- Email: [email protected]
One-step hydrothermal method to fabricate drag reduction superhydrophobic surface on aluminum foil
Applied Surface Science xxx (2018) xxx–xxx
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
One-step hydrothermal method to fabricate drag reduction superhydrophobic surface on aluminum foil Yanjing Tuo a, Weiping Chen b, Haifeng Zhang a,⇑, Pujun Li a, Xiaowei Liu a,b a b
MEMS Center, Harbin Institute of Technology, Harbin 150001, China State Key Laboratory of Urban Water Resource & Environment (Harbin Institute of Technology), Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 10 October 2017 Revised 1 January 2018 Accepted 5 January 2018 Available online xxxx Keywords: Hydrothermal reaction Superhydrophobic Drag reduction
a b s t r a c t Superhydrophobic surface, covered by micro or nano textured layer, can trap and retain air pockets and possesses unique excellent properties. Here we propose a novel one-step hydrothermal method to fabricate superhydrophobic surface on aluminum foil. The film of sheet structure is Al[CF3(CF2)12COO]3, which can provide micro structures and reduce surface energy. The as-prepared surface with high contact angle and low sliding angle has some interesting characteristics. In the test, the powder dirt can be easily removed from the surface and the formation of ice is inhibited on it. The drag reduction ratio of the superhydrophobic surface is about 20–30% at the velocity of 2–5 m/s. We envision this superhydrophobic surface has a great prospect in industrial applications. Ó 2018 Published by Elsevier B.V.
1. Introduction Inspired from the natural phenomenon that the lotus leaf is not dyed out of the sludge and the water strider can walk on the water, researchers have found the superhydrophobic phenomenon. In recent years, superhydrophobic surface has aroused widespread concern, owing to its excellent properties such as self-cleaning, corrosion resistance, anti-icing and drag reduction [1–4]. Generally, there are two ways to prepare superhydrophobic surface, the first is to create a suitable roughness on the hydrophobic surface, the second is to fabricate superhydrophilic surface with micro-nano structures and then modify the sample with chemical reagent to reduce surface energy [5,6]. The superhydrophobic surface can trap and retain air film underwater to reduce the contact area between solid surface and water. Therefore, this superhydrophobic surface is in Cassie-Baxter wetting state, which is defined as a high contact angle (>150°) and a low contact angle hysteresis [7,8]. So far various technologies have been used to fabricate superhydrophobic surface, such as chemical etching, laser micromachining, electrodeposition method and sol-gel method, etc. [9–12]. Maciej et al. have prepared superhydrophobic surface by laser ablation, replication, and RF plasma treatment, to improve the ability of industrial application [10]. Song et al. have fabricated superhydrophobic surface via electrochemical machining and fluoroalkylsilane modification and proved that rough structures and ⇑ Corresponding author.
low surface energy is the crucial factors for superhydrophobicity [12]. In this paper, we present a novel one-step method to fabricate superhydrophobic surface on an aluminum substrate. The surface is evenly distributed with nanosheets, which form microcavities to trap air. The nanosheets are prepared by one-step hydrothermal reaction in a high pressure autoclave. With the increase of the reaction time, the density of the nanosheets becomes larger. When the reaction time is 1 h, the contact angle (CA) is 158° and the sliding angle (SA) is 3°. Compared with other common methods, this onestep method does not require expensive equipment, high voltage, strong acid or alkali. Its process is simple and causes less harm to the operators and environment. When the as-prepared superhydrophobic surface is tilted in a certain angle, the dust can be easily removed by rolling droplets of water and the superhydrophobicity can inhibit the surface freezing at low temperature. Another interesting property of the as-prepared surface is drag reduction, which is due to the composite surface of air and solid underwater. The air pockets trapped by the micro-structures on the superhydrophobic surface can reduce the liquid-solid contact area, thus reducing the frictional resistance between water and substrate. The drag reduction property of superhydrophobic surface can help to reduce the energy consumption. For the marine ships, more than half of the energy used for propulsion is wasted on overcoming the surface friction [13]. In the pipeline of liquid transport, the energy of pump is mainly used for the wall friction. The application of superhydrophobic surfaces in these industrial fields will save energy and improve national
E-mail address: [email protected] (H. Zhang). https://doi.org/10.1016/j.apsusc.2018.01.046 0169-4332/Ó 2018 Published by Elsevier B.V.
