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Temperature-based adhesion tuning and superwettability switching on superhydrophobic aluminum surface for droplet manipulations
Surface & Coatings Technology 375 (2019) 527–533
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Temperature-based adhesion tuning and superwettability switching on superhydrophobic aluminum surface for droplet manipulations
T ⁎
Ziai Liua,1, Xiaolong Yangb,1, Guibing Pangc, Fan Zhanga, Yuqi Hana, Xuyue Wanga, Xin Liua, , ⁎ Lin Xuea, a
Key Laboratory for Precision and Non-traditional Machining Technology of the Ministry of Education, Dalian University of Technology, Dalian 116024, PR China National Key Laboratory of Science and Technology on Helicopter Transmission, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China c College of Mechanical Engineering and Automation, Dalian Polytechnic University, Dalian 116024, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Superhydrophobic surface Aluminum Temperature-responsive characteristic Adhesion tuning Superwettability switching
Superhydrophobic surfaces with tunable water adhesion and adjustable wettability have the capacity to manipulate water droplets under complicated conditions and, therefore, have great prospects in various domains. Hence, developing a facile method to prepare highly stable superhydrophobic surfaces with tunable water adhesion and adjustable wettability is of great importance. Herein, we prepare the superhydrophobic aluminum surface by combining laser etching and stearic acid modification. The variations of water contact angle, water sliding angle and water adhesive force with temperature of the prepared surface are characterized during the adhesion tuning process to investigate its temperature-responsive characteristics. A low/high adhesion switching of water on the prepared surface is subsequently achieved under alternating temperature. In addition, a facile approach is designed to achieve the superwettability switching process (from superhydrophobic state to superhydrophilic state) on the prepared surface. The prepared superhydrophobic surface displays excellent recoverability, stability and repeatability in the adhesion switching process and the superwettability switching process, even after being cycled for 10 times.
1. Introduction Superhydrophobic surfaces repelling water with a static water contact angle (WCA) larger than 150° have been extensively investigated in various areas, such as self-cleaning [1], directional transportation [2], oil/water separation [3], drag reduction [4], anticorrosion [5,6], etc. In the past decades, numerous techniques have been proposed to prepare superhydrophobic surfaces, including laser etching [7], chemical etching [8–10], electrochemical etching [11], chemical deposition [12], chemical oxidation [13], spray-coating [14,15]. Generally, superhydrophobic surfaces can be classified into superhydrophobic surfaces with low adhesion and superhydrophobic surfaces with high adhesion according to the state of water droplets on them. Water droplets are in Cassie state on the superhydrophobic surfaces with low adhesion [16]. Droplets are able to roll off the surfaces very easily with a water sliding angle (WSA) lower than 10°. Artificial superhydrophobic surfaces with low adhesion can be fabricated directly by integrating ordinary microstructures construction with low surface
energy modification [17–19]. For the superhydrophobic surfaces with high adhesion, water droplets on them are in Wenzel state [20,21]. Owing to their sticky behaviors towards water, WSA on such surfaces is larger than 10°. Superhydrophobic surfaces with high adhesion can be prepared by combining multiple microstructures construction and low surface energy modification [22,23]. Additionally, they can also be prepared by fabricating (super)hydrophilic patterns with high adhesion on superhydrophobic surface with low adhesion [24,25]. Superhydrophobic surfaces with stable wettability and adhesion have attracted extensive interest in various domains, such as lossless transfer of microdroplets [26], biomimetic organs fabrication [27], and droplets manipulation [28]. In recent years, researchers have found that the wettability and adhesion of some superhydrophobic surfaces can be tuned when internal and external conditions are changed [29–33]. For instance, Raturi et al. [34] endowed ZnO-nanowirescoated surface mesh with reversible wettability by annealing the mesh alternatively under hydrogen and oxygen environment for oil/water separation. Liu et al. [35] reported an intelligent surface that could fast and reversibly switch its wettability through macroscopic shape
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Corresponding authors. E-mail addresses: [email protected] (X. Liu), [email protected] (L. Xue). 1 Contribute equally. https://doi.org/10.1016/j.surfcoat.2019.07.041 Received 8 May 2019; Received in revised form 9 July 2019; Accepted 21 July 2019 Available online 22 July 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
Surface & Coatings Technology 375 (2019) 527–533
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spectroscopy (EDS, INCA Energy, Oxford Instruments), X-ray diffraction (XRD, Shimadzu XRD-6000, Japan) and Fourier transform infrared spectroscopy (FTIR, JACSCO, Japan). Surface roughness value, Ra, was measured with a roughness tester (Talysurf CLI2000, UK). The average roughness value (Ra) was based on 10 measurements obtained from 10 different directions because the laser etched surface was anisotropic. Water contact angle (WCA) was measured according to the sessile-drop method using an optical contact angle meter (KINO SL200KS, USA). Water sliding angle (WSA) was investigated using a precision turntable. To measure the WSA, a water droplet was put on the surface of a sample which was placed on the turntable. Then, WSA was measured by tilting the turntable to a certain angle at which the water droplet started to slide off the surface. A temperature controller (Hanbang HP-2525, China) was fixed with the sample by thermal conductive silicone (Kafuter K-5203K, China) to heat the sample surface to a certain temperature during WCA measurement, WSA measurement, droplet manipulation processes and reversible superwettability switching processes. The WCA and WSA measurements were performed using 14 μL deionized water droplets at ambient temperature (27 °C, 78% relative humidity). It is necessary to note that WCA and WSA were measured as soon as possible when a droplet was deposited on a sample surface. Therefore, the influence of the mass loss of a droplet is negligible. Error bars of WCA and WSA showed the variation from the average value based on five measurements obtained at different positions. In this manuscript, dyed water droplets were only used to take digital images for visualization. Undyed water droplets were used to measure WCA and WSA. Experimental optical images were captured using a camera equipped with a Nikon 105 mm f/2.8G lens (Nikon D7200, Japan).
