Facile fabrication of biomimetic superhydrophobic surface with anti-frosting on stainless steel substrate

Applied Surface Science 355 (2015) 1238–1244

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Facile fabrication of biomimetic superhydrophobic surface with anti-frosting on stainless steel substrate Yan Liu a , Yuan Bai a , Jingfu Jin b , Limei Tian a,∗ , Zhiwu Han a , Luquan Ren a a b

Key Laboratory of Bionic Engineering(Ministry of Education), Jilin University, Changchun 130022, PR China College of Biological and Agricultural Engineering, Jilin University, Changchun 130022, PR China

a r t i c l e

i n f o

Article history: Received 25 April 2015 Received in revised form 28 June 2015 Accepted 4 August 2015 Available online 6 August 2015 Keywords: Biomimetic Superhydrophobic Stainless steel Anti-frosting Chemical etching

a b s t r a c t Inspired by typical plant surfaces with super-hydrophobic character such as lotus leaves and rose petals, a superhydrophobic surface was achieved successfully by a chemical immersion process. Here, 304 SS (stainless steel) was used as substrates and a micro-nano hierarchical structure was obtained by chemical etching with a mixed solution containing ferric chloride. The results showed that the water contact angle (WAC) decreased obviously due to surface morphology changing after chemical etching process. However, we obtained a superhydrophobic surface with a WAC of 158.3 ± 2.8◦ after modification by DTS (CH3 (CH2 )11 Si(OCH3 )3 ). Furthermore, the superhydrophobic surface showed an excellent anti-frosting character compared to pure staining steel. The surface morphology, chemical composition and wettability are characterized by means of SEM, XPS and water contact angle measurements. This method could provide a facile, low-cost and stable route to fabricate a large-area superhydrophobic surface with antifrosting for application in various environments including in humid condition. © 2015 Elsevier B.V. All rights reserved.

1. Introduction In nature, various plants and insects, including lotus leaves [1–4], rose petals [5], and legs of water striders [6,7], exhibit the unusual phenomenon of wettability. Wettability is one of the most important characters of material surfaces from both fundamental and practical aspects [8], which is governed by both the chemical composition and the geometrical microstructure of the surface [9–13]. Superhydrophobic surfaces, with a static contact angle (CA) higher than 150◦ and low contact angle hysteresis [14], have been extensively studied and possess very important applications in preventing ice/frost [15,16], microfluidics [17–19], collecting water [20,21], anti-corrosion [22,23] and self-cleaning [24–26] etc. Stainless steel as a common metallic materials, for its superior corrosion resistance and decorative function, has been employed for applications in various fields, including petrochemical, construction, maritime and aviation industries.[27] However, ice and frost formation and accumulation on substance surfaces are known to cause serious problems, such as hindering the operation and impairing the efficiency of infrastructural components and machines, including aircrafts, ships, electrical power plants, transportations telecoms equipment and aeronautics and astronautics.[28] In recent decades, efforts have been made to

∗ Corresponding author. http://dx.doi.org/10.1016/j.apsusc.2015.08.027 0169-4332/© 2015 Elsevier B.V. All rights reserved.

look for anti-ice/frost materials which can effectively retard and prevent ice/frost formation on cold surfaces. Relevant results have confirmed that the super-hydrophobic materials are promising candidates [29]. However, the highest water contact angles reported on smooth, low energy surfaces are in the range of 120◦ [14]. Inspired from the lotus leaf, researchers have demonstrated the wettability of solid surface is determined by both the chemical compositions and geometrical microstructures of the surfaces [1,30–34]. Therefore, a rough surface texture is necessary for the preparation of super-hydrophobic surface. Yan et al. [35] systematically researched the characteristics of the surface patterns based on nanoparticles and the formed wettability. Conventionally, two approaches can be used to develop the superhydrophobic surface: one is to fabricate surface with micro-nano hierarchical structures on low surface-energy materials; the other is to modify rough solid surface by low surface-energy molecules. By now, a variety of biomimetic surfaces with superhydrophobicity have been fabricated based on the combination of surface micro-nano structures and chemical compositions by using many different synthetic methods, including sandblasting [36,37], sol–gel methods [38,39], chemical vapor deposition [40,41], electrochemical deposition [42,43], chemical etching [44,45] and laser surface treatment [46,47] etc. Recently, some achievements on the creation and characterization of stable superhydrophobic surfaces on stainless steel have been made. Bizi et al. [46] obtained a multi-scale corrugated structure by femtosecond laser, making transformation

