Anti-biofouling superhydrophobic surface fabricated by picosecond laser texturing of stainless steel

Applied Surface Science 436 (2018) 263–267

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Anti-biofouling superhydrophobic surface fabricated by picosecond laser texturing of stainless steel Ke Sun a,1 , Huan Yang a,b,1 , Wei Xue a , An He a , Dehua Zhu a , Wenwen Liu a , Kenneth Adeyemi a , Yu Cao a,∗ a

Zhejiang Key Laboratory of Laser Processing Robot, College of Mechanical & Electrical Engineering, Wenzhou University, Wenzhou, 325035, China International Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology of Ministry of Education, College of Optoelectronic Engineering, Shenzhen University, Shenzhen, 518060, China b

a r t i c l e

i n f o

Article history: Received 4 September 2017 Received in revised form 30 November 2017 Accepted 2 December 2017 Available online 7 December 2017 Keywords: Anti-biofouling Superhydrophobic surface Laser ablation Laser induced surface hierarchical micro-nano structures Stainless steel

a b s t r a c t Anti-biofouling technology is based on specifically designed materials and coatings. This is an enduring goal in the maritime industries, such as shipping, offshore oil exploration, and aquaculture. Recently, research of the relationship between wettability and antifouling effectiveness has attracted considerable attention, due to the anti-biofouling properties of the lotus leaf and shark skin. In this study, super-hydrophobic surfaces (SHSs) with controllable periodic structures were fabricated on AISI304 stainless steel by a picosecond laser, and their anti-biofouling performance were investigated by seawater immersion for five weeks in summertime. The results showed that the specimens with SHS demonstrate significant anti-biofouling effect as compared with the bare stainless steel plate. We observed that nearly 50% decrease of the average microbe attachment area ratio (Avg. MAAR) could be obtained. The micro-groove SHS with more abundant hierarchical micro-nano structures showed better anti-biofouling performance than the micro-pit SHS. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Biofouling is the accumulation of microorganisms, plants, algae, or animals on wetted surfaces. As commented by Salta et al. [1], whatever surface (natural or artificial) immersed in seawater, is affected by biofilm attachment. Marine biofouling has become a worldwide problem affecting maritime and aquatic industries such as marine vessels, underwater constructions, and desalination plants [2–4]. Biofouling on ship’s hull can lead to increased hydrodynamic friction drag of up to 60%, which may require up to a 40% increase in fuel to compensate [3], in addition, it could cause significant increase in maintenance costs, the emissions of carbon dioxide, and sulfur dioxide [5]. Superhydrophobic surface (SHS) that mimics the lotus leaf and shark skin has been proved to be a promising technique for suppressing biofouling [6–8], and the artificial micro-nanoscale hierarchical structures with low wettability could effectively

∗ Corresponding author. E-mail address: [email protected] (Y. Cao). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.apsusc.2017.12.012 0169-4332/© 2017 Elsevier B.V. All rights reserved.

reduce the microbes attached to the material’s surface. Spasova et al. [9] designed a low wettability plastic (Polyvinylidene Fluoride) which not only improved the thermal stability of nano-fibrous materials, but imparted the materials with anti-adhesive and antimicrobial properties. Liu et al. produced a SHS loaded by silver nanoparticles (Ag-NPs), which could effectively prevent corrosion of the Al substrate and bacterial absorption, as confirmed by electrochemical impedance spectroscopy [10]. Furthermore, polyelectrolyte multilayers [11,12], fiber glass [13], polymers [14] and hydrogels [15] were also used to form antimicrobial films on metal substrates. However, as to the above-mentioned methods, the disadvantages such as insecure metal-to-material adhesion [16], complex preparation steps and expensive materials requirements imposed restrictions on practical applications [17]. For metal materials, SHSs are usually achieved by creating a rough surface structure, and then combined with depositing a layer of low surface energy chemical molecules [18,19]. In this work, the SHSs with hierarchical structures on stainless steels were fabricated by a picosecond (ps) laser. The anti-fouling behaviors of the SHS specimens with different surface morphologies were investigated in seawater environments, and the effect of the surface