change. However, the wettability of these surfaces could only be varied between superhydrophobicity and superhydrophilicity, revealing poor controllability in adhesion tuning. Chen et al. [36] proposed an allwater-based self-repairing superhydrophobic coating based on UV-responsive microcapsules by Pickering emulsion polymerization, the coating showed recoverability under UV light. Nevertheless, the nanoparticles contained in the experiment were harmful to the operators and the preparation processes were quite complicated. Liu et al. [37] reported switchable water adhesion on superhydrophobic surface by atmospheric pressure maskless microplasma jet treatment. However, this method required expensive equipment and the low surface energy modification was needed to recover superhydrophobicity. Yao et al. [31] reported a temperature-driven switching of water adhesion on nparaffin-swollen organogel via the thermo-responsive phase change of n-paraffin. However, the white substrate was transformed into transparent when the switching of water adhesion occurred at the melting temperature of n-paraffin, which was inconvenient for practical applications. It is, therefore, essential to design a high-performance wettability and adhesion tuning strategy that can solve the aforementioned problems. Aluminum (Al) is extensively used for fundamental science and industrial applications owing to its outstanding mechanical properties, good heat dissipating performance and low cost. Thus, endowing Al surface with tunable water adhesion and adjustable wettability is of great significance. In the present study, superhydrophobic Al surface with 167.5 ± 2.0° WCA and 1.7 ± 0.6° WSA was facilely fabricated by integrating laser etching with stearic acid modification. We investigated the effect of the temperature on the wettability of the resulting surface by measuring WCA and WSA. The results showed that water droplet was in sticky state with 137.5 ± 3.0° WCA when the superhydrophobic surface was heated at 70 °C. Moreover, water droplet could be adjusted between sliding (low adhesion) and sticky state (high adhesion) on the surface under alternating temperature. Additionally, a facile approach induced by temperature and pressure was devised to achieve the reversible superhydrophobic to superhydrophilic conversion on the resulting surface. Compared with the existing methods, we developed a facile method to achieve the precise adhesion tuning and reversible superwettability switching, which ensures good reliability and paves ways for stable droplet manipulations.
3. Result and discussion 3.1. Characterization of the superhydrophobic surface Fig. 1(a) shows the micro-morphology of the superhydrophobic Al surface with three-dimensional antler-like microstructures, which were covered by 1–5 μm spherical particles (Fig. 1(b)). The generation of these spherical particles were owing to the melting and solidification of Al during the laser etching process [38]. The surface roughness (Ra) of the original Al surface before the laser etching process was about 1 μm, while the Ra value increased to about 5 μm after being etched by laser as shown in Fig. 1(c). It is necessary to note that the stearic acid modification process did not have an influence on the Ra value (Fig. S1). EDS image shown in Fig. 1(d) detected the peaks from Al and O on the superhydrophobic surface. Moreover, XRD pattern of the surface confirmed that the main compositions of the microstructures on the superhydrophobic surface was Al (Fig. 1(e)). As the FTIR spectra illustrated in Fig. 1(f), the absorption bands assigned to the CeH stretching vibration of the –CHn groups of the stearic acid molecules were detected on the superhydrophobic surface. The absorption bands at around 2919 and 2850 cm−1 were assigned to the asymmetric and symmetric CeH stretching modes of –CH2 groups of stearic acid, respectively [39]. The absorption band at around 2958 cm−1 was ascribed to the asymmetric in-plane CeH stretching mode of the –CH3 groups [39]. Both the –CH2 and –CH3 groups could significantly lower the surface energy. As shown in Fig. 1(g), water droplets bead up with 167.5 ± 2.0° WCA. In addition, the superhydrophobic surface also shows repellence to other water-containing droplets with WCA > 150° (Fig. S2).
2. Experimental and methods 2.1. Materials Al plates were purchased from Suzhou Metal Material Manufacturer (China). Stearic acid was purchased from Tianjin Guangfu Fine Chemical Research Institute (China). Ethanol was purchased from Tianjin Kermel Chemical Reagent Co (China). 2.2. Superhydrophobic surface fabrication Al surface was first ultrasonically cleaned by anhydrous ethanol for 5 min to remove the contaminations on surface. After drying, Al surface was etched by a fiber laser marking system (SK-CX30, Shanghai Sanke Laser Technology Co., China) at 24 W power, 20 kHz frequency, and 200 mm/s traverse speed to construct micrometer-scale rough structures. The pulse width and the wave length are 100 nm and 1064 nm, respectively. Then the laser-etched surface was immersed in 0.05 mol/L stearic acid ethanol solution for 30 min at room temperature to lower the surface energy.
3.2. Adhesion tuning on the superhydrophobic surface We investigated the temperature-responsive characteristics of the resulting surface. Variation of WCA with temperature is shown in Fig. 2(a), and the insets display that water droplet maintained quasisphere shape at different temperature (i.e., from 27 °C to 70 °C). Although WCA decreased with increased temperature, WCA on the prepared surface remained > 150° up to 49 °C and started dropping with a
2.3. Characterization Surface morphologies and chemical compositions of the sample surfaces were characterized by environmental scanning electron microscope (SEM, SUPRA 55 SAPPHIRE, Germany), energy dispersive 528
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Fig. 1. Characterizations of the superhydrophobic surface. (a, b) SEM images of superhydrophobic surface with different magnifications. (c) Surface roughness of the superhydrophobic surface. (d, e, f) EDS, XRD and FTIR images of the superhydrophobic surface. (g) Digital image of water droplets on the superhydrophobic surface.
minor decrease up to 70 °C at which point the WCA is ≈137 ± 3°. A further increase in temperature led to the flash evaporation of water droplet so the WCA of droplet under such unstable condition was not characterized. We also studied the influence of temperature on the WSA of the surface. As shown in Fig. 2(b), WSA was dramatically affected by temperature and WSA underwent an increase with increased temperature, from 1.7 ± 0.6° at 27 °C to 88.2 ± 1.2° at 50 °C. Insets of Fig. 2(b) show that the water droplet rolled off the 2° tilted surface easily at 27 °C but was in the sticky state on the vertical surface at 50 °C, which means the droplet was adjusted from sliding to sticky state. Experimental results proved that the temperature had a great effect on the adhesion of the surface. Adhesive force (Fadh) is obtained by the projection of water droplet's gravity and can be calculated by the relationship (Eq. (1)),
Fadh = m⋅g⋅sin α
(1)
where m, g and α denote the water droplet mass, gravitational acceleration and WSA. Therefore, water adhesion on the resulting surface at changing temperatures can be obtained (details in Fig. S3, Supporting Information). Digital image of 14 μL colorized droplet arrays on a largescale superhydrophobic surface placed horizontally at 27 °C is shown in Fig. 2(c). As the surface was heated to 50 °C and given a tilt angle of 90°, water droplets still firmly adhered on the surface without sliding away owing to the high adhesive force as shown in Fig. 2(d). According to Eq. (1), the magnitude of the adhesion force between the 50 °C surface and a 14 μL droplet was about 137 μN. Since the wettability and adhesion of the prepared superhydrophobic surface can be controlled via tuning temperature, such surface can potentially act as a mechanical hand for some droplet manipulation processes: capture, merging, and transfer. Fig. 3(a–d) and
Fig. 2. Wettability of the superhydrophobic surface at different temperature. Variations of the (a) WCA and (b) WSA with temperature. Digital images of colorized water droplet arrays on (c) a horizontal surface at 27 °C and (d) a vertical surface at 50 °C, respectively. 529
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Fig. 3. Droplet manipulation processes. (a–d) Capture process using Plate A (superhydrophobicity with low adhesion, 27 °C) and Plate B (hydrophobicity with high adhesion, 50 °C). (f–i) Merging process using Plate A (superhydrophobicity with low adhesion, 27 °C) and Plate B (hydrophobicity with high adhesion, 50 °C). (j–m) Transfer process using Plate B (hydrophobicity with high adhesion, 50 °C) and Plate C (hydrophilicity, 27 °C).