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from hydrophilicity to hydrophobicity on the surface of AISI 316L SS. Han et al. [42] fabricated lotus-leaf-like superhydrophobic metal surfaces by a simple electrochemical reaction on steel sheets with sulfur gas, and a subsequent fluorosilane treatment. Liang et al. [43] prepared superhydrophobic nickel films on SS316L substrates by a combined electrodeposition and fluorinated modification approach. However, most approaches involve severe conditions, including dangerous chemicals, low processing efficiency, complex devices and high processing cost. To solve the aforementioned disadvantages, a facile, highly effective, and low-cost approach was adopted to fabricate the surperhydrophobic surface on SS substrate. In our paper, we present a simple method to fabricate superhydrophobic surfaces on 304 SS with micro-nano structure which were then modified by DTS. Furthermore, the anti-frosting behavior of superhydrophobic SS surfaces was investigated, which indicated that the frosting of superhydrophobic surface was greatly retarded compared with pristine SS surface. Fig. 1. The schematic representation of the experimental setup.

2. Experimental 2.1. Chemical etching SS304 sheets (0.08 wt% C, 18.17 wt% Cr, 9.3wt% Ni, 1.54 wt % Mn, 0.8 wt%Si, and the balance iron) with the size of 20 mm × 20 mm × 1 mm were polished with 500# and 800# sandpapers in turn, and then cleaned with acetone in an ultrasonic bath for 15 min. Samples of SS were immersed in a solution mixture of ferric trichloride aqueous solution (FeCl3 , 1.65 mol/L), hydrochloric acid (HCl, 37%) and hydrogen peroxide (H2 O2 , 30%) (15:1:1, vol%) for 20 min. Here the (FHH FeCl3 + HCl + H2 O2 (15:1:1, vol%)) solution was used in this work as etching solution. After etching treatment, samples were again rinsed with DI and dried in atmosphere condition. The treated samples desiccated in an oven at room temperature (26 ◦ C) for 15 min. 2.2. Hydrophobic film modification In order to obtain superhydrophobic surface, DTS was used to modify SS film. The samples was placed in a modified solution prepared by adding 60 ␮L DTS to 40 mL Toluene (C6 H5 CH3 ) for 60 min, and then desiccated in an oven at 30 ◦ C. 2.3. Characterization The obtained samples were characterized by scanning electron microscopy (SEM) (JBM-7500F, Japan Electronic). The surface chemical composition was examined by X-ray photoelectron spectroscopy (XPS, SPECS XR50). The contact angles were measured with a contact angle meter (JC2000A Powereach, China), where 3 ␮L droplets were placed at three different places of the surface for investigation and the average value was taken as the contact angle used to measure the static water contact angle on the SS film. 2.4. Experiments of anti-frosting property The experimental apparatus was composed of temperature control, image acquisition and data collection systems. The schematic diagram of the experimental setup is shown in Fig. 1. This apparatus included a thermostatic bath, K-type thermocouple, data acquisition (DAQ11625, Quatronix, China), a microscope (MZDM0745, MT, China) and a computer etc. The frosting processes were monitored and collected by a CCD camera (73X11H, Mintron, China). The surface temperature of the samples on the cold plate (40 mm × 40 mm × 3 mm) was precisely maintained in a range of 20–35 ◦ C. The temperature was measured by a K-type thermocouples which was