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Fig. 1. Schematic diagram of the picosecond laser galvo-scanning system and the designed patterns of micro-groove/micro-pit array for the stainless steel SHSs.

topography on the anti-fouling characteristics of the SHSs was also presented in this paper. 2. Materials and methods A 50 × 50 mm2 AISI304 stainless steel sheet with thickness of 2 mm was used as the specimen substrate. The specimens were ultrasonically cleaned in an ethanol bath for 10 min before and after being textured by the ps laser. The ultrafast laser micro-machining system includes a 515 nm wavelength laser (TRUMPF, micro 5000) with pulse width of 10 ps at 400 kHz PRF (pulse repetition frequency) and a 2-D galvo-scanner system, as shown in Fig. 1. The focal plane was fixed on the specimen’s surface and the focused laser beam was approximately 20 ␮m in diameter. Micro-groove and micro-pit array with the designed geometric profile are shown in Fig. 1, the designed depth of micro-groove and micro-pit was 5 ␮m and the line width/diameter was 25 ␮m, the spacing of microgroove and micro-pit array was set at 25 ␮m. For fabricating the micro-pit array, a 2500 mm/s scanning speed was used with a PRF of 100 kHz, and the scanning time was 30 s. Because the distance between the adjacent pulses (25 ␮m) was bigger than the laser spot, the micro-pit was obtained by punching the pulses at the same location. A 400 mm/s scanning speed combined with a PRF of 400 kHz was chosen to fabricate the micro-groove. The laser ablated specimens were chemically modified by spin coated with silicone sol (30 wt.% spherical silica NPs with the diameter ranges from 10 nm to 20 nm; provided by Aladdin Co.), then being heat treated at 270 ◦ in air for 30 min. Silica NPs are wide used for modifying metal surface to achieve superhydrophobicity [20]. The solution of silica nano particles (silicone sol) used in this paper is carried out with the reaction of the proportional mixture of tetraethoxysilane (TEOS), ammonium hydroxide, absolute ethanol and methyltriethoxysilane under certain conditions. The heat treatment promotes the hydrolysis and polycondensation reactions which leads to form Si—CH3 groups on the silica NPs that contribute to lower the solid surface energy and achieve a firm adhesion between the substrate and the NPs [21]. Static contact angles (SCAs), advancing contact angles (␪adv ), receding contact angles (␪rec ) [inserted in Fig. 4(a)] and sliding angles (SAs) were measured five times at different tested area using a video optic CA instrument (Dataphysics, OCA15EC) equipped with a goniometer.

Fig. 2. Schematic diagram of the seawater immersion test.

The images of a 3 ␮L distilled water droplet placed on the specimen’s surface were analyzed by a Low-Band Axisymmetric Drop Shape Analysis (LBADSA) software. The temperature and relative humidity in the experiment were 25 ◦ and 65% RH, respectively. The geometric profiles of the laser ablated surfaces were then measured by a profilometer, and the surface topographies were investigated using a scanning electron microscope (SEM, Zeiss, supra 55). As to describe the progress of biofouling on the specimen surfaces over time, a laser scanning confocal microscopy (OLYMPUS, OSL 4100) was used to detect and estimate the area of the attached microbes. In each round of the biofouling progress test, 5 rectangle areas with the same size of 1 mm2 on the specimen surface were randomly selected to calculate the average microbe attachment area ratio (Avg. MAAR). The rate is defined as the following equation: Avg.MAAR = Af /At