with ~2° WSA, demonstrating a good recoverability. Stability and repeatability of the prepared surface were tested by cycling the heating and cooling process. As shown in Fig. 4(a, b), WCA and WSA were still stabilized at 162.5 ± 2.2° and 2.2 ± 0.4° after 10 cycles at 27 °C, respectively. When the temperature was increased up to 49 °C, WCA and WSA maintained at 149.3 ± 2.3° and 66.0 ± 5.8°. Similar to the dramatical variation of WSA on the surface under alternating temperature cycling, the adhesive force (Fadv) also changed steeply as shown in Fig. S4 (Supporting Information), demonstrating the sharp adhesion switching between sliding superhydrophobicity at 27 °C and sticky superhydrophobicity at 49 °C. Fig. S5 (Supporting Information) shows the characterization of the resulting surface after 10 cycles of low/high adhesion switching. The microstructures were retained and no obvious changes in chemical compositions coated on the surface could be observed, which demonstrated that the stearic acid molecules did not leave the surface. Consequently, these results indicated that water adhesion on the prepared surface could be controlled readily between sliding and sticky. Moreover, the surface revealed excellent recoverability, stability and repeatability at alternating temperature. It can be envisioned that the reversible adhesion switching of the superhydrophobic surface illustrates the feasibility in the applications of smart surface fabrication and in-situ droplet manipulations.
Video S1 (Supporting Information) show a water droplet capture process by taking advantage of adhesion contrast. A 5 μL droplet was first placed on Plate A (superhydrophobicity with low adhesion, 27 °C). After contacting with Plate B (hydrophobicity with high adhesion, 50 °C), the droplet was picked up by Plate B without any water residue left on Plate A (Fig. 3(d)). Similarly, water droplets merging and transfer can also be achieved using surfaces with significant adhesion contrast. As shown in Fig. 3(f–i) and Video S2 (Supporting Information), a 5 μL red droplet adhering to the Plate B (hydrophobicity with high adhesion, 50 °C) was brought into contact with a 5 μL green droplet on Plate A (superhydrophobicity with low adhesion, 27 °C). In the merging process, the green droplet coalesced with the red droplet and firmly adhered to Plate B, demonstrating the lossless transportation from Plate A to Plate B. Subsequently, Plate C (hydrophilicity, 27 °C) was applied to achieve the transfer of the coalesced droplet. The droplet was successfully transferred from low adhesive surface to high adhesive target surface as shown in Fig. 3(j–m) and Video S3 (Supporting Information). Owing to the high surface energy of hydrophilic Plate C surface, the coalesced droplet detached completely from Plate B and no water was remained Fig. 3(m). 3.3. Adhesion switching on the superhydrophobic surface
3.4. Superwettability switching on the superhydrophobic surface Then we carefully studied the influence of the alternating temperature on superhydrophobicity of the prepared surface. According to Fig. 2(a) and (b), the critical temperature that the droplet had a WCA larger than 150° and still could roll off the surface was 49 °C. Hence, the recoverability test of the superhydrophobic surface was performed within the temperature range from 27 °C to 49 °C. During the cycle testing under alternating temperature, the sample was heated from 27 °C to 49 °C, and then cooled back to 27 °C. In the first cycle shown in the red dotted box of Fig. 4(a), WCA decreased from 166.8 ± 1.2° at 27 °C to 147.2 ± 1.4° at 49 °C, revealing a slight degradation of water repellence. When the temperature naturally cooled back to 27 °C, WCA returned to above 160°. However, a dramatical variation of WSA was observed as shown in the red dotted box of Fig. 4(b). Notably, the 1.7 ± 0.6° WSA at 27 °C drastically increased to 71.2 ± 2.9° at 49 °C, which indicated the occurrence of a sharp transition from low adhesion to high adhesion. Subsequently, after cooling back to ambient temperature, the surface recovered to the previous low adhesive superhydrophobic state and water droplet could roll off the recovered surface
In the previous section (Fig. 2(a)), the WCA of the prepared surface at 70 °C was still above 90°, which indicated that the wettability of the prepared surface could not convert from superhydrophobicity to superhydrophilicity by increasing temperature. As shown in Fig. 5(a–h) and Video S4 (Supporting Information), a facile approach was introduced to realize the superwettability switching on the prepared surface, which was from superhydrophobic to superhydrophilic conversion. A 5 μL water droplet was first placed on the Plate D (superhydrophobicity, 27 °C), then superhydrophobic Plate E was heated at 100 °C and was brought into contact with the droplet. Under a mechanical compression applied by the two plates as shown in Fig. 5(c), the droplet was gradually squeezed, deformed with a decreasing WCA (Fig. 5(d–g)), and finally spread out on the surface of Plate D (Fig. 5(h)). After this process, the surfaces of Plate D and Plate E demonstrated ~0° WCA (the left images of Fig. 5(i, j)), achieving the superwettability switching. Moreover, the superhydrophilicity of both surfaces could be maintained for 90 min (Fig. 5(k)). The mechanical compression 530
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Fig. 4. Recoverability, stability and repeatability of the superhydrophobic surface at alternating temperature. Multiple cycles of switching low/high (a) WCA and (b) WSA on the prepared surface.