Please cite this article in press as: Y. Tuo et al., One-step hydrothermal method to fabricate drag reduction superhydrophobic surface on aluminum foil, Appl. Surf. Sci. (2018), https://doi.org/10.1016/j.apsusc.2018.01.046
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economic efficiency. But most of the drag reduction research on superhydrophobic surface is focused on the theory of model simulation and numerical analysis. Koji et al. have investigated the effects of superhydrophobic surface on friction drag through direct numerical simulations of a turbulent channel flow, and found that the superhydrophobic surface inducing a slip length on the order of ten wall units or more is possible to have a large drag reduction [14]. There are a few studies on the experimental test of drag reduction property, and most of them are not directly. Gao et al. have found the drag reduction effect in polydimethylsiloxane microchannels when the flow is pulled by depressurization at the inlet, and drag reduction is expressed by the change of pressure and flow [15]. In our research, we use a self-designed friction resistance testing device to measure the friction drag at solid-liquid interface. The testing device can convert the mechanical signal into an electrical signal and achieve a real-time testing of solid-liquid interface friction drag. Through the test, we find that at the velocity of 2–5 m/s, drag reduction ratio of the superhydrophobic surface is about 20–30%. 2. Experimental 2.1. Materials and reagents The substrate material is aluminum foil that is industrial grade (99% purity) and the thickness is 0.8 mm. The chemical reagents are aluminum oxide (Al2O3), perfluorotetradecanoic acid (CF3(CF2)12COOH) and ethanol. These regents are utilized without further purification. In addition, deionized water is used throughout the experiment. 2.2. Fabrication of superhydrophobic surface The aluminum foil is cut into small pieces (50 mm 30 mm) and then polished by #800, #1700 sandpapers. After cleaning by acetone, ethanol and deionized water, the aluminum piece is vertically put into a high pressure autoclave where contains 0.1 g Al2O3 powder, 0.05 g perfluorotetradecanoic acid, 10 ml deionized water and 5 ml ethanol. Hydrothermal reaction takes place at 150 °C for a period, after that a superhydrophobic surface can be obtained. The illustration for the fabrication of the superhydrophobic surface is showed in Fig. 1. 2.3. Surface characterization A field-emission scanning electron microscope (FE-SEM, TESCAN VEGA) is used to observe the morphology of the as-prepared
aluminum samples. Energy dispersive spectroscopy (EDS) and Xray photoelectron spectroscopy (XPS) are used to characterize the chemical composition of the as-prepared samples. A contact angle meter system (JC2000D2A, Shanghai Zhongchen Digital Technic Apparatus Co., Ltd.) is used to measure CAs and SAs of the superhydrophobic surface under ambient conditions. 3. Results and discussion 3.1. Surface morphology and wettability After hydrothermal reaction, the sheet-like structures have been formed on the aluminum surface. Fig. 2 shows the SEM images of aluminum samples after hydrothermal reaction for different time. When the reaction time is 20 min, a few sheet structures are randomly distributed on the aluminum surface, and the CA of water is 145° ± 2°. Under this condition, the water droplets cannot slide on the sample, so the as-prepared surface is not superhydrophobic. As the duration of the hydrothermal reaction increases, more of the sheet structures are formed on aluminum surface. When the reaction time reaches to 1 h, the sheet structures are dense and evenly distributed on the aluminum surface. The asprepared sample exhibits a well superhydrophobic property with the CA of 158° ± 2° and the SA of 3.5° ± 0.5°. As the reaction time continues to lengthen, larger flake structures begin to appear at substrate surface, and the CA of water decreases slightly. In addition, when the growth solution does not contain Al2O3 particles, the superhydrophobic surface also can be obtained. The SEM images are shown in Fig. 2e and f, when the reaction time is 1 h, the flaky clusters form flower-like structures and evenly distribute on the surface. When the reaction time is 2 h, the flake is denser and CA reaches to 160°. Compared Fig. 2c with e, the aluminum particles reduce the formation rate of the flake structures and ensure that the microstructures are formed by chemical bonds rather than physical deposition in a short reaction time. The SAs of different samples are in the supplementary materials. 3.2. Composition of superhydrophobic film The chemical composition of the superhydrophobic surface is characterized by EDS and XPS. Fig. 3 shows the distribution of elements on the substrate after hydrothermal reaction for 1 h. The sheet structures consist of four elements about Al, C, O, and F, and each element is evenly distributed on the substrate. Fig. 4 shows the XPS results of the superhydrophobic surface. There are four signals: Al2p, C1s, O1s and F1s. The Al2p spectrum of the as-prepared sample is divided into two components: the signal
Fig. 1. Illustration for the fabrication of the superhydrophobic surface.
Please cite this article in press as: Y. Tuo et al., One-step hydrothermal method to fabricate drag reduction superhydrophobic surface on aluminum foil, Appl. Surf. Sci. (2018), https://doi.org/10.1016/j.apsusc.2018.01.046
Y. Tuo et al. / Applied Surface Science xxx (2018) xxx–xxx
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284.6 eV, 289.8 eV, 291.6 eV and 293.7 eV are assigned to CAC, C@O, ACF2 and ACF3, respectively [17,18]. Therefore, the sheet structures film is formed on the aluminum surface through two step reactions: the first is hydrolysis reaction of Al and Al2O3 to generate AlOOH and Al(OH)3, the second is neutralization reaction to generate Al[CF3(CF2)12COO]3 with low surface energy. 3.3. Surface durability Surface durability is an important index to measure the quality of superhydrophobic surface. Ludmila et al. have studied the deterioration of wetting at long-term continuous contact of superhydrophobic surface with deionized water [19]. Khorsand et al. also have proved that the anticorrosion performance of the superhydrophobic surface is degraded gradually with immersion time [20]. In this paper, the as-prepared surfaces are immersed in 3.5% NaCl aqueous solution for different time. The result shows that increasing the immersion time leads to a degradation of superhydrophobicity. As shown in Fig. 5, the sample reacts in growth solution without Al2O3 particles for 2 h, whose CA and SA become worse with the increase of the contact time between the surface and the neutral corrosion solution. When the immersion time comes to 8 d, the as-prepared surface has no superhydrophobicity. But for the sample which is reacted for 1 h in the growth solution containing Al2O3 particles, the degradation rate of superhydrophobicity is rather slower. When the immersion time is 20 d, the CA is 153° and SA is 8°, and the sample still has superhydrophobic property. The results show that the microstructures of the physical deposition are unstable and nondurable. Therefore, all the samples used in the following experiments are hydrothermal reacted for 1 h in the growth solution containing Al2O3 particles. 3.4. Drag reduction property Fig. 2. SEM images of aluminum samples after hydrothermal reaction. (a) reaction for 20 min, (b) reaction for 40 min, (c) reaction for 1 h, (d) reaction for 2 h, (e) without Al2O3 and reaction for 1 h, (f) without Al2O3 and reaction for 2 h.