connected with the cold plate and the measurement error was ±1.0%. 3. Results and discussion 3.1. Microstructure To obtain rough microstructure on SS surface, the samples were chemically etched for 20 min in FHH solution with 0.5 mol/L, 1 mol/L, 1.5 mol/L, 2 mol/L and 2.5 mol/L FeCl3 , respectively. During the FHH etch process, SS samples change from their well-known, shiny silver appearance to black due to the added surface roughness and changes in chemical surface composition. Through the redeposition of metal chloride and oxides, roughness is created with micro/nano scale on SS surface. Fig. 2 shows the SEM images of an as-prepared surface. It can be found that FeCl3 , as an etching agent, plays a major role in the forming process of the hierarchical structure. With the reaction going on, the SS surface was gradually etched and irregularly-shaped micro-nano islands and concave pits were formed in various sizes on the specimen surface at different FeCl3 solution concentration. As shown in Fig. 2a, the micro-scale pits structure appeared on the surface of SS, which was arising from the lack of iron on SS surface after FeCl3 etching. As the etching concentration was increased, the pits become deeper and smaller, constituting a rough micro-nano multiscale structure on the surface. The micro-nano structure gradually increased with the increasing of FeCl3 solution concentration, as shown in Fig. 2b and c. When further increasing the concentration of ferric chloride, the solution concentration reached 2 mol and the rough micro structures were relatively small and more intensive. The structured surfaces were of a combination of a coarse microstructure and submicrometer-sized structures, as shows in Fig. 2d, and the structure was lamellar films interlaced whose structure can trap a large amount of air. However, as the FeCl3 density is continuously increased and reaches 2.5 mol/L, it can be found that the hierarchical structure on the SS surface is destroyed. We further analyzed the reasons for the formation of micronano structures on SS surface. In the etching process, besides FeCl3 concentration, there are a number of other factors determining the SS morphology, such as etching time, etchant composition, the characteristics of the metal being etched, and so forth. The reaction mechanism of stainless steel in etching solution can be represented as: 2FeCl3 + Fe → 3FeCl2

(1)

As is known to all, there are many microscopic defects and faults between the crystals of stainless steel. When the etching agent

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Fig. 2. SEM images of the stainless steel surface under different FeCl3 concentration for (a–e) is 0.5, 1, 1.5, 2 and 2.5 mol/L, respectively.

reaches a certain concentration, the stainless steel with microscopic defects and faults priority dissolves and the concentration of ferrous ion increases in the etching solution. As the etching proceeds, Fe2+ increases and deposition on the surface being etched makes penetration of the etchant to the etched surface difficult. The effect of HCl was studied by keeping the concentrations of FeCl3 and H2 O2 in the etching solution constant. By addition of hydrochloric acid, the solubility of ferrous ion in the etchant solution can be increased and therefore results in a slight increase in the etch rate of SS surface. Furthermore, the addition of H2 O2 is needed to oxidize Fe2+ into Fe3+ , keeping the etching reaction sustained. As shown in Fig. 3, after etching treatment with FHH solution for 10 and 20 min respectively, there is no significant difference in the morphology of sample surfaces and the contact angle just increased from 154◦ to 158◦ after DTS modification. The experiment results show that at the initial stage of etching reaction, which is very intense and accompanied by a lot of bubbles, the micro-nano structure appeared and gradually increased. However, with the decrease of the ion concentration, the etch rate greatly slowed down when the etching time exceeded 10 min. The results also indicate that to obtain micro-nanostructure on stainless steel substrates, an etchant consisting of FeCl3 , HCl and H2 O2 are needed. 3.2. Chemical composition XPS is used to test the chemical composition of the as-prepared surfaces modified by DTS after etching. Fig. 4a and b show the XPS survey spectrum of the chemical composition on the modified surfaces. It reveals the presence of C, Si and O on the as-prepared surfaces. The modified surface shows significant peaks at 284.7 eV and 102.75 eV corresponding to the C1s peak and Si 2p peak. The composition of Si, C increased remarkably compared to the substrate, implying that the SS surface has been covered with silane film. As a result, the obtained surfaces were able to possess both micro-nano hierarchical structure and low surface energy at the same time, thus providing basic conditions for superhydrophobicity. Furthermore, the molecules grafted SS substrates through the surface reaction of the hydrolysis silane species with the functional groups (Fe–OH) of metallic surface to form self-assembled monolayer (SAM) with low surface energy as shown in Fig. 5. The formation mechanism of SAMs is as follows: first, the hydrolysis reaction is initiated between methoxy groups (–OCH3 ) and water