The converted superhydrophilic plates also could recover superhydrophobicity, as shown in the right images of Fig. 5(i, j). The superhydrophilic plates were first heated at 70 °C for 2 min. Then, superhydrophobicity of the surfaces with the WCA of 161.6 ± 1.2° were regained when the plates were cooled down to 27 °C. Such a reversible superhydrophobic to superhydrophilic switching on the prepared
performed by two unheated plates (both superhydrophobicity, 27 °C) was also investigated. As shown in Video S5 (Supporting Information), the droplet was squeezed out from the interspace between the two plates, and the two plates maintained their superhydrophobicity after compression. Therefore, we attributed the superwettability switching to the synergistic effects of the combination of temperature and pressure.
Fig. 5. The reversible superhydrophobic to superhydrophilic conversion process. (a–h) The conversion process from superhydrophobic to superhydrophilic. (i, j) Recover process of regaining superhydrophobicity of Plate D and Plate E. (k) Durability of the converted superhydrophilic plates. (l, m) Multiple cycles of switching superhydrophobicity and superhydrophilicity on Plate D and Plate E. 531
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Fig. 6. Schematics of the reversible superhydrophobic to superhydrophilic conversion. (a–d) Mechanism of the conversion process from superhydrophobic to superhydrophilic. (e–g) Mechanism of the recover process from superhydrophilic to superhydrophobic.
conversion was also demonstrated. The prepared superhydrophobic surface exhibited excellent recoverability, stability and repeatability during the adhesion switching process and the superwettability switching process. The technical simplicity, environmental friendliness and controllability of our method meet the demands of efficient wettability and adhesion control, and the results offer a new perspective on designing smart mechanical hands and intelligent devices. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.surfcoat.2019.07.041.
surfaces was very well retained after 10 cycles (Fig. 5(l, m)), demonstrating good stability and recoverability. This is primarily attributed to the robust surface micro morphologies and the stable chemical compositions, as shown in Figs. S6 and S7 (Supporting Information). The mechanism explaining the synergism between temperature and pressure during the reversible superhydrophobic to superhydrophilic conversion process is shown in Fig. 6. Water droplet is supported as a sphere on Plate D (Fig. 6(a)) and the high WCA on the surface (superhydrophobicity, 27 °C) would be explained considering CassieBaxter model [16]. Therein to, WCA is determined by the microstructures and the trapped air in the microstructures. Employing the mechanical compression by the superhydrophobic Plate E heated at 100 °C, the droplet is pressed into pancake shape, as shown in Fig. 6(b). Then, the high temperature of Plate E surface generates fog formation around the contact area between liquid and solid. After the small fog drops enter the microstructures of Plate D and Plate E and displace the trapped air, the water and the fog drops will immediately merge. As a result, the microstructures are wetted by water and water droplet spread completely on the surfaces of Plate D and Plate E, exhibiting superhydrophilicity and achieving the conversion to Wenzel state [20,21]. Except the superhydrophobic to superhydrophilic (Cassie to Wenzel) conversion, the recover process (Wenzel to Cassie) is also analyzed, as the schematics shown in Fig. 6(e–g). A heating-up process enable water and fog trapped in the microstructures to be thoroughly evaporated. Since the microstructures and chemical compositions are intact, the surfaces can recover their superhydrophobicity.
Declaration of Competing Interest There are no conflicts to declare. Acknowledgements This project was financially supported by National Natural Science Foundation of China (Grant No. 51605078 and 51621064), National Basic Research Program of China (Grant No. 2015CB057304), Young Elite Scientists Sponsorship Program by CAST (Grant No. 2017QNRC001), Aviation Science Fund (Grant No. 2017ZE63012), Fundamental Research Funds for the Central Universities (No. DUT17JC25 and DUT18JC19). References [1] Y. Lu, S. Sathasivam, J. Song, C.R. Crick, C.J. Carmalt, I.P. Parkin, Robust selfcleaning surfaces that function when exposed to either air or oil, Science 347 (6226) (2015) 1132–1135, https://doi.org/10.1126/science.aaa0946. [2] X. Xue, C. Yu, J. Wang, L. Jiang, Superhydrophobic cones for continuous collection and directional transportation of CO2 microbubbles in CO2 supersaturated solutions, ACS Nano 10 (12) (2016) 10887–10893, https://doi.org/10.1021/acsnano. 6b05371. [3] Z. Chu, Y. Feng, S. Seeger, Oil/water separation with selective superantiwetting/ superwetting surface materials, Angew. Chem. Int. Ed. 54 (8) (2015) 2328–2338, https://doi.org/10.1002/anie.201405785. [4] S. Srinivasan, J.A. Kleingartner, J.B. Gilbert, R.E. Cohen, A.J.B. Milne, G.H. Mckinley, Sustainable drag reduction in turbulent Taylor-Couette flows by depositing sprayable superhydrophobic surfaces, Phys. Rev. Lett. 114 (0145011) (2015), https://doi.org/10.1103/PhysRevLett.114.014501. [5] H. Qian, D. Xu, C. Du, D. Zhang, X. Li, L. Huang, L. Deng, Y. Tu, J.M.C. Mol, H.A. Terryn, Dual-action smart coatings with a self-healing superhydrophobic surface and anti-corrosion properties, J. Mater. Chem. A 5 (5) (2017) 2355–2364, https://doi.org/10.1039/C6TA10903A. [6] J. Li, R. Wu, Z. Jing, L. Yan, F. Zha, Z. Lei, One-step spray-coating process for the fabrication of colorful superhydrophobic coatings with excellent corrosion resistance, Langmuir 31 (39) (2015) 10702–10707, https://doi.org/10.1021/acs. langmuir.5b02734.