at around 75.5 eV is Al3+, which indicates the occurrence of hydrolysis reaction on the aluminum substrate [16]. The C1s spectrum of the as-prepared sample is shown in Fig. 4c. The peaks at around
The friction resistance at the liquid and solid interface is tested by a self-designed measurement device. As shown in Fig. 6a, the testing device is mainly composed of a cantilever, two strain gauges, constant current source and data collection system. When the liquid ejected from the nozzle flows over the surface of the sample, the cantilever produces mechanical signals and leads to the strain gauges generate electrical signals. The data collection
Fig. 3. Distribution of elements and EDS spectrum on the substrate after hydrothermal reaction for 1 h.
Please cite this article in press as: Y. Tuo et al., One-step hydrothermal method to fabricate drag reduction superhydrophobic surface on aluminum foil, Appl. Surf. Sci. (2018), https://doi.org/10.1016/j.apsusc.2018.01.046
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Fig. 4. XPS results of aluminum sample after hydrothermal reaction for 1 h. (a) survey spectra, (b) Al2p spectra, (c) C1s spectra and (d) O1s spectra.
Fig. 5. The CA and SA of as-prepared samples immersed in 3.5% NaCl aqueous solution.
system processes the electrical signals and outputs them in the form of frictional drag. Therefore, by adjusting the flowmeter, the friction resistance at solid-liquid interface can be tested at different flow velocities. Through the relation between Reynolds coefficient and flow velocity, we calculate that when the velocity of the nozzle outlet is less than 6.3 m/s, the Reynolds coefficient is less than 2000 and the flow is in laminar state. In Fig. 6b, the friction resistance of untreated aluminum and superhydrophobic surface is tested under different flow velocity. The friction resistance becomes larger with the increase of flow velocity. At the velocity of 2–5 m/s, the drag reduction ratio for the superhydrophobic surface is about 20–30%. In the range of test velocity, the water flow is always in laminar state, and the friction drag between water flow and untreated aluminum is always bigger than the friction drag between water flow and superhydrophobic surface. Therefore,
the effect of drag reduction can be realized after the superhydrophobic treatment. Fig. 6e exhibits the state of a water droplet at the superhydrophobic surface of Cassie-Baxter model. The air is trapped in the cavities of the micro-structures, which can be proved by the photographs in Fig. 6c and d. According to the theory of total reflection in physics, when incident light passes through the water into the interface of air with an incident angle higher than the critical angle, the light is completely reflected [21–23]. The superhydrophobic surface immersed in water is exceptionally bright. However, the untreated aluminum surface cannot reflect all incident light, therefore the surface is dark when it is underwater. This means that the as-prepared superhydrophobic surface can capture air by the cavities of micro-structures. So, at the interface, a part of the water droplet is contact with solid and other part is contact
Please cite this article in press as: Y. Tuo et al., One-step hydrothermal method to fabricate drag reduction superhydrophobic surface on aluminum foil, Appl. Surf. Sci. (2018), https://doi.org/10.1016/j.apsusc.2018.01.046
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Fig. 6. (a) Testing device of friction resistance, (b) friction resistance of untreated aluminum and superhydrophobic surface, (c) optical photograph of untreated aluminum, (d) optical photograph of superhydrophobic surface, (e) model of Cassie-Baxter, (f) slippage of water on the superhydrophobic surface.
with air. When the water flows through the composite surface of solid and gas, the phenomenon of slippage occurs. The velocity at the interface between water and solid is 0 m/s, but it is not zero at the interface between water and gas. As shown in Fig. 6f, the slip length is b. In addition, the friction drag between water flow and solid is much bigger than that between water flow and air, so the superhydrophobic surface composited of gas and solid shows the effect of drag reduction. 3.5. Anti-icing properties Finally, we investigate the anti-icing effect of the as-prepared surface. Anti-icing property has also attracted considerable attention, because the industrial equipment is often affected by freezing at low temperatures, such as aircraft, high tension cable, and wind turbine. Superhydrophobic coatings can capture tiny air pockets at the ice/solid interface in order to reduce the real ice/coating surface area and disrupt bonding by creating stress concentrations [24]. Alexandre et al. have found that icephobicity of superhydrophobic materials essentially depends on the mechanical stability of the textured layer [25]. To test the anti-icing ability of the superhydrophobic surface, the as-prepared sample and untreated aluminum foil are put into a refrigerator for 10 d. The samples are placed at 45°, the relative humidity is about 50%, and the temperature is reduced from 20 °C to 10 °C. Fig. 7 shows the optical images of ice formation on aluminum surface and superhydrophobic surface under cool condition. In Fig. 7a, the untreated aluminum foil is covered with a thin layer of ice, however, there is almost no freezing point on the superhydrophobic surface. After taking the samples from refrigerator for 10 min, the ice on the aluminum surface begin to melt into small droplets, while there still no small droplet occurs on the superhydrophobic surface as shown in Fig. 7b. In further tests, the samples are placed in a closed space. By spraying water into the space everyday, the relative humidity is maintained at 80–90%. In addition, the temperature in refrigerator is reduced from 20 °C to 17 °C and remains constant. Table 1 shows the increase in the weight of samples under low tempera-
Fig. 7. Optical images of ice formation on aluminum surface and as-prepared surface under cool condition. (a) 10 °C, (b) at room temperature for 10 min.
ture and humid conditions for different periods of time. The weight of the superhydrophobic sample is almost constant within 10 days, while it increase rapidly about the untreated aluminum sample. Previous studies have shown that during the formation of the freezing point, the small condensate droplets converge into a critical droplet size and roll from the superhydrophobic surface [26–28]. So the superhydrophobic surface film can inhibit the formation of ice under cool and humid conditions. In addition, the asprepared superhydrophobic surface has a self-cleaning property, and the relevant test is in the supplementary materials.
