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Two–step fabrication of superhydrophobic surfaces with anti–adhesion
Optics and Laser Technology 113 (2019) 273–280
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Full length article
Two–step fabrication of superhydrophobic surfaces with anti–adhesion a,⁎
b
a
a
Jing Li , Feng Du , Yanhui Zhao , Shicai Zhao , Huadong Yu a b
T
a
College of Mechanical and Electric Engineering, Changchun University of Science and Technology, Changchun 130022, PR China Army Academy of Amored Forces, Changchun 130117, PR China
H I GH L IG H T S
superhydrophobic surfaces were fabricated by coupling processing. • The contact angle was as high as 156.3° and the sliding angle was 4.8°. • The superhydrophobic surface exhibited an excellent low adhesion property. • The • The superhydrophobic surface had excellent self–cleaning property.
A R T I C LE I N FO
A B S T R A C T
Keywords: Electro–brush plating Laser processing Superhydrophobicity Anti–adhesion
A self–cleaning property with anti–adhesion and superhydrophobicity was fabricated on aluminum alloy surface via a facile and low–cost method coupling electro–brush plating and laser processing. The surface morphology, chemical composition, wettability, anti–adhesion and self–cleaning effect were analyzed by multiple ways. The results indicated that an array of micro–scale grooves and typical hierarchical protrusion structure were covered on the surface, and the edge of the groove was covered by micro–scale sputtering particles. The special composite structures played an important role in amplifying the hydrophobic property of surface. The surface showed excellent superhydrophobic properties. The contact angle was as high as 156.3° and the sliding angle was 4.8°. The water droplet has approached onto the superhydrophobic surface and completely bounced back to the tip of the needle on the coupling surface. The adhesion between the coupling surface and the water droplet is very weakly, the surface has excellent anti–adhesion property. Through the comparative test, the composite method also has excellent low adhesion property to the polyethylene material. In this case, the low adhesion led to a rolling and self–cleaning behavior. The approach presented in this study provides a facile, low–cost and efficient method to fabricate superhydrophobic surfaces on all types of metallic materials for commercial applications.
1. Introduction
observed on surfaces of some plants and animals in nature, such as rice leaves [18], lotus leaves [19], rose petals [20–23] and butterfly wings [24], which exhibited excellent hydrophobicity. Their hydrophobicity was determined by the cooperation of geometric microstructure and chemical composition of surfaces. Therefore, those natural surface structures, as mimetic models, could be used to design and fabricated on the engineering materials surfaces. So far, a large number of approaches have been successfully used to develop hydrophobic surface, including chemical etching [25,26], solution immersion [27], interference lithography [28–31], high speed wire electrical discharge machining processing (HS–WEDM) [32] and laser fabrication [33–35]. Up to now, the various methods have been reported for fabricating superhydrophobic surface on aluminum and its alloys material. The preparation method mainly comes down to two types: one of the ways is
As the continuous exploration of the special morphologies and properties of some animal skins and plant leaf surfaces in nature, new hydrophobic surfaces are explored through imitation of the observed surface effects. The hydrophobic [1,2] theory has been analyzed by many people, and hydrophobic surface has been investigated by numerous researchers because of the potential in self–cleaning [3–6], anti–adhesion [7–10], anti–icing [11–13], water repellency [14], drag reduction in micro–fluidics [15], prevention of corrosion [16], etc. A surface with static water contact angle (CA) more than 150° and contact angle hysteresis less than 10° was defined as superhydrophobic surface [17]. The preparation of the superhydrophobic surface has very important significance and value. The hydrophobicity has been originally ⁎
Corresponding author. E-mail address: [email protected] (J. Li).
https://doi.org/10.1016/j.optlastec.2018.12.045 Received 4 June 2018; Received in revised form 12 November 2018; Accepted 22 December 2018 0030-3992/ © 2018 Elsevier Ltd. All rights reserved.
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fabricating rough structure on low energy surface, the other way is fabricating rough structures on high energy surface and using chemical modification. The chemical modification could improve processing costs or other problem of usage. So the second method has more difficulty than the first one. Liu et al. [36] fabricated lotus–leaf–like superhydrophobic surface on carbon steel substrate via two–layer nano–composite electro–brush plating and subsequent surface modification with low free energy materials and the contact angle is up to 155.5°, the sliding angle is 5°. Zhong et al. [37] used a microstructure array template to transform a polydimethylsiloxane surface into a superhydrophobic surface with high transparency and stable mechanical properties using a femtosecond laser. By adjusting the size and separation of the microstructure, the hydrophobicity and transparency of the surface could be controlled. Jagdheesh et al. [38] fabricated micro channels and pillars by a nanosecond laser source on thin aluminum surface. They found that the frequency power, density and pulse width could significantly influence the new aluminum oxide surfaces formed after laser processing. Liu et al. [34] reviewed facile methods to fabricate large–area biomimetic hierarchical structures via laser processing, a diversity of large area hierarchical structures can be obtained and show rich biomimetic functions such as superhydrophobicity. Gu et al. [39] obtained a superhydrophobic surface by repeatedly immersing the copper sheet in a 0.05 M silver nitrate aqueous solution and then modifying it in an ethanol solution of n-dodecanethiol. In this study, the superhydrophobic surface was successfully fabricated on aluminum alloy. An array of micro–scale grooves and typical hierarchical protrusions structure were fabricated on the surface coupling electro–brush plating and laser processing. The wettability of the surface had been changed from hydrophilicity to superhydrophobicity, and the superhydrophobic surface has excellent low–adhesion and self–cleaning property.
