structures were occupied by PFPE lubricant and the contact angles on the three kinds of SLIPS were decreased to about 120° (Figure 4 (c)). The sliding angles and the sliding speed of water droplets (5 μL) were about 2° and 8.5~9.5 mm/s on the three kinds of SLIPS, revealing the ultra-slippery characteristic of the lubricant layer. The contamination could be prevented from contacting the substrate by this ultra-slippery characteristic and taken away by water. To evaluate the long term stability of the lubricant layer in seawater, the SLIPS were immersed in 3.5 wt.% NaCl solution with continuous agitation to accelerate the failure process of lubricant layer. With the increase of immersion time, the contact angles and sliding speed on the three kinds of SLIPS were decreased and the sliding angles on the three kinds of SLIPS were increased for the running off of the lubricant (Figure 4 (c,d)). Due to the loss of the most of lubricant in the nanopores, the contact angle on SLIPS-10V was decreased to 96° and the sliding angle was increased to 45°, so the water droplet was hard to slide on the surface as the substrate was titled by 20° on the 10th day. On the 15th day, the sliding angle of SLIPS-15V was increased 30° and the water droplet was also hard to slide on the surface as the substrate was titled by 20°. While the nanotubes with the largest diameter and depth were conductive to storing more lubricant and releasing lubricant slowly, for this reason, SLIPS-30V displayed better stability under NaCl solution with the sliding speed of about 0.24 mm·s-1 on the 15th day.

11



Figure 4. Contact angles of water droplets on bare TC4 alloys (a1), anodized TC4 alloys at different voltages: (a2)10 V, (a3)15 V, (a4)30 V and POTS modified TC4 anodic films after exposure to E. coli solution for different days (b). Contact angles of water droplets on SLIPS after exposure to 3.5 wt.% NaCl solution for different days (c). Sliding angles and sliding speed of water droplets on SLIPS as the substrate was titled by 20° after exposure to 3.5 wt.% NaCl solution for different days (d). 3.4. Effect of SLIPS on E. coli Settlement. In order to evaluate the adhesion amount of E. coli on titanium alloys, TC4 alloys after exposure to E. coli solution were ultrasonically treated in 0.9 wt.% NaCl solution to remove adherent bacteria. The bacteria solution was then diluted and coated on the agar plates to observe the number of colony. As can be seen from Figure 5, the colony number of bare TC4 alloys, anodized TC4 alloys with different nanostructures and POTS modified TC4 anodic films was increased with the immersion time in E. coli solution, but there was no significant difference in colony number between them at the same immersion time, which indicated that the surface morphologies and wettability of the TC4 alloys showed negligible influence on the settlement of E. coli. Nevertheless, it was worth noting that the colony number of the three kinds of SLIPS was decreased 12


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significantly. In particular, there was no colony exist for SLIPS-15V and SLIPS-30V after exposure to E. coli solution for one day. This was due to the ultra-slippery behavior of lubricant layer, E. coli were hardly adhered to the substrate and easily taken away by water. The colony number of the SLIPS was increased as prolonging the exposure time in E. coli solution which caused by the running off of lubricant in nanostructures under the shear force of water. Among them, the colony number of SLIPS-10V was increased the most due to the loss of the most of lubricant. The colony number of the SLIPS-30V was still very little on the 10th day which was corresponded with the low sliding angle and high sliding speed of water on the SLIPS-30V on the 10th day. As a consequence, SLIPS could effectively suppress the adhesion of E. coli. and more lubricant stored nanostructures showed a long term effect on preventing bacteria adhesion.