4. Conclusion In summary, a superhydrophobic Al surface was fabricated via laser etching and stearic acid modification. We investigated the effect of temperature on the wettability of the resulting surface by measuring WCA and WSA. The superhydrophobicity of the surface was degraded with the increase of temperature. In addition, the effects of alternating temperature on the superhydrophobicity were studied. Water adhesion on the superhydrophobic surface could be readily tuned between lowadhesive ability (167.5 ± 2.2° WCA, 1.7 ± 0.6° WSA) at 27 °C and high-adhesive ability (149.3 ± 2.3° WCA, 66.0 ± 5.8° WSA) at 49 °C. Utilizing the adhesion contrast, various water droplet manipulations involving capture, merging and transfer were readily achieved. Moreover, a facile approach induced by temperature and pressure to achieve the reversible superhydrophobic to superhydrophilic 532
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Temperature-based adhesion tuning and superwettability switching on superhydrophobic aluminum surface for droplet manipulations
T ⁎
Ziai Liua,1, Xiaolong Yangb,1, Guibing Pangc, Fan Zhanga, Yuqi Hana, Xuyue Wanga, Xin Liua, , ⁎ Lin Xuea, a
Key Laboratory for Precision and Non-traditional Machining Technology of the Ministry of Education, Dalian University of Technology, Dalian 116024, PR China National Key Laboratory of Science and Technology on Helicopter Transmission, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China c College of Mechanical Engineering and Automation, Dalian Polytechnic University, Dalian 116024, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Superhydrophobic surface Aluminum Temperature-responsive characteristic Adhesion tuning Superwettability switching
Superhydrophobic surfaces with tunable water adhesion and adjustable wettability have the capacity to manipulate water droplets under complicated conditions and, therefore, have great prospects in various domains. Hence, developing a facile method to prepare highly stable superhydrophobic surfaces with tunable water adhesion and adjustable wettability is of great importance. Herein, we prepare the superhydrophobic aluminum surface by combining laser etching and stearic acid modification. The variations of water contact angle, water sliding angle and water adhesive force with temperature of the prepared surface are characterized during the adhesion tuning process to investigate its temperature-responsive characteristics. A low/high adhesion switching of water on the prepared surface is subsequently achieved under alternating temperature. In addition, a facile approach is designed to achieve the superwettability switching process (from superhydrophobic state to superhydrophilic state) on the prepared surface. The prepared superhydrophobic surface displays excellent recoverability, stability and repeatability in the adhesion switching process and the superwettability switching process, even after being cycled for 10 times.
1. Introduction Superhydrophobic surfaces repelling water with a static water contact angle (WCA) larger than 150° have been extensively investigated in various areas, such as self-cleaning [1], directional transportation [2], oil/water separation [3], drag reduction [4], anticorrosion [5,6], etc. In the past decades, numerous techniques have been proposed to prepare superhydrophobic surfaces, including laser etching [7], chemical etching [8–10], electrochemical etching [11], chemical deposition [12], chemical oxidation [13], spray-coating [14,15]. Generally, superhydrophobic surfaces can be classified into superhydrophobic surfaces with low adhesion and superhydrophobic surfaces with high adhesion according to the state of water droplets on them. Water droplets are in Cassie state on the superhydrophobic surfaces with low adhesion [16]. Droplets are able to roll off the surfaces very easily with a water sliding angle (WSA) lower than 10°. Artificial superhydrophobic surfaces with low adhesion can be fabricated directly by integrating ordinary microstructures construction with low surface
energy modification [17–19]. For the superhydrophobic surfaces with high adhesion, water droplets on them are in Wenzel state [20,21]. Owing to their sticky behaviors towards water, WSA on such surfaces is larger than 10°. Superhydrophobic surfaces with high adhesion can be prepared by combining multiple microstructures construction and low surface energy modification [22,23]. Additionally, they can also be prepared by fabricating (super)hydrophilic patterns with high adhesion on superhydrophobic surface with low adhesion [24,25]. Superhydrophobic surfaces with stable wettability and adhesion have attracted extensive interest in various domains, such as lossless transfer of microdroplets [26], biomimetic organs fabrication [27], and droplets manipulation [28]. In recent years, researchers have found that the wettability and adhesion of some superhydrophobic surfaces can be tuned when internal and external conditions are changed [29–33]. For instance, Raturi et al. [34] endowed ZnO-nanowirescoated surface mesh with reversible wettability by annealing the mesh alternatively under hydrogen and oxygen environment for oil/water separation. Liu et al. [35] reported an intelligent surface that could fast and reversibly switch its wettability through macroscopic shape
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Corresponding authors. E-mail addresses: [email protected] (X. Liu), [email protected] (L. Xue). 1 Contribute equally. https://doi.org/10.1016/j.surfcoat.2019.07.041 Received 8 May 2019; Received in revised form 9 July 2019; Accepted 21 July 2019 Available online 22 July 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
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spectroscopy (EDS, INCA Energy, Oxford Instruments), X-ray diffraction (XRD, Shimadzu XRD-6000, Japan) and Fourier transform infrared spectroscopy (FTIR, JACSCO, Japan). Surface roughness value, Ra, was measured with a roughness tester (Talysurf CLI2000, UK). The average roughness value (Ra) was based on 10 measurements obtained from 10 different directions because the laser etched surface was anisotropic. Water contact angle (WCA) was measured according to the sessile-drop method using an optical contact angle meter (KINO SL200KS, USA). Water sliding angle (WSA) was investigated using a precision turntable. To measure the WSA, a water droplet was put on the surface of a sample which was placed on the turntable. Then, WSA was measured by tilting the turntable to a certain angle at which the water droplet started to slide off the surface. A temperature controller (Hanbang HP-2525, China) was fixed with the sample by thermal conductive silicone (Kafuter K-5203K, China) to heat the sample surface to a certain temperature during WCA measurement, WSA measurement, droplet manipulation processes and reversible superwettability switching processes. The WCA and WSA measurements were performed using 14 μL deionized water droplets at ambient temperature (27 °C, 78% relative humidity). It is necessary to note that WCA and WSA were measured as soon as possible when a droplet was deposited on a sample surface. Therefore, the influence of the mass loss of a droplet is negligible. Error bars of WCA and WSA showed the variation from the average value based on five measurements obtained at different positions. In this manuscript, dyed water droplets were only used to take digital images for visualization. Undyed water droplets were used to measure WCA and WSA. Experimental optical images were captured using a camera equipped with a Nikon 105 mm f/2.8G lens (Nikon D7200, Japan).