(1)

where Af is the total area of the attached microbes; At is the total sampling area, specifically 5 mm2 in this study. Seawater immersion tests were conducted for 36 days during summertime in the Dongtou island, Zhejiang province, China (27◦ 51 N, 121◦ 08 E, East China Sea), the location was selected in a small village close to the marine protected area, far away from the sources polluted by harbors and heavy anthropic activities. The natural conditions of the seawater is listed in Table 1. The specimens were installed on a plastic floating anchor which made the SHSs lie vertically at a depth of about 30cm, as shown in Fig. 2. 3. Results and discussions Fig. 3 shows the effect of the laser fluence on the ablation depth and groove-width/ pit-diameter. When the laser fluence is lower than the threshold value of 0.9 J/cm2 , the surface gets very shallow laser marking traces. Along with the increasing laser fluence, a continuous increase of the ablation depth occurs, while the ablation width and pit diameter increase rapidly to the saturation values which is determined by the focused laser spot diameter and the Gaussian energy distribution [22]. Therefore, the optimized laser fluence for fabricating the micro-groove and micro-pit array were selected as 4 J/cm2 and 9 J/cm2 , respectively.

Table 1 The natural conditions of seawater for immersion test. Months

DIN (mg/L)

Phosphate (mg/L)

N/P

Water Temp. (◦ C)

pH Value

Salinity (‰)

JUNE JULY

0.30 0.24

0.05 0.10

6.15 2.48

20.92 27.00

8.10 8.01

30.34 33.91

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Fig. 3. Ablation depth and width/diameter as function of the laser fluence for (a) micro-groove, (b) micro-pit.

Table 2 The static contact angle (SCA), advancing contact angles (␪adv ), receding contact angles (␪rec ), sliding angles (SAs) of the specimens before and after seawater immersion test. Before immersion test Specimen Bare SS plate Micro-groove SHS Micro-pit SHS a b

SCA 73◦ 152◦ 151◦

␪adv 77◦ 156◦ 154◦

␪rec 54◦ 149◦ 145◦

SA 23◦ a 3◦ b 6◦ b

After immersion test SCA 51◦ 92◦ 87◦

␪adv 63◦ 103◦ 96◦

␪rec 31◦ 79◦ 69◦

SA 32◦ a 24◦ a 27◦ a

Contact angle hysteresis. Sliding angles.

Fig. 4. Images of laser textured specimen surface. (a)–(d) SEM images of microgroove and micro-pit arrays, and inserts are ␪adv , ␪rec and SCA images, (e) and (f) are the cross-sectional profiles of micro-groove and micro-pit arrays respectively.

The SEM images of the laser textured stainless surface morphologies are shown in Fig. 4, where the laser induced hierarchical micro-nano structures appear on the stainless surfaces. The periodic patterns of micron-sized groove [Fig. 4(a)] and pit [Fig. 4(b)] arrays are decided by the laser processing parameters, such as laser fluence, scanning speed and spacing. Meanwhile, spherical nanoparticles with diameter of a little more than 100 nm can be observed in the higher magnification SEM images [Fig. 4(c) and (d)], which were attributed to the rapid cooling of the melt ejected liquid in the localized melt region [23,31]. Moreover, the surface with the micro-groove array presents much more abundant nano structures than that with micro-pit array. This disparity is caused by the different amount of laser induced recast layer since ps laser ablation always has minor but inevitable metal melting effect. The micropit is formed by multiple laser pulses punched at the same location, where the overlapping heat accumulation leads to more recast layer at the side wall and bottom surface than the micro-groove formed by one-time laser scanning. However, the wettability test results inserted in Fig. 2(c) and (d) show that the specimens with microgroove array and micro-pit array have the quite similar SCAs which are 151 ± 2◦ and 152 ± 2◦ respectively, besides the SAs are 3◦ and

Fig. 5. The evolution of Avg. MAAR of specimen surfaces with different surface morphologies during seawater immersion test.