Fig. 3. SEM images of the stainless steel surface under different etching time for (a) 10 and (b) 20 min, respectively. The inset of (b) shows the high magnifications of the stainless steel etched by 2 mol FeCl3 solution.

(H2 O) to form silanols (Si–OH) (Eq. (2)), and then the silanols react with the surface functional groups of hydroxyl (–OH) on SS surface to form a self-assembled film (Eq. (3)). In addition, the silanols can also bond with other silanol by forming a siloxane bond (Si–O–Si)

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Fig. 4. XPS spectrum of the as-prepared superhydrophobic stainless steel surface surface of (a) full-spectrum and (b) Si 2p.

Fig. 6. Contact angles at different FeCl3 concentration.

Si–OH + CH3 OH

(2)

between the droplets and the hierarchical structure of the coating surface. Afterwards, with further increase of FeCl3 concentration, the excessive etch of SS resulted in the destroying the complex hierarchical structure, and therefore the contact angle values decreased slightly to 154.5 ± 3.2◦ . Fig. 6 indicates that as FeCl3 concentration was raised from 0.5 to 2 mol of FHH solution, the density of the surface microstructures gradually changed, and the superhydrophobic property was slightly enhanced.



(3)

3.4. Anti-frosting properties

(4)

In order to reveal the anti-frosting property of superhydrophobic surface, a study on the frost formation on plain SS surface and superhydrophobic SS surface with a size of 20 mm × 20 mm were carried out at room temperature of 14 ◦ C, relative humidity of 60% and cold surface temperature of −22 ◦ C. Fig. 6a–d shows optical photographs of frost formation on plain SS with a CA of 66◦ at different time. As a comparison, the superhydrophobic SS surface with a CA of 158.3◦ was obtained by etching process and then modified with DTS (Fig. 7e–h). In Fig. 7 the frost-free regions are separated from the regions covered by frost with red lines. It was found that the frost size and distribution were completely different from each other on the superhydrophobic surface and the plain SS surface. The starting time for the observable frost crystal was also different. The frost particles of several microns formed on plain SS surfaces after

Fig. 5. Schematic illustrating the formation of DTS self-assembled monolayers on stainless steel.

(Eq. (4)). After modified with self-assembled films, the hydrophilic SS surface becomes hydrophobic. ≡ Si–OCH3



Hydrolysis

≡ SiOH + FeOH

Si–O–Fe ≡ +H2 O

Condensation

≡ SiOH + SiOH 

≡ Si–O–Si ≡ +H2 O

3.3. Wettability Fig. 6 shows the CA of SS etched with FHH solution and the following modification with DTS. The samples are hydrophilic (CA < 20◦ ) without the DTS coating. The CA reached 117.6 ± 2◦ after 20 min etch reaction in 0.5 M FeCl3 and DTS modification. As the etchant concentration increased to 1 M, the CA grew to 134.2 ± 2.2◦ and when the etchant concentration reached 1.5 and 2 M, the CA rose to 142.9 ± 2◦ and 158.3 ± 2.8◦ after DTS modification and CA hysteresis was less than 3 ± 1.2◦ . Therefore, we can infer that the micro–nano structures with low surface energy contributed greatly to the superhydrophobic property because the air is trapped