Fig. 1. Schematic views of formation for the superhydrophobic surface.
were placed in air for many days. The processing flowchart is shown in Fig. 1. 2.3. Characterization
2. Experimental
The morphologies of the aluminum alloys were observed by scanning electron microscope (COXEM, EM–30 Korea) and laser scanning confocal microscopy (LSCM, LSM 700, ZEISS, Germany). The composition of the prepared surfaces was analyzed using X–ray diffraction (XRD, D/max–2500PC, Rigaku, Japan). The wettability of various surfaces were measured by statics contact angle and slide angle on the contact angle meter (OCA15 Pro, Dataphysics, Germany) with a water drop volume 4 μl at room temperature. All water contact angles and slide angle were measured at 4–6 different points and were averaged.
2.1. Materials 20 mm × 20 mm × 2 mm 7075 aluminum alloy (major elements, wt %: Si 0.4, Fe 0.50, Cu 1.2–2.0, Mn 0.30, Mg 2.1–2.9, Cr 0.18–0.28, Zn 5.1–6.1, Ti 0.20 and remaining element Al) purchased from Shanghai Ao feng metal product co, ltd. China. The Ni coatings were prepared by an improved electro–brush plating technique. The used the plating solution mainly consists of nickel sulfate hexahydrate (265 g/L), purchase from Beijing Zhen hai Institute of chemistry China, ammonium citrate (100 g/L), ammonium acetate (23 g/L), ammonium hydroxide (25–28%, 100–200 mL/L) purchase from Qidong Ming into chemical co, ltd. China, the pH value is 7.3–7.8.
3. Results and discussion 3.1. Morphology It is well known that hydrophobic surfaces are usually associated with geometrical micro–structure and surface roughness. The different images of surfaces were observed with different processing methods from Fig. 2. The morphology of the prepared surface is shown in the SEM images. It can be seen that the smooth substrate surface is very flat, which doesn’t have other raise or the appearance of micro–scale structures (Fig. 2a). Fig. 2b showed the micro–topography of the Ni coating prepared by electro–brush plating technique with the plating voltage of 14 V. Many micro–scale protrusions and pits are uniformly distributed on the surface. The high–resolution shows that the size of pit is 5–20 μm. The diameter in top of the protrusion is 25–40 μm. The micro–groove structures are distributed on the Ni coatings, the surface becomes a composite structure with grooves and micro–protrusions. The protrusion structures are distributed between adjacent grooves. As shown in Fig. 2c, the distance between adjacent grooves is generally in the range 90–100 μm, the width of one groove is 45–55 μm, and the width of one bare protrusion is about 45–60 μm. This image also shows that the feature of coating do not effect directly by laser processing.
2.2. Fabrication First, aluminum alloy surfaces were polished by No. 800–2000 SiC paper to obtain a relatively smooth substrate, and subsequently cleaned with acetone, alcohol and de–ionized water, and then dried in air. Second, the micro–scale asperities and pits are uniformly distributed on surface with a special DC power (NBD–150, Jinlin, China). And the hierarchical composite structures which were consisted of micro–protrusions and submicro–grains were formed on the sample surfaces by electric brush–plating technique. When preparing coatings, graphite anode should move back and forth along a direction relative to the substrate. The working parameters were determined: plating voltage is 14 V and its chosen is according to plating solution concentration, the speed of plating is 60–130 mm/s, working time is 10 min. The Ni coating surface was dried completely at 150 °C for 2 h. Finally, the sample was machined at distance 100 μm between the adjacent grooves centrality by laser (HBS–GQ–20, Beijing, China), the speed of laser scanning is 500 mm/s and scans twice in a row. The prepared surfaces 274
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Fig. 2. SEM micrographs of different surfaces, (a) Smooth substrate, (b) Ni coating surface, (c) Coupling processing surface; Three–dimensional LSCM images of different surfaces, (d) Smooth substrate, (e) Ni coating surface, (f) Coupling processing surface.