Figure 5. Colony number of E. coli of different samples after exposure to E. coli solution for 1 day (a, b), 5 days (c, d) and 10 days (e, f): (b1, d1, f1) bare TC4 alloys, 13



(b2, d2, f2) 10V, (b3, d3, f3) 15V, (b4, d4, f4) 30V, (b5, d5, f5) 10V+POTS, (b6, d6, f6) 15V+POTS, (b7, d7, f7) 30V+POTS, (b8, d8, f8) SLIPS-10V, (b9, d9, f9) SLIPS-15V, (b10, d10, f10) SLIPS-30V. The charts on the left are corresponding to the pictures of bacterial colonies on the right. 3.5. The green inhibition mechanism of SLIPS to E. coli. In Figure 6, compared with the growth curve of E. coli, the addition of POTS and PFPE didn’t influence the growth of E. coli indicating that POTS and PFPE were not toxic to E. coli. At the same time, the nanostructures with rutile TiO2 also have no bactericidal effect on E. coli. Hence, the suppress of E. coli settlement on SLIPS was due to the ultra-slippery behavior of lubricant layer which offered a surface E. coli can hardly attach under a low incline angle. At the same time, nanostructures played an important role in storage of lubricant. The nanotubes with the largest internal volumes could store more lubricant by the capillary forces and also possessed stronger slow-release effect so that a long-term bacteriostatic effect was obtained after lubricant infusion.

Figure 6. Growth curves of E. coli cultivated with POTS and PFPE. 3.6. Effect of SLIPS on Navicula exigua Settlement. Figure 7 showed the images of the Navicula exigua settled on bare TC4 alloys and SLIPS. The average number of Navicula exigua on all coatings was showed in Figure 8. It can be seen 14


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that, after cultivating in Navicula exigua solution for 7 days, the quantity of Navicula exigua on SLPS was obviously much lower than that on bare TC4 alloys which caused by the ultra-slippery behavior of lubricant layer so that the Navicula exigua was hard to adhere to the TC4 alloys. Among the three kinds of SLIPS, the SLIPS-30V revealed the lowest adhesion quantity with 3 cells per 0.126 mm2 and the SLIPS-10V revealed the highest adhesion quantity with 14 cells per 0.126 mm2. The reason may be that there was more lubricant left in the largest nanotubes than in nanopores. So the largest nanotubes played positive effect on inhibiting the settlement of Navicula exigua. After cultivating in Navicula exigua solution for 14 days, the anti-adhesion behavior of SLIPS-30V was also maintained and the reduction ratios was 88%. As a consequence, the SLIPS displayed a significant reduction of Navicula exigua compared to the bare TC4 alloys.

Figure 7. The FE-SEM images of the Navicula exigua settled on bare TC4 alloys (a1,a2), SLIPS-10V (b1,b2), SLIPS-15V (c1,c2) and SLIPS-30V (d1,d2) after cultivating for 7 days (a1,b1,c1,d1) and 14 days (a2,b2,c2,d2).

15



Figure 8. The average number of Navicula exigua on bareTC4 alloys and SLIPS after cultivating for 7 days and 14 days. 3.7. Electrochemical Tests. Electrochemical tests of bare TC4 alloys, anodized TC4 alloys, POTS modified TC4 anodic films and SLIPS were carried to evaluate the effect of anodic oxidation, chemical modification and lubricant infusion on corrosion resistance of TC4 alloys (Figure 9(a, b)). The electrochemical parameters (Ecorr, icorr and Z) obtained from Tafel polarization curves and electrochemical impedance spectroscopic were showed in table 1. In Figure 9(a1), compared with the bare TC4 alloys, the impedance of TC4 alloys anodized at 10 V was increased one order of magnitude, indicating that the corrosion resistance of TC4 alloys was improved for the barrier effect of anodic films on corrosive medium. After POTS modification, the impedance of the TC4 alloys was further improved because of the repellency against water of hydrophobic surface. Significantly, after lubricant infusion, the impedance of SLIPS-10V was two orders of magnitude larger than that of bare TC4 alloys because the hydrophobic lubricant layer effectively prevented the infiltration of corrosive medium. So the corrosion resistance of SLIPS-10V was improved by anodic oxidation, chemical modification and lubricant infusion. The same result was showed in Tafel polarization curves (Figure 9(b1)), the corrosive potential (Ecorr) of SLIPS-10V was 0.523 V higher than of bare TC4 alloys and the corrosive current density (icorr) of SLIPS-10V was two order magnitudes lower than that of bare TC4 alloys. For the TC4 alloys anodized at 15 V and 30 V, after POTS modification and 16