change. However, the wettability of these surfaces could only be varied between superhydrophobicity and superhydrophilicity, revealing poor controllability in adhesion tuning. Chen et al. [36] proposed an allwater-based self-repairing superhydrophobic coating based on UV-responsive microcapsules by Pickering emulsion polymerization, the coating showed recoverability under UV light. Nevertheless, the nanoparticles contained in the experiment were harmful to the operators and the preparation processes were quite complicated. Liu et al. [37] reported switchable water adhesion on superhydrophobic surface by atmospheric pressure maskless microplasma jet treatment. However, this method required expensive equipment and the low surface energy modification was needed to recover superhydrophobicity. Yao et al. [31] reported a temperature-driven switching of water adhesion on nparaffin-swollen organogel via the thermo-responsive phase change of n-paraffin. However, the white substrate was transformed into transparent when the switching of water adhesion occurred at the melting temperature of n-paraffin, which was inconvenient for practical applications. It is, therefore, essential to design a high-performance wettability and adhesion tuning strategy that can solve the aforementioned problems. Aluminum (Al) is extensively used for fundamental science and industrial applications owing to its outstanding mechanical properties, good heat dissipating performance and low cost. Thus, endowing Al surface with tunable water adhesion and adjustable wettability is of great significance. In the present study, superhydrophobic Al surface with 167.5 ± 2.0° WCA and 1.7 ± 0.6° WSA was facilely fabricated by integrating laser etching with stearic acid modification. We investigated the effect of the temperature on the wettability of the resulting surface by measuring WCA and WSA. The results showed that water droplet was in sticky state with 137.5 ± 3.0° WCA when the superhydrophobic surface was heated at 70 °C. Moreover, water droplet could be adjusted between sliding (low adhesion) and sticky state (high adhesion) on the surface under alternating temperature. Additionally, a facile approach induced by temperature and pressure was devised to achieve the reversible superhydrophobic to superhydrophilic conversion on the resulting surface. Compared with the existing methods, we developed a facile method to achieve the precise adhesion tuning and reversible superwettability switching, which ensures good reliability and paves ways for stable droplet manipulations.
3. Result and discussion 3.1. Characterization of the superhydrophobic surface Fig. 1(a) shows the micro-morphology of the superhydrophobic Al surface with three-dimensional antler-like microstructures, which were covered by 1–5 μm spherical particles (Fig. 1(b)). The generation of these spherical particles were owing to the melting and solidification of Al during the laser etching process [38]. The surface roughness (Ra) of the original Al surface before the laser etching process was about 1 μm, while the Ra value increased to about 5 μm after being etched by laser as shown in Fig. 1(c). It is necessary to note that the stearic acid modification process did not have an influence on the Ra value (Fig. S1). EDS image shown in Fig. 1(d) detected the peaks from Al and O on the superhydrophobic surface. Moreover, XRD pattern of the surface confirmed that the main compositions of the microstructures on the superhydrophobic surface was Al (Fig. 1(e)). As the FTIR spectra illustrated in Fig. 1(f), the absorption bands assigned to the CeH stretching vibration of the –CHn groups of the stearic acid molecules were detected on the superhydrophobic surface. The absorption bands at around 2919 and 2850 cm−1 were assigned to the asymmetric and symmetric CeH stretching modes of –CH2 groups of stearic acid, respectively [39]. The absorption band at around 2958 cm−1 was ascribed to the asymmetric in-plane CeH stretching mode of the –CH3 groups [39]. Both the –CH2 and –CH3 groups could significantly lower the surface energy. As shown in Fig. 1(g), water droplets bead up with 167.5 ± 2.0° WCA. In addition, the superhydrophobic surface also shows repellence to other water-containing droplets with WCA > 150° (Fig. S2).
2. Experimental and methods 2.1. Materials Al plates were purchased from Suzhou Metal Material Manufacturer (China). Stearic acid was purchased from Tianjin Guangfu Fine Chemical Research Institute (China). Ethanol was purchased from Tianjin Kermel Chemical Reagent Co (China). 2.2. Superhydrophobic surface fabrication Al surface was first ultrasonically cleaned by anhydrous ethanol for 5 min to remove the contaminations on surface. After drying, Al surface was etched by a fiber laser marking system (SK-CX30, Shanghai Sanke Laser Technology Co., China) at 24 W power, 20 kHz frequency, and 200 mm/s traverse speed to construct micrometer-scale rough structures. The pulse width and the wave length are 100 nm and 1064 nm, respectively. Then the laser-etched surface was immersed in 0.05 mol/L stearic acid ethanol solution for 30 min at room temperature to lower the surface energy.
3.2. Adhesion tuning on the superhydrophobic surface We investigated the temperature-responsive characteristics of the resulting surface. Variation of WCA with temperature is shown in Fig. 2(a), and the insets display that water droplet maintained quasisphere shape at different temperature (i.e., from 27 °C to 70 °C). Although WCA decreased with increased temperature, WCA on the prepared surface remained > 150° up to 49 °C and started dropping with a
2.3. Characterization Surface morphologies and chemical compositions of the sample surfaces were characterized by environmental scanning electron microscope (SEM, SUPRA 55 SAPPHIRE, Germany), energy dispersive 528
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Fig. 1. Characterizations of the superhydrophobic surface. (a, b) SEM images of superhydrophobic surface with different magnifications. (c) Surface roughness of the superhydrophobic surface. (d, e, f) EDS, XRD and FTIR images of the superhydrophobic surface. (g) Digital image of water droplets on the superhydrophobic surface.
minor decrease up to 70 °C at which point the WCA is ≈137 ± 3°. A further increase in temperature led to the flash evaporation of water droplet so the WCA of droplet under such unstable condition was not characterized. We also studied the influence of temperature on the WSA of the surface. As shown in Fig. 2(b), WSA was dramatically affected by temperature and WSA underwent an increase with increased temperature, from 1.7 ± 0.6° at 27 °C to 88.2 ± 1.2° at 50 °C. Insets of Fig. 2(b) show that the water droplet rolled off the 2° tilted surface easily at 27 °C but was in the sticky state on the vertical surface at 50 °C, which means the droplet was adjusted from sliding to sticky state. Experimental results proved that the temperature had a great effect on the adhesion of the surface. Adhesive force (Fadh) is obtained by the projection of water droplet's gravity and can be calculated by the relationship (Eq. (1)),
Fadh = m⋅g⋅sin α
(1)
where m, g and α denote the water droplet mass, gravitational acceleration and WSA. Therefore, water adhesion on the resulting surface at changing temperatures can be obtained (details in Fig. S3, Supporting Information). Digital image of 14 μL colorized droplet arrays on a largescale superhydrophobic surface placed horizontally at 27 °C is shown in Fig. 2(c). As the surface was heated to 50 °C and given a tilt angle of 90°, water droplets still firmly adhered on the surface without sliding away owing to the high adhesive force as shown in Fig. 2(d). According to Eq. (1), the magnitude of the adhesion force between the 50 °C surface and a 14 μL droplet was about 137 μN. Since the wettability and adhesion of the prepared superhydrophobic surface can be controlled via tuning temperature, such surface can potentially act as a mechanical hand for some droplet manipulation processes: capture, merging, and transfer. Fig. 3(a–d) and
Fig. 2. Wettability of the superhydrophobic surface at different temperature. Variations of the (a) WCA and (b) WSA with temperature. Digital images of colorized water droplet arrays on (c) a horizontal surface at 27 °C and (d) a vertical surface at 50 °C, respectively. 529
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Fig. 3. Droplet manipulation processes. (a–d) Capture process using Plate A (superhydrophobicity with low adhesion, 27 °C) and Plate B (hydrophobicity with high adhesion, 50 °C). (f–i) Merging process using Plate A (superhydrophobicity with low adhesion, 27 °C) and Plate B (hydrophobicity with high adhesion, 50 °C). (j–m) Transfer process using Plate B (hydrophobicity with high adhesion, 50 °C) and Plate C (hydrophilicity, 27 °C).