Table 1 Increase in the weight of samples under
17 °C for different periods of time.
Time
5d
10 d
15 d
20 d
Untreated aluminum Superhydrophobic surface
5 mg 0 mg
22.6 mg 0.5 mg
34.6 mg 2.8 mg
47 mg 6.8 mg
Please cite this article in press as: Y. Tuo et al., One-step hydrothermal method to fabricate drag reduction superhydrophobic surface on aluminum foil, Appl. Surf. Sci. (2018), https://doi.org/10.1016/j.apsusc.2018.01.046
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4. Conclusion In summary, we have used a novel one-step hydrothermal method to fabricate superhydrophobic surface on aluminum foil. When the reaction time is 1 h, the distribution of microstructures is even. The CA of the as-prepared surface is 158° ± 2° and the SA is 3.5° ± 0.5°. Through experimental test, we have demonstrated that the as-prepared surface has drag reduction, self-cleaning and anti-icing properties. The superhydrophobic surface can reduce the friction resistance between solid-liquid interface, inhibit the formation of freezing points and the dust on it can be easily removed. With further refinement, we hope that the one-step hydrothermal method reported preparing superhydrophobic surface on aluminum can be used to practical applications. Acknowledgements The work is supported by National Science Foundation of China (No. 61474034), National Basic Research Program of China (No. 2012CB934100), Natural Science Foundation of Heilongjiang Province of China (No. F201418), State Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology) (No. 2016TS 06). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.apsusc.2018.01.046. References [1] Z. Yang, L. Wang, W. Sun, Superhydrophobic epoxy coating modified by fluorographene used for anti-corrosion and self-cleaning, Appl. Surf. Sci. 401 (2017) 146–155. [2] P. Wang, D. Zhang, R. Qiu, J. Wu, Y. Wan, Super-hydrophobic film prepared on zinc and its effect on corrosion in simulated marine atmosphere, Corros. Sci. 69 (2013) 23–30. [3] Y. Wu, Q. Wei, M. Cai, Interfacial friction control, Adv. Mater. Interfaces 2 (2015) 686–687. [4] R. Liao, Z. Zuo, C. Guo, Fabrication of superhydrophobic surface on aluminum by continuous chemical etching and its anti-icing property, Appl. Surf. Sci. 317 (2014) 701–709. [5] B. Su, S. Wang, J. Ma, Elaborate positioning of nanowire arrays contributed by highly adhesive superhydrophobic pillar-structured substrates, Adv. Mater. 24 (2012) 559–564. [6] L. Yao, M. Zheng, M. Li, Self-assembly of diverse alumina architectures and their morphology-dependent wettability, Mater. Res. Bull. 46 (2011) 1403– 1408.
[7] Y. Tian, B. Su, L. Jiang, Interfacial material system exhibiting superwettability, Adv. Mater. 26 (2014) 6872–6897. [8] J.G. Buijnsters, R. Zhong, N. Tsyntsaru, Surface wettability of macroporous anodized aluminum oxide, Acs Appl. Mater. Interfaces 5 (2013) 3224–3233. [9] H. Wang, D. Dai, X. Wu, Fabrication of superhydrophobic surfaces on aluminum, Appl. Surf. Sci. 254 (2008) 5599–5601. [10] M. Psarski, J. Marczak, G. Celichowski, Superhydrophobic surface by replication of laser micromachined pattern in epoxy/alumina nanoparticle composite, J. Nanomater. 2 (2014) 41–51. [11] N. Shirtcliffe, G. Mchale, M. Newton, Dual-scale roughness produces unusually water-repellent surfaces, Adv. Mater. 16 (2014) 1929–1932. [12] J.L. Song, W.J. Xu, X. Liu, Electrochemical machining of super-hydrophobic Al surfaces and effect of processing parameters on wettability, Appl. Phys. A 108 (2012) 559–568. [13] M.N. Kavalenka, F. Vüllers, S. Lischker, Bioinspired air-retaining nanofur for drag reduction, Acs Appl. Mater. Interfaces 7 (2015) 10651–10655. [14] K. Fukagata, N. Kasagi, P. Koumoutsakos, A theoretical prediction of friction drag reduction in turbulent flow by superhydrophobic surfaces, Phys. Fluids 18 (2006) 360–363. [15] G. Yang, L. Jiang, H.C. Shum, Drag reduction by bubble-covered surfaces found in PDMS microchannel through depressurization, Langmuir 32 (2016) 4815– 4819. [16] L. Feng, H. Zhang, Z. Wang, Superhydrophobic aluminum alloy surface: fabrication, structure, and corrosion resistance, Colloids Surfaces A Physicochem. Eng. Aspects 441 (2014) 319–325. [17] P. Wang, Z. Lu, D. Zhang, Slippery liquid-infused porous surfaces fabricated on aluminum as a barrier to corrosion induced by sulfate reducing bacteria, Corros. Sci. 93 (2015) 159–166. [18] L. Feng, Y. Che, Y. Liu, One-step immersion method for fabricating superhydrophobic aluminum alloy with excellent corrosion resistance, Surf. Interface Anal. 48 (2016) 1320–1327. [19] L. Boinovich, A.M. Emelyanenko, A.S. Pashinin, Analysis of long-term durability of superhydrophobic properties under continuous contact with