Many spurting particles are located on the edge of grooves, made the surface much rougher. In laser processing, the surface of the material would be generated high temperature instantly. Then part of the surface could appear melting phenomenon around laser beam point with the moving of the laser beam, the melting area is going to be cooled far from the center of laser. So the groove structures are formed by laser beam. These micro–structures can capture a large amount of air and significantly reduce the actual contact area between water droplet and the surface, which can prevent the wetting between water droplet and solid surface. The morphology of the surface was fabricated by different processing. To more intuitively show the morphology of the surface composite structure, the images were characterized by LSCM, as shown in Fig. 2(d–f). It can be seen that the surface is very smooth, just have many slight polishing traces (Fig. 2a). Fig. 2b shows that many protrusions are uniform and dense distribution on the surface, the height is about 15 μm. There are pit structures between different protrusions. Fig. 2c clearly reveals the groove structures and protrusion structures on the surface, and the depth of groove is 6–10 μm.
Fig. 3. Comparison of the XRD patterns obtained for: (a) Coupling processing surface, (b) Ni coating surface, (c) Substrate surface.
diffraction angles are all fit well with the standard pattern of polycrystalline Ni (ICSD PDF NO: 70–1849). As inferred from the strength of the diffraction peak, the laser processing made the crystal grow better along (200) crystal face.
3.2. Chemical composition The XRD pattern of substrate surface, Ni coating surface and coupling processing surface are shown in Fig. 3. It is obvious that the diffraction peak characteristic of smooth substrate surface is different from the other two surfaces. The diffraction peaks of smooth surface diffraction angle 2θ are 38.441°, 44.737°, 65.025°, 78.141° and 82.343°, respectively. All the diffraction peaks are fit well with those of the standard pattern of polycrystalline Al (ICSD PDF No: 89–0437) of smooth surface. The diffraction angle on Ni coating surface are 44.500°, 51.802°, 76.421° and on coupling processing surface are 44.480°, 51.859°, 76.282°, respectively. The diffraction angles and the diffraction peaks are changed on the sample surfaces. This result indicates that the electro–brush plating processing has completely changed the position of the diffraction peak, and then new phase appeared. By comparing the diffraction peak about Ni coating surface and coupling processing surface, it can be found that the laser processing on the Ni coating has not changed the crystal texture and other production phases, but the diffraction peak intensity enhanced obviously. The
3.3. Wettability Wettability is a very important property of the surface, static contact angle (CA) and sliding angle (SA) are used to describe it. The images show the wettability of these surfaces fabricated by different methods in Fig. 4. The CA measurement showed that the contact angle was about 78° on the bare aluminum alloy surface, water droplet spread on its surface and quickly penetrated into sample. The surface revealed a hydrophilic property (Fig. 4a). The CA was obviously improved after electro–brush plating and was up to 143°. The structures of close arrays micro–papillae endow the coating surface with sufficient roughness for hydrophobicity. The rough structures effectively reduced the actual contact area between the solid surface and water droplet. The surface has transformed from hydrophilicity to hydrophobicity (Fig. 4b). Due to 275
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Fig. 5. Contact angles of coupling surface with different stored times.
Here, f is the area fraction of solid surfaces in contact with water, the value of θ0 (78°) and θr (156.3°) are the CA on the smooth surface and coupling processing surface, and r (0.989) is the roughness of sample surface. From the equation, it is easy to calculate the fraction of the trapped air, which is about 93% on the superhydrophobic surface. It indicates that the water droplet contact with air and plenty of air immerse into the interspaces of rough surface, which can prevent the water droplet immersing into the inner of the surface. This result demonstrates that the rough structure with abundant air fraction is the major factor to achieve superhydrophobicity. We also discussed the exchange of the energy, the energy equation is:
Fig. 4. Physical map of the sample and corresponding contact angle profiles: (a) Smooth substrate, (b) Ni coating and (c) Coupling processing.