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covered with frost on the superhydrophobic surface (Fig. 7g). The results show that the small droplet size on superhydrophobic surface delayed the freezing of droplet, retarded the appearance of frost crystal in the early stage about 60 s under the temperature of −22 ◦ C, and reduced the density of the frost layer growth on the surface of hydrophobic coating. The frost formation process on cold surface is dynamic and complex and the reaction process is not easy to control. The main factors affecting frost formation is the temperature of cold surface and environmental humidity. The initial stage of frost formation was accompanied by atomization. Therefore, the droplets formation and growth are not able to be observed at this temperature. Furthermore, the water droplets on plain SS surface has been completely frozen and a large number of small frost sticks grow on its surface after 30s, and the volume of frozen drops at this stage is very small, leading to an increase in the density of frost layer in the later stage. The superhydrophobic SS surface was horizontally placed on a cold plate with the temperature being kept at −22 ◦ C for 3 min. Then it was moved out and placed in ambient environment until the surface was completely dried. The freezing-thawing process was repeated 10 times on the same sample, which could still retard the appearance of frost crystal after 60 s. Moreover, the prepared sample surface still showed superhydrophobicity after exposed to the air for one month. This stable character is important for the surface to be applied to achieve practical application. The profile of water in contact with the as-prepared surface are schematically shown in Fig. 8a and b Water on these surfaces is primarily in contact with air pockets trapped in the rough surfaces. The contact angle is calculated in terms of the Cassie–Baxter equation [48]: cos r = f1 cos o − f2

Fig. 7. The optical photographs of frost formation on (a–d) plain stainless steel surface and (e–h) superhydrophobic surface with contact angle of 158.3◦ at room temperature 14 ◦ C, humidity 60% and cold surface −22 ◦ C with magnification of ×4.5.

only 30 s (the bright areas in Fig. 7b) and the surface was fully covered by a continuous frost layer only after 90 s (Fig. 7d). In contrast, almost no frost is found on the superhydrophobic surface even after 60 s and frost firstly nucleates and grows at such areas as marked by the red line in Fig. 7f–g. Comparing the frost and frost-free area on superhydrophobic surfaces, it shows that the area covered with frost on these surfaces increases with the increase of time. What is more, once the water droplets on the superhydrophobic surface were frozen, the frost particles tend to grow on the top of the original frost layer with sizes from a few microns to tens of microns. About 90 s later, the surface is covered almost completely by frost on plain SS surface (Fig. 7d) while only part of the area is

(5)

where f1 and f2 are the fractions of the solid surface and air in contact with the liquid, respectively;  r (158.3◦ ) and  o (66◦ ) are the contact angles on the as-prepared surface by Chemical etching and hydrophobic thin film deposition and on the primary surface, respectively. Given that f1 + f2 = 1, we calculated the corresponding f1 to be 0.05. The low value of f1 implies that only about 5% of the surface is in direct contact with water. Therefore, the air trapped in the micro–nano structures surface plays an important role in increasing the contact angle and enhancing the hydrophobicity. Frost formation on a cold surface is a typical crystal growth process and the contact angle will certainly affect the frost growth by influencing vapor condensation and nucleation on the cold surface. According to classic nucleation theory [49], the nucleation free-energy barrier (Gc ) for the heterogeneous nucleation is: Gc =

4rc 2 1␯   f  3

(6)

Fig. 8. Schematic illustrations of a water droplet on the as-prepared surface. (a) The model of a water droplet on the surface. (b) Cross-sectional profile of water in contact with a superhydrophobic surface.