water, Acs Appl. Mater. Interfaces 2 (2010) 1754–1758. [20] S. Khorsand, K. Raeissi, F. Ashrafizadeh, Corrosion resistance and long-term durability of super-hydrophobic nickel film prepared by electrodeposition process, Appl. Surf. Sci. 305 (2014) 498–505. [21] P. Wang, D. Zhang, R. Qiu, Liquid/solid contact mode of super-hydrophobic film in aqueous solution and its effect on corrosion resistance, Corros. Sci. 54 (2012) 77–84. [22] P. Wang, D. Zhang, R. Qiu, Y. Wan, J. Wu, Green approach to fabrication of a super-hydrophobic film on copper and the consequent corrosion resistance, Corros. Sci. 80 (2014) 366–373. [23] P. Wang, D. Zhang, Z. Lu, Advantage of super-hydrophobic surface as a barrier against atmospheric corrosion induced by salt deliquescence, Corros. Sci. 90 (2015) 23–32. [24] Richard Menini, Masoud Farzaneh, Advanced icephobic coatings, J. Adhes. Sci. Technol. 25 (2011) 971–992. [25] A.M. Emelyanenko, L.B. Boinovich, A.A. Bezdomnikov, Reinforced superhydrophobic coating on silicone rubber for longstanding anti-icing performance in severe conditions, Acs Appl. Mater. Interfaces 9 (2017) 24210–24219. [26] S. Anand, A.T. Paxson, R. Dhiman, Enhanced condensation on lubricantimpregnated nanotextured surfaces, Acs Nano 6 (2012) 10122–10129. [27] H.A. Stone, Ice-phobic surfaces that are wet, Acs Nano 6 (2012) 6536–6540. [28] Y. Wang, H. Zhang, X. Liu, Slippery liquid-infused substrates: a versatile preparation, unique anti-wetting and drag-reduction effect on water, J. Mater. Chem. A 4 (2016) 2524–2529.
Please cite this article in press as: Y. Tuo et al., One-step hydrothermal method to fabricate drag reduction superhydrophobic surface on aluminum foil, Appl. Surf. Sci. (2018), https://doi.org/10.1016/j.apsusc.2018.01.046
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
One-step hydrothermal method to fabricate drag reduction superhydrophobic surface on aluminum foil Yanjing Tuo a, Weiping Chen b, Haifeng Zhang a,⇑, Pujun Li a, Xiaowei Liu a,b a b
MEMS Center, Harbin Institute of Technology, Harbin 150001, China State Key Laboratory of Urban Water Resource & Environment (Harbin Institute of Technology), Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 10 October 2017 Revised 1 January 2018 Accepted 5 January 2018 Available online xxxx Keywords: Hydrothermal reaction Superhydrophobic Drag reduction
a b s t r a c t Superhydrophobic surface, covered by micro or nano textured layer, can trap and retain air pockets and possesses unique excellent properties. Here we propose a novel one-step hydrothermal method to fabricate superhydrophobic surface on aluminum foil. The film of sheet structure is Al[CF3(CF2)12COO]3, which can provide micro structures and reduce surface energy. The as-prepared surface with high contact angle and low sliding angle has some interesting characteristics. In the test, the powder dirt can be easily removed from the surface and the formation of ice is inhibited on it. The drag reduction ratio of the superhydrophobic surface is about 20–30% at the velocity of 2–5 m/s. We envision this superhydrophobic surface has a great prospect in industrial applications. Ó 2018 Published by Elsevier B.V.
1. Introduction Inspired from the natural phenomenon that the lotus leaf is not dyed out of the sludge and the water strider can walk on the water, researchers have found the superhydrophobic phenomenon. In recent years, superhydrophobic surface has aroused widespread concern, owing to its excellent properties such as self-cleaning, corrosion resistance, anti-icing and drag reduction [1–4]. Generally, there are two ways to prepare superhydrophobic surface, the first is to create a suitable roughness on the hydrophobic surface, the second is to fabricate superhydrophilic surface with micro-nano structures and then modify the sample with chemical reagent to reduce surface energy [5,6]. The superhydrophobic surface can trap and retain air film underwater to reduce the contact area between solid surface and water. Therefore, this superhydrophobic surface is in Cassie-Baxter wetting state, which is defined as a high contact angle (>150°) and a low contact angle hysteresis [7,8]. So far various technologies have been used to fabricate superhydrophobic surface, such as chemical etching, laser micromachining, electrodeposition method and sol-gel method, etc. [9–12]. Maciej et al. have prepared superhydrophobic surface by laser ablation, replication, and RF plasma treatment, to improve the ability of industrial application [10]. Song et al. have fabricated superhydrophobic surface via electrochemical machining and fluoroalkylsilane modification and proved that rough structures and ⇑ Corresponding author.