the roughness increasing, the Ni coating became more hydrophobic. But the air–bubble extrusion phenomenon almost cannot be observed between the water droplet and the surface. The water droplet nearly kept Cassie state on the surface. After further testing the wettability of the coupling processing surface, the result showed that the biggest CA has reached 156.3° on the surface along laser–made groove direction (Fig. 4c). The profile of water droplet was approaching perfect spherical at the moment, and the CA was almost not changed after the water droplet kept on the surface for 5 min. The wettability of the surface is stable. The increasement of the roughness strongly amplified the hydrophobicity of the surface. The surface oxidation has been accelerated after laser processing, it made the surface form a dense oxide films, which can reduce the surface energy. And the special groove surface made the water droplet contact with more air cushion. The air cushion prevented the penetration of water molecules. The surface had better hydrophobicity compared with other surfaces. The durability and stability are very important factors for surface applications. In this study, the change tendency of CA was investigated on the coupling processing surface (Fig. 5). The surfaces were dried completely at 150 °C for 2 h and stored at room temperature for 1–60 days. The CA of surface was about 20° in the first day, showed an obvious hydrophilicity. The CA was found to enhance from 20° to around 90° when the stored time was 10 days, the state has changed from hydrophilicity to hydrophobicity. And the CA was reached 147° ± 4.2° when the surface stored about 20 days. After 30 days, the CA rose to 155.8° ± 0.5°. However, the surface which was placed in air for a long time had not significantly change tendency for the CA, which were stained over 155° after 30–60 days. Obviously, the prepared superhydrophobic surfaces have good durability and stability in air. In order to explain the wettability on the surface better, the contact angle can be determined through the Cassie equation:
cos θr = −1 + f (1 + rcos θ0)
G = π 1/3 (3V )2/3 (1 − cos θr )2/3 (2 + cos θr )1/3γLV
(2)
In this equation, G is the needed energy of water at the steady state, V (4 μl) is the volume of water, γLV is the surface tension between liquid and air. The liquid is water and tension is 72. Therefore, the energy changed from the initial state to the eventual state (θ) is a monotonic increasing quantity of θ, which is 0–180°, at a given water volume V. If there has an energy barrier for the transition between homogeneous and heterogeneous wetting, this energy barrier should be bigger than either of them. So the energy which added to the water droplet can counteract this energy difference and has a plus energy. Taking the θr volume (78°, 143° and 156.3°) into this equation, the G are 0.061614 J, 0.086900 J and 0.0875865 J, respectively. The results indicated that the superhydrophobic surface needs more energy to maintain balance. The energy difference is 0.025286 J between 78° and 143°. The energy difference is 0.0006865 J between 143° and 156.3°. The energy differences between smooth surfaces and rough surfaces have been counteracted and the surfaces changed from hydrophilicity to hydrophobicity. The microstructure is the main structure of the hydrophobic surface. We investigated the wettability of the superhydrophobic surface by measured its sliding angle (SA). The surface shows an excellent rollability with a sliding angle less than 4.8°. The roll–off process of water droplet is shown in Fig. 6a. The water droplet is sliding along the arrows direction. The water droplet is in instable state when it lands on the surface. It became instable with the increasing of tilting angle. Finally, it immediately slides across the surface when the angle reach about 4.8°, the sliding speeds is about 11.56 mm/s. The wetting condition had not been obviously changed after rolling. The superhydrophobic surface has excellent water–repellent performance. In the test, we can find that the smooth substrate and Ni coating surfaces have strong adhesion. The profiles of water droplet with different tilting angles on the surface are shown in Fig. 6(b and c). The water droplet
(1) 276
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was very difficult to drop on the surface in all state (Fig. 7d). When the surface was separated from the water droplet, the water droplet returns to the head of microsyringe with a slowly speed. But the water droplet doesn’t continue to move with the surface moving, it still stay on the head part of microsyringe. The water droplet did not adhere to the surface even if it was pushed onto the surface. There was no wet trace on the surface (Fig. 7e–g). But this phenomenon did not happen on the other two surfaces. The result shows that the adhesion between the coupling processing surface and the water droplet is very weakly, the surface has excellent anti–adhesion property. The experiment is designed to further explain this anti–adhesion property on different surfaces, and the results are shown in Fig. 8. In the typical experiment, water droplet is placed on three kinds of surfaces for 5 min. The maximal volume for the water droplet hanged on the microsyringe is about 10 μl. The volume of water on the surface and pinhead are 4 μl. The water droplet was moved along the arrow’s direction on the surface. By comparing the three groups of images, we can find that the water droplet on the smooth substrate and Ni coating surface can be absorbed by the surface and away from the pinhead, only the water droplet on the Ni coating surface was more elongated while it left the pinhead. This indicates that the surface has stronger adhesion (Fig. 8b). When the surface was kept away from the water droplet, the water droplet completely bounced back to the tip of the needle (Fig. 8c), this phenomenon is a typical Cassie state. The phenomenon indicated that the superhydrophobic surface had obvious low adhesion. Obviously, the experimentally prepared aluminum–based surface has a very low adsorption force on water, and the water droplet is in a highly mobile state on the surface, demonstrating that the surface has low adhesion to water. In order to better explain the low adhesion properties of the surface, the adhesion of the surface to the polyethylene material was further tested. After the plastic adhesive stick containing polyethylene melted into a liquid state under high temperature conditions, it was quickly dripped onto the surface of the prepared sample and the smooth surface, and then cooled and solidified for 3 