with ~2° WSA, demonstrating a good recoverability. Stability and repeatability of the prepared surface were tested by cycling the heating and cooling process. As shown in Fig. 4(a, b), WCA and WSA were still stabilized at 162.5 ± 2.2° and 2.2 ± 0.4° after 10 cycles at 27 °C, respectively. When the temperature was increased up to 49 °C, WCA and WSA maintained at 149.3 ± 2.3° and 66.0 ± 5.8°. Similar to the dramatical variation of WSA on the surface under alternating temperature cycling, the adhesive force (Fadv) also changed steeply as shown in Fig. S4 (Supporting Information), demonstrating the sharp adhesion switching between sliding superhydrophobicity at 27 °C and sticky superhydrophobicity at 49 °C. Fig. S5 (Supporting Information) shows the characterization of the resulting surface after 10 cycles of low/high adhesion switching. The microstructures were retained and no obvious changes in chemical compositions coated on the surface could be observed, which demonstrated that the stearic acid molecules did not leave the surface. Consequently, these results indicated that water adhesion on the prepared surface could be controlled readily between sliding and sticky. Moreover, the surface revealed excellent recoverability, stability and repeatability at alternating temperature. It can be envisioned that the reversible adhesion switching of the superhydrophobic surface illustrates the feasibility in the applications of smart surface fabrication and in-situ droplet manipulations.
Video S1 (Supporting Information) show a water droplet capture process by taking advantage of adhesion contrast. A 5 μL droplet was first placed on Plate A (superhydrophobicity with low adhesion, 27 °C). After contacting with Plate B (hydrophobicity with high adhesion, 50 °C), the droplet was picked up by Plate B without any water residue left on Plate A (Fig. 3(d)). Similarly, water droplets merging and transfer can also be achieved using surfaces with significant adhesion contrast. As shown in Fig. 3(f–i) and Video S2 (Supporting Information), a 5 μL red droplet adhering to the Plate B (hydrophobicity with high adhesion, 50 °C) was brought into contact with a 5 μL green droplet on Plate A (superhydrophobicity with low adhesion, 27 °C). In the merging process, the green droplet coalesced with the red droplet and firmly adhered to Plate B, demonstrating the lossless transportation from Plate A to Plate B. Subsequently, Plate C (hydrophilicity, 27 °C) was applied to achieve the transfer of the coalesced droplet. The droplet was successfully transferred from low adhesive surface to high adhesive target surface as shown in Fig. 3(j–m) and Video S3 (Supporting Information). Owing to the high surface energy of hydrophilic Plate C surface, the coalesced droplet detached completely from Plate B and no water was remained Fig. 3(m). 3.3. Adhesion switching on the superhydrophobic surface
3.4. Superwettability switching on the superhydrophobic surface Then we carefully studied the influence of the alternating temperature on superhydrophobicity of the prepared surface. According to Fig. 2(a) and (b), the critical temperature that the droplet had a WCA larger than 150° and still could roll off the surface was 49 °C. Hence, the recoverability test of the superhydrophobic surface was performed within the temperature range from 27 °C to 49 °C. During the cycle testing under alternating temperature, the sample was heated from 27 °C to 49 °C, and then cooled back to 27 °C. In the first cycle shown in the red dotted box of Fig. 4(a), WCA decreased from 166.8 ± 1.2° at 27 °C to 147.2 ± 1.4° at 49 °C, revealing a slight degradation of water repellence. When the temperature naturally cooled back to 27 °C, WCA returned to above 160°. However, a dramatical variation of WSA was observed as shown in the red dotted box of Fig. 4(b). Notably, the 1.7 ± 0.6° WSA at 27 °C drastically increased to 71.2 ± 2.9° at 49 °C, which indicated the occurrence of a sharp transition from low adhesion to high adhesion. Subsequently, after cooling back to ambient temperature, the surface recovered to the previous low adhesive superhydrophobic state and water droplet could roll off the recovered surface
In the previous section (Fig. 2(a)), the WCA of the prepared surface at 70 °C was still above 90°, which indicated that the wettability of the prepared surface could not convert from superhydrophobicity to superhydrophilicity by increasing temperature. As shown in Fig. 5(a–h) and Video S4 (Supporting Information), a facile approach was introduced to realize the superwettability switching on the prepared surface, which was from superhydrophobic to superhydrophilic conversion. A 5 μL water droplet was first placed on the Plate D (superhydrophobicity, 27 °C), then superhydrophobic Plate E was heated at 100 °C and was brought into contact with the droplet. Under a mechanical compression applied by the two plates as shown in Fig. 5(c), the droplet was gradually squeezed, deformed with a decreasing WCA (Fig. 5(d–g)), and finally spread out on the surface of Plate D (Fig. 5(h)). After this process, the surfaces of Plate D and Plate E demonstrated ~0° WCA (the left images of Fig. 5(i, j)), achieving the superwettability switching. Moreover, the superhydrophilicity of both surfaces could be maintained for 90 min (Fig. 5(k)). The mechanical compression 530
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Fig. 4. Recoverability, stability and repeatability of the superhydrophobic surface at alternating temperature. Multiple cycles of switching low/high (a) WCA and (b) WSA on the prepared surface.