low surface energy is the crucial factors for superhydrophobicity [12]. In this paper, we present a novel one-step method to fabricate superhydrophobic surface on an aluminum substrate. The surface is evenly distributed with nanosheets, which form microcavities to trap air. The nanosheets are prepared by one-step hydrothermal reaction in a high pressure autoclave. With the increase of the reaction time, the density of the nanosheets becomes larger. When the reaction time is 1 h, the contact angle (CA) is 158° and the sliding angle (SA) is 3°. Compared with other common methods, this onestep method does not require expensive equipment, high voltage, strong acid or alkali. Its process is simple and causes less harm to the operators and environment. When the as-prepared superhydrophobic surface is tilted in a certain angle, the dust can be easily removed by rolling droplets of water and the superhydrophobicity can inhibit the surface freezing at low temperature. Another interesting property of the as-prepared surface is drag reduction, which is due to the composite surface of air and solid underwater. The air pockets trapped by the micro-structures on the superhydrophobic surface can reduce the liquid-solid contact area, thus reducing the frictional resistance between water and substrate. The drag reduction property of superhydrophobic surface can help to reduce the energy consumption. For the marine ships, more than half of the energy used for propulsion is wasted on overcoming the surface friction [13]. In the pipeline of liquid transport, the energy of pump is mainly used for the wall friction. The application of superhydrophobic surfaces in these industrial fields will save energy and improve national
E-mail address: [email protected] (H. Zhang). https://doi.org/10.1016/j.apsusc.2018.01.046 0169-4332/Ó 2018 Published by Elsevier B.V.
Please cite this article in press as: Y. Tuo et al., One-step hydrothermal method to fabricate drag reduction superhydrophobic surface on aluminum foil, Appl. Surf. Sci. (2018), https://doi.org/10.1016/j.apsusc.2018.01.046
2
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economic efficiency. But most of the drag reduction research on superhydrophobic surface is focused on the theory of model simulation and numerical analysis. Koji et al. have investigated the effects of superhydrophobic surface on friction drag through direct numerical simulations of a turbulent channel flow, and found that the superhydrophobic surface inducing a slip length on the order of ten wall units or more is possible to have a large drag reduction [14]. There are a few studies on the experimental test of drag reduction property, and most of them are not directly. Gao et al. have found the drag reduction effect in polydimethylsiloxane microchannels when the flow is pulled by depressurization at the inlet, and drag reduction is expressed by the change of pressure and flow [15]. In our research, we use a self-designed friction resistance testing device to measure the friction drag at solid-liquid interface. The testing device can convert the mechanical signal into an electrical signal and achieve a real-time testing of solid-liquid interface friction drag. Through the test, we find that at the velocity of 2–5 m/s, drag reduction ratio of the superhydrophobic surface is about 20–30%. 2. Experimental 2.1. Materials and reagents The substrate material is aluminum foil that is industrial grade (99% purity) and the thickness is 0.8 mm. The chemical reagents are aluminum oxide (Al2O3), perfluorotetradecanoic acid (CF3(CF2)12COOH) and ethanol. These regents are utilized without further purification. In addition, deionized water is used throughout the experiment. 2.2. Fabrication of superhydrophobic surface The aluminum foil is cut into small pieces (50 mm 30 mm) and then polished by #800, #1700 sandpapers. After cleaning by acetone, ethanol and deionized water, the aluminum piece is vertically put into a high pressure autoclave where contains 0.1 g Al2O3 powder, 0.05 g perfluorotetradecanoic acid, 10 ml deionized water and 5 ml ethanol. Hydrothermal reaction takes place at 150 °C for a period, after that a superhydrophobic surface can be obtained. The illustration for the fabrication of the superhydrophobic surface is showed in Fig. 1. 2.3. Surface characterization A field-emission scanning electron microscope (FE-SEM, TESCAN VEGA) is used to observe the morphology of the as-prepared
aluminum samples. Energy dispersive spectroscopy (EDS) and Xray photoelectron spectroscopy (XPS) are used to characterize the chemical composition of the as-prepared samples. A contact angle meter system (JC2000D2A, Shanghai Zhongchen Digital Technic Apparatus Co., Ltd.) is used to measure CAs and SAs of the superhydrophobic surface under ambient conditions. 3. Results and discussion 3.1. Surface morphology and wettability After hydrothermal reaction, the sheet-like structures have been formed on the aluminum surface. Fig. 2 shows the SEM images of aluminum samples after hydrothermal reaction for different time. When the reaction time is 20 min, a few sheet structures are randomly distributed on the aluminum surface, and the CA of water is 145° ± 2°. Under this condition, the water droplets cannot slide on the sample, so the as-prepared surface is not superhydrophobic. As the duration of the hydrothermal reaction increases, more of the sheet structures are formed on aluminum surface. When the reaction time reaches to 1 h, the sheet structures are dense and evenly distributed on the aluminum surface. The asprepared sample exhibits a well superhydrophobic property with the CA of 158° ± 2° and the SA of 3.5° ± 0.5°. As the reaction time continues to lengthen, larger flake structures begin to appear at substrate surface, and the CA of water decreases slightly. In addition, when the growth solution does not contain Al2O3 particles, the superhydrophobic surface also can be obtained. The SEM images are shown in Fig. 2e and f, when the reaction time is 1 h, the flaky clusters form flower-like structures and evenly distribute on the surface. When the reaction time is 2 h, the flake is denser and CA reaches to 160°. Compared Fig. 2c with e, the aluminum particles reduce the formation rate of the flake structures and ensure that the microstructures are formed by chemical bonds rather than physical deposition in a short reaction time. The SAs of different samples are in the supplementary materials. 3.2. Composition of superhydrophobic film The chemical composition of the superhydrophobic surface is characterized by EDS and XPS. Fig. 3 shows the distribution of elements on the substrate after hydrothermal reaction for 1 h. The sheet structures consist of four elements about Al, C, O, and F, and each element is evenly distributed on the substrate. Fig. 4 shows the XPS results of the superhydrophobic surface. There are four signals: Al2p, C1s, O1s and F1s. The Al2p spectrum of the as-prepared sample is divided into two components: the signal
Fig. 1. Illustration for the fabrication of the superhydrophobic surface.