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lubricant infusion, the change trend of impedance, Ecorr and icorr was the same as that of TC4 alloys anodized at 10 V, indicating that anodic films, hydrophobic surface and lubricant layer had synergistic effect on the improvement of corrosion resistance of TC4 alloys (Figure 9(a2, a3, b2, b3)). In particular, the difference of corrosion resistance of the three kinds of SLIPS was small, revealing that the lubricant layer played an important role in preventing corrosive medium. Nevertheless, the corrosion resistance of three kinds of SLIPS was decreased after immersing in NaCl solution for 10 days and the difference in corrosion resistance of the three kinds of SLIPS was significant caused by the loss of lubricant. The SLIPS-30V displayed the highest corrosion resistance for the behavior of releasing lubricant slowly and the most of the lubricant still remained in the nanotubes. The corrosion resistance of SLIPS-10V was the lowest but still higher than that of bare TC4 alloys. Hence, the SLIPS displayed a positive effect on improving the corrosion resistance of TC4 alloys

Figure 9. Electrochemical impedance spectroscopic and Tafel polarization curves for bare TC4 alloys, anodized TC4 alloys, POTS modified TC4 anodic films, SLIPS and SLIPS immersed in 3.5 wt.% NaCl solution for 10 days: 10 V (a1, b1), 15 V (a2, b2), 30 V(a3, b3). Table 1 Electrochemical parameters of bare TC4 alloys and treated TC4 alloys. Samples

log(icorr/(A·cm-2))

Ecorr(V) 17


log(Z/Ohm)


Page 18 of 25

Bare TC4

-0.560

-6.068

4.561

10V

-0.190

-6.468

5.226

15V

-0.041

-6.486

5.478

30V

-0.351

-6.281

4.972

10V+POTS

-0.129

-7.207

5.741

15V+POTS

0.000

-7.465

6.059

30V+POTS

-0.020

-7.025

5.585

SLIPS-10V

-0.037

-8.366

6.804

SLIPS-15V

0.016

-8.592

6.980

SLIPS-30V

-0.028

-8.348

6.876

SLIPS-10V-10days

-0.214

-7.599

5.695

SLIPS-15V-10days

-0.153

-7.613

5.637

SLIPS-30V-10days

-0.053

-7.732

6.055

4. CONCLUSIONS In summary, SLIPS obtained on TC4 alloys by the anodic oxidation, POTS modification and PFPE lubricant infusion showed the significant fouling resistance performance for E. coli and Navicula exigua on account of the ultra-slippery performance of lubricant layer. Simultaneously, the SLIPS could improve the corrosion resistance of TC4 alloys for synergistic effect of anodic films, POTS layer and lubricant layer. Nevertheless, the three kinds of nanostructures prepared by anodic oxidation and the hydrophobic surface themselves had negligible effect on preventing bacterial adhesion. Among three kinds of SLIPS, the SLIPS-30V performed the best anti-fouling performance and corrosion resistance because the nanotubes utilized had the largest internal volumes of 14.287×10-1 μm3 for per square micron on TC4 anodic film, which were conducive to storing more lubricant and also 18



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holding the lubricant for a long time. Hence, this work provided a simple and efficient anti-fouling method for titanium alloys, which also would be potential to apply to other alloys. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Tel: +86 574 86694901; Fax: +86 574 86685159 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is supported by the National Key Basic Research Program of China (973) (2014CB643305), the National Natural Science Foundation of China (51775540), and the Youth Innovation Promotion Association, CAS (2017338).

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Fabricating bionic ultra-slippery surface on titanium alloys with

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