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f  =



2 + cos 



1 − cos 

4

2 (7)

where rc is the critical radius,  lv is the solid–liquid interfacial tension, g is the Gibb’s energy density difference between ice and liquid water and  is the apparent contact angle. From Eqs. (6) and (7), it can be seen that Gc is proportional to f(), indicating an increase in Gc with the contact angle. Therefore, increasing the contact angle of a surface will augment the energy barrier, and thus restrain the frost crystals nucleation. Therefore, the superhydrophobic SS surface is more inclined to restrain frozen droplets and frost growth than the plain surface. 4. Conclusion In summary, we have developed a facile, highly effective, and low-cost two-step methodology to fabricate the biomimetic superhydrophobic surface on 304 SS. First, the SS surface with binary micro–nanoscale structures can be obtained by changing the morphology using a chemical etching process with FHH solution, and then a superhydrophobic surface with a CA for water of 158.3 ± 2.8 ◦ can be prepared by modifying the surface with low free energy material of DTS. Furthermore, the as-prepared films have excellent anti-frosting property. The facile, highly effective, and low-cost approach can be adopted to fabricate the surperhydrophobic SS surface which may be a good super-hydrophobic engineering material in the industrial applications even in humid condition. Acknowledgements The authors thank the National Natural Science Foundation of China (nos. 51275555, 51475200 and 51325501), Science and Technology Development Project of Jilin Province (no. 20150519007JH) and Basically Science Research Foundation of Jilin University (no. 2013ZY09). References [1] W. Barthlott, C. Neinhuis, Purity of the sacred lotus, or escape from contamination in biological surfaces, Planta 202 (1997) 1–8. [2] S. Herminghaus, Roughness-induced non-wetting, Europhysics Letters 52 (2000) 165. [3] A. Otten, S. Herminghaus, How plants keep dry: a physicist’s point of view, Langmuir 20 (2004) 2405–2408. [4] X. Zhang, F. Shi, J. Niu, Y. Jiang, Z. Wang, Superhydrophobic surfaces: from structural control to functional application, Journal of Materials Chemistry 18 (2008) 621–633. [5] L. Feng, Y. Zhang, J. Xi, Y. Zhu, N. Wang, F. Xia, L. Jiang, Petal effect: a superhydrophobic state with high adhesive force, Langmuir 24 (2008) 4114–4119. [6] X. Gao, L. Jiang, Biophysics: water-repellent legs of water striders, Nature 432 (2004) 36. [7] X.-Q. Feng, X. Gao, Z. Wu, L. Jiang, Q.-S. Zheng, Superior water repellency of water strider legs with hierarchical structures: experiments and analysis, Langmuir 23 (2007) 4892–4896. [8] J. Lin, H. Chen, T. Fei, C. Liu, J. Zhang, Highly transparent and thermally stable superhydrophobic coatings from the deposition of silica aerogels, Applied Surface Science 273 (2013) 776–786. [9] L. Jiang, R. Wang, B. Yang, T. Li, D. Tryk, A. Fujishima, K. Hashimoto, D. Zhu, Binary cooperative complementary nanoscale interfacial materials, Pure and Applied Chemistry 72 (2000) 73–81. [10] A. Lafuma, D. Quéré, Superhydrophobic states, Nature Materials 2 (2003) 457–460. [11] N. Gao, Y. Yan, Characterisation of surface wettability based on nanoparticles, Nanoscale 4 (2012) 2202–2218. [12] N. Gao, Y. Yan, X. Chen, D. Mee, Superhydrophobic surfaces with hierarchical structure, Materials Letters 65 (2011) 2902–2905. [13] N. Gao, Y. Yan, X. Chen, X. Zheng, Superhydrophobic composite films based on THS and nanoparticles, Journal of Bionic Engineering 7 (2010) S59–S66. [14] N.J. Shirtcliffe, G. McHale, M.I. Newton, C.C. Perry, Wetting and wetting transitions on copper-based super-hydrophobic surfaces, Langmuir 21 (2005) 937–943. [15] S. Farhadi, M. Farzaneh, S. Kulinich, Anti-icing performance of superhydrophobic surfaces, Applied Surface Science 257 (2011) 6264–6269.

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