Please cite this article in press as: Y. Tuo et al., One-step hydrothermal method to fabricate drag reduction superhydrophobic surface on aluminum foil, Appl. Surf. Sci. (2018), https://doi.org/10.1016/j.apsusc.2018.01.046
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284.6 eV, 289.8 eV, 291.6 eV and 293.7 eV are assigned to CAC, C@O, ACF2 and ACF3, respectively [17,18]. Therefore, the sheet structures film is formed on the aluminum surface through two step reactions: the first is hydrolysis reaction of Al and Al2O3 to generate AlOOH and Al(OH)3, the second is neutralization reaction to generate Al[CF3(CF2)12COO]3 with low surface energy. 3.3. Surface durability Surface durability is an important index to measure the quality of superhydrophobic surface. Ludmila et al. have studied the deterioration of wetting at long-term continuous contact of superhydrophobic surface with deionized water [19]. Khorsand et al. also have proved that the anticorrosion performance of the superhydrophobic surface is degraded gradually with immersion time [20]. In this paper, the as-prepared surfaces are immersed in 3.5% NaCl aqueous solution for different time. The result shows that increasing the immersion time leads to a degradation of superhydrophobicity. As shown in Fig. 5, the sample reacts in growth solution without Al2O3 particles for 2 h, whose CA and SA become worse with the increase of the contact time between the surface and the neutral corrosion solution. When the immersion time comes to 8 d, the as-prepared surface has no superhydrophobicity. But for the sample which is reacted for 1 h in the growth solution containing Al2O3 particles, the degradation rate of superhydrophobicity is rather slower. When the immersion time is 20 d, the CA is 153° and SA is 8°, and the sample still has superhydrophobic property. The results show that the microstructures of the physical deposition are unstable and nondurable. Therefore, all the samples used in the following experiments are hydrothermal reacted for 1 h in the growth solution containing Al2O3 particles. 3.4. Drag reduction property Fig. 2. SEM images of aluminum samples after hydrothermal reaction. (a) reaction for 20 min, (b) reaction for 40 min, (c) reaction for 1 h, (d) reaction for 2 h, (e) without Al2O3 and reaction for 1 h, (f) without Al2O3 and reaction for 2 h.
at around 75.5 eV is Al3+, which indicates the occurrence of hydrolysis reaction on the aluminum substrate [16]. The C1s spectrum of the as-prepared sample is shown in Fig. 4c. The peaks at around
The friction resistance at the liquid and solid interface is tested by a self-designed measurement device. As shown in Fig. 6a, the testing device is mainly composed of a cantilever, two strain gauges, constant current source and data collection system. When the liquid ejected from the nozzle flows over the surface of the sample, the cantilever produces mechanical signals and leads to the strain gauges generate electrical signals. The data collection
Fig. 3. Distribution of elements and EDS spectrum on the substrate after hydrothermal reaction for 1 h.
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Fig. 4. XPS results of aluminum sample after hydrothermal reaction for 1 h. (a) survey spectra, (b) Al2p spectra, (c) C1s spectra and (d) O1s spectra.
Fig. 5. The CA and SA of as-prepared samples immersed in 3.5% NaCl aqueous solution.
system processes the electrical signals and outputs them in the form of frictional drag. Therefore, by adjusting the flowmeter, the friction resistance at solid-liquid interface can be tested at different flow velocities. Through the relation between Reynolds coefficient and flow velocity, we calculate that when the velocity of the nozzle outlet is less than 6.3 m/s, the Reynolds coefficient is less than 2000 and the flow is in laminar state. In Fig. 6b, the friction resistance of untreated aluminum and superhydrophobic surface is tested under different flow velocity. The friction resistance becomes larger with the increase of flow velocity. At the velocity of 2–5 m/s, the drag reduction ratio for the superhydrophobic surface is about 20–30%. In the range of test velocity, the water flow is always in laminar state, and the friction drag between water flow and untreated aluminum is always bigger than the friction drag between water flow and superhydrophobic surface. Therefore,
the effect of drag reduction can be realized after the superhydrophobic treatment. Fig. 6e exhibits the state of a water droplet at the superhydrophobic surface of Cassie-Baxter model. The air is trapped in the cavities of the micro-structures, which can be proved by the photographs in Fig. 6c and d. According to the theory of total reflection in physics, when incident light passes through the water into the interface of air with an incident angle higher than the critical angle, the light is completely reflected [21–23]. The superhydrophobic surface immersed in water is exceptionally bright. However, the untreated aluminum surface cannot reflect all incident light, therefore the surface is dark when it is underwater. This means that the as-prepared superhydrophobic surface can capture air by the cavities of micro-structures. So, at the interface, a part of the water droplet is contact with solid and other part is contact
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Fig. 6. (a) Testing device of friction resistance, (b) friction resistance of untreated aluminum and superhydrophobic surface, (c) optical photograph of untreated aluminum, (d) optical photograph of superhydrophobic surface, (e) model of Cassie-Baxter, (f) slippage of water on the superhydrophobic surface.