The converted superhydrophilic plates also could recover superhydrophobicity, as shown in the right images of Fig. 5(i, j). The superhydrophilic plates were first heated at 70 °C for 2 min. Then, superhydrophobicity of the surfaces with the WCA of 161.6 ± 1.2° were regained when the plates were cooled down to 27 °C. Such a reversible superhydrophobic to superhydrophilic switching on the prepared
performed by two unheated plates (both superhydrophobicity, 27 °C) was also investigated. As shown in Video S5 (Supporting Information), the droplet was squeezed out from the interspace between the two plates, and the two plates maintained their superhydrophobicity after compression. Therefore, we attributed the superwettability switching to the synergistic effects of the combination of temperature and pressure.
Fig. 5. The reversible superhydrophobic to superhydrophilic conversion process. (a–h) The conversion process from superhydrophobic to superhydrophilic. (i, j) Recover process of regaining superhydrophobicity of Plate D and Plate E. (k) Durability of the converted superhydrophilic plates. (l, m) Multiple cycles of switching superhydrophobicity and superhydrophilicity on Plate D and Plate E. 531
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Fig. 6. Schematics of the reversible superhydrophobic to superhydrophilic conversion. (a–d) Mechanism of the conversion process from superhydrophobic to superhydrophilic. (e–g) Mechanism of the recover process from superhydrophilic to superhydrophobic.
conversion was also demonstrated. The prepared superhydrophobic surface exhibited excellent recoverability, stability and repeatability during the adhesion switching process and the superwettability switching process. The technical simplicity, environmental friendliness and controllability of our method meet the demands of efficient wettability and adhesion control, and the results offer a new perspective on designing smart mechanical hands and intelligent devices. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.surfcoat.2019.07.041.
surfaces was very well retained after 10 cycles (Fig. 5(l, m)), demonstrating good stability and recoverability. This is primarily attributed to the robust surface micro morphologies and the stable chemical compositions, as shown in Figs. S6 and S7 (Supporting Information). The mechanism explaining the synergism between temperature and pressure during the reversible superhydrophobic to superhydrophilic conversion process is shown in Fig. 6. Water droplet is supported as a sphere on Plate D (Fig. 6(a)) and the high WCA on the surface (superhydrophobicity, 27 °C) would be explained considering CassieBaxter model [16]. Therein to, WCA is determined by the microstructures and the trapped air in the microstructures. Employing the mechanical compression by the superhydrophobic Plate E heated at 100 °C, the droplet is pressed into pancake shape, as shown in Fig. 6(b). Then, the high temperature of Plate E surface generates fog formation around the contact area between liquid and solid. After the small fog drops enter the microstructures of Plate D and Plate E and displace the trapped air, the water and the fog drops will immediately merge. As a result, the microstructures are wetted by water and water droplet spread completely on the surfaces of Plate D and Plate E, exhibiting superhydrophilicity and achieving the conversion to Wenzel state [20,21]. Except the superhydrophobic to superhydrophilic (Cassie to Wenzel) conversion, the recover process (Wenzel to Cassie) is also analyzed, as the schematics shown in Fig. 6(e–g). A heating-up process enable water and fog trapped in the microstructures to be thoroughly evaporated. Since the microstructures and chemical compositions are intact, the surfaces can recover their superhydrophobicity.
Declaration of Competing Interest There are no conflicts to declare. Acknowledgements This project was financially supported by National Natural Science Foundation of China (Grant No. 51605078 and 51621064), National Basic Research Program of China (Grant No. 2015CB057304), Young Elite Scientists Sponsorship Program by CAST (Grant No. 2017QNRC001), Aviation Science Fund (Grant No. 2017ZE63012), Fundamental Research Funds for the Central Universities (No. DUT17JC25 and DUT18JC19). References [1] Y. Lu, S. Sathasivam, J. Song, C.R. Crick, C.J. Carmalt, I.P. Parkin, Robust selfcleaning surfaces that function when exposed to either air or oil, Science 347 (6226) (2015) 1132–1135, https://doi.org/10.1126/science.aaa0946. [2] X. Xue, C. Yu, J. Wang, L. Jiang, Superhydrophobic cones for continuous collection and directional transportation of CO2 microbubbles in CO2 supersaturated solutions, ACS Nano 10 (12) (2016) 10887–10893, https://doi.org/10.1021/acsnano. 6b05371. [3] Z. Chu, Y. Feng, S. Seeger, Oil/water separation with selective superantiwetting/ superwetting surface materials, Angew. Chem. Int. Ed. 54 (8) (2015) 2328–2338, https://doi.org/10.1002/anie.201405785. [4] S. Srinivasan, J.A. Kleingartner, J.B. Gilbert, R.E. Cohen, A.J.B. Milne, G.H. Mckinley, Sustainable drag reduction in turbulent Taylor-Couette flows by depositing sprayable superhydrophobic surfaces, Phys. Rev. Lett. 114 (0145011) (2015), https://doi.org/10.1103/PhysRevLett.114.014501. [5] H. Qian, D. Xu, C. Du, D. Zhang, X. Li, L. Huang, L. Deng, Y. Tu, J.M.C. Mol, H.A. Terryn, Dual-action smart coatings with a self-healing superhydrophobic surface and anti-corrosion properties, J. Mater. Chem. A 5 (5) (2017) 2355–2364, https://doi.org/10.1039/C6TA10903A. [6] J. Li, R. Wu, Z. Jing, L. Yan, F. Zha, Z. Lei, One-step spray-coating process for the fabrication of colorful superhydrophobic coatings with excellent corrosion resistance, Langmuir 31 (39) (2015) 10702–10707, https://doi.org/10.1021/acs. langmuir.5b02734.
4. Conclusion In summary, a superhydrophobic Al surface was fabricated via laser etching and stearic acid modification. We investigated the effect of temperature on the wettability of the resulting surface by measuring WCA and WSA. The superhydrophobicity of the surface was degraded with the increase of temperature. In addition, the effects of alternating temperature on the superhydrophobicity were studied. Water adhesion on the superhydrophobic surface could be readily tuned between lowadhesive ability (167.5 ± 2.2° WCA, 1.7 ± 0.6° WSA) at 27 °C and high-adhesive ability (149.3 ± 2.3° WCA, 66.0 ± 5.8° WSA) at 49 °C. Utilizing the adhesion contrast, various water droplet manipulations involving capture, merging and transfer were readily achieved. Moreover, a facile approach induced by temperature and pressure to achieve the reversible superhydrophobic to superhydrophilic 532
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