6◦ (Table 2). The mechanism can be described by the Cassie state wetting model, for the metal SHSs fabricated by the ps laser, the water droplets could not completely wet the roughened substrate, only contacting the peak of the micro-scale burrs. Therefore, the micro-pit’s smooth side wall and bottom surface that lack of nano structures have no attenuation to its superhydrophobicity, as compared to the surface of micro-groove array. Fig. 5 shows the Avg. MAAR evolution of the specimen surfaces with different surface morphologies. Within 5 weeks immersed in the seawater, the specimens with SHS demonstrate significant anti-biofouling effect as compared with the bare stainless steel plate, specifically, nearly 50% decrease of the Avg. MAAR could be obtained. Moreover, the SHS with the micro-groove pattern shows better anti-biofouling performance than that with the micro-pit pattern. After 5 weeks of immersion, the Avg. MAARs of the bare specimen, the micro-pit array SHS and the micro-groove array SHS were 92%, 57% and 43%, respectively.

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anism agrees with the reported anti-fouling property of the SH coating with chemical layer and composite materials [19]. 4. Conclusion A fast, highly controllable ps laser patterning way for preparing hierarchical micro/nanostructures, combined with the chemical modification by silica sol, was proposed to fabricate the antibiofouling stainless steel SHSs. The results of five weeks seawater immersion test showed that the specimens with SHS demonstrate significant anti-biofouling effect as compared with the bare stainless steel plate, the SHSs with micro-pit array and micro-groove array lead to the Avg. MAARs decrease from 92% to 57% and 43%, respectively. The laser induced micro-nano structures of the microgroove array were much more abundant than that of the micro-pit array, which led to the better anti-biofouling performance. It indicated that the interconnected micro-structure and abundant nano-structure are very important elements to anti-biofouling SHSs. The anti-biofouling SHS fabricated by the picosecond laser could be used for suppressing the biofouling of marine equipment, especially for the communication devices, and detection components like sonar dome, and the underwater electromagnetic window covers. Fig. 6. Photographs of SHSs after being immersed for about five weeks. (a)SHS with micro-groove pattern; (b) SHS with micro-pit pattern.

Fig. 6 shows the photographs of the SHS specimens immersed in seawater for five weeks. The microbial organisms fouling appears more frequently in the specimen with micro-pit structure. The reasons could be concluded as follows; firstly, the micro-pit provides a “harbor” for the microbial organisms to avoid being washed away by the water current, but in the interconnected micro-grooves, the microbial organisms would be more easily to be washed away. On the other hand, the nano-structure in the micro-groove SHS is more abundant than that in the micro-pit SHS, leading to a lower SA which is induced by an air film trapped inside the nano-structure [24], and the air film may prevent the nano-scale biological entities from adhering to material surface, which could enhance the anti-biofouling property of the SHS. In addition, the interspace in the nano-structure of the micro-groove SHS is significantly smaller than the micro-scaled biological entities [25], which might lead the micro-groove SHS to be more unlikely settled with the microorganisms [26]. The SCAs and SAs with advancing contact angles (␪adv ), receding contact angles (␪rec ) of the specimens were tested before and after seawater immersion, as listed in Table 2. All the specimens were ultrasonically cleaned before the tests. Before the immersion test in seawater, the water droplet adopt the Cassie state on the superhydrophobic surfaces (SHSs), and it can slide off from the surface while we slant the specimens at a finite small angle (3◦ and 6◦ , respectively); However, after immersion test, the chemical modified layer is peeled off gradually by the flowing seawater [27], resulting in the loss of superhydrophobicity, and the SCA decreasing to 92◦ and 87◦ , respectively. The droplets are adhered tightly to the surface and could not slide off from the surface at a finite angle, indicating that the wetting state of the SHSs is changed from Cassie state to Wenzel state [28,29]. To clarify the changes in surface wettability above, the SA after seawater immersion test is represented by the contact angle hysteresis, which is obtained by subtracting ␪rec from ␪adv [30]. The SCA of the micro-groove pattern SHS decreases from 152◦ to 92◦ and the SA increases from 7◦ to 24◦ , meanwhile the CA of SHS with micro-pit pattern decreases from 151◦ to 87◦ , and the SA increases from 9◦ to 27◦ . The increasing Avg. MAAR with the immersion time is ascribed to the gradual deterioration of the SCA and SA properties. The mech-

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