with air. When the water flows through the composite surface of solid and gas, the phenomenon of slippage occurs. The velocity at the interface between water and solid is 0 m/s, but it is not zero at the interface between water and gas. As shown in Fig. 6f, the slip length is b. In addition, the friction drag between water flow and solid is much bigger than that between water flow and air, so the superhydrophobic surface composited of gas and solid shows the effect of drag reduction. 3.5. Anti-icing properties Finally, we investigate the anti-icing effect of the as-prepared surface. Anti-icing property has also attracted considerable attention, because the industrial equipment is often affected by freezing at low temperatures, such as aircraft, high tension cable, and wind turbine. Superhydrophobic coatings can capture tiny air pockets at the ice/solid interface in order to reduce the real ice/coating surface area and disrupt bonding by creating stress concentrations [24]. Alexandre et al. have found that icephobicity of superhydrophobic materials essentially depends on the mechanical stability of the textured layer [25]. To test the anti-icing ability of the superhydrophobic surface, the as-prepared sample and untreated aluminum foil are put into a refrigerator for 10 d. The samples are placed at 45°, the relative humidity is about 50%, and the temperature is reduced from 20 °C to 10 °C. Fig. 7 shows the optical images of ice formation on aluminum surface and superhydrophobic surface under cool condition. In Fig. 7a, the untreated aluminum foil is covered with a thin layer of ice, however, there is almost no freezing point on the superhydrophobic surface. After taking the samples from refrigerator for 10 min, the ice on the aluminum surface begin to melt into small droplets, while there still no small droplet occurs on the superhydrophobic surface as shown in Fig. 7b. In further tests, the samples are placed in a closed space. By spraying water into the space everyday, the relative humidity is maintained at 80–90%. In addition, the temperature in refrigerator is reduced from 20 °C to 17 °C and remains constant. Table 1 shows the increase in the weight of samples under low tempera-
Fig. 7. Optical images of ice formation on aluminum surface and as-prepared surface under cool condition. (a) 10 °C, (b) at room temperature for 10 min.
ture and humid conditions for different periods of time. The weight of the superhydrophobic sample is almost constant within 10 days, while it increase rapidly about the untreated aluminum sample. Previous studies have shown that during the formation of the freezing point, the small condensate droplets converge into a critical droplet size and roll from the superhydrophobic surface [26–28]. So the superhydrophobic surface film can inhibit the formation of ice under cool and humid conditions. In addition, the asprepared superhydrophobic surface has a self-cleaning property, and the relevant test is in the supplementary materials.
Table 1 Increase in the weight of samples under
17 °C for different periods of time.
Time
5d
10 d
15 d
20 d
Untreated aluminum Superhydrophobic surface
5 mg 0 mg
22.6 mg 0.5 mg
34.6 mg 2.8 mg
47 mg 6.8 mg
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4. Conclusion In summary, we have used a novel one-step hydrothermal method to fabricate superhydrophobic surface on aluminum foil. When the reaction time is 1 h, the distribution of microstructures is even. The CA of the as-prepared surface is 158° ± 2° and the SA is 3.5° ± 0.5°. Through experimental test, we have demonstrated that the as-prepared surface has drag reduction, self-cleaning and anti-icing properties. The superhydrophobic surface can reduce the friction resistance between solid-liquid interface, inhibit the formation of freezing points and the dust on it can be easily removed. With further refinement, we hope that the one-step hydrothermal method reported preparing superhydrophobic surface on aluminum can be used to practical applications. Acknowledgements The work is supported by National Science Foundation of China (No. 61474034), National Basic Research Program of China (No. 2012CB934100), Natural Science Foundation of Heilongjiang Province of China (No. F201418), State Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology) (No. 2016TS 06). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.apsusc.2018.01.046. References [1] Z. Yang, L. Wang, W. Sun, Superhydrophobic epoxy coating modified by fluorographene used for anti-corrosion and self-cleaning, Appl. Surf. Sci. 401 (2017) 146–155. [2] P. Wang, D. Zhang, R. Qiu, J. Wu, Y. Wan, Super-hydrophobic film prepared on zinc and its effect on corrosion in simulated marine atmosphere, Corros. Sci. 69 (2013) 23–30. [3] Y. Wu, Q. Wei, M. Cai, Interfacial friction control, Adv. Mater. Interfaces 2 (2015) 686–687. [4] R. Liao, Z. Zuo, C. Guo, Fabrication of superhydrophobic surface on aluminum by continuous chemical etching and its anti-icing property, Appl. Surf. Sci. 317 (2014) 701–709. [5] B. Su, S. Wang, J. Ma, Elaborate positioning of nanowire arrays contributed by highly adhesive superhydrophobic pillar-structured substrates, Adv. Mater. 24 (2012) 559–564. [6] L. Yao, M. Zheng, M. Li, Self-assembly of diverse alumina architectures and their morphology-dependent wettability, Mater. Res. Bull. 46 (2011) 1403– 1408.
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Please cite this article in press as: Y. Tuo et al., One-step hydrothermal method to fabricate drag reduction superhydrophobic surface on aluminum foil, Appl. Surf. Sci. (2018), https://doi.org/10.1016/j.apsusc.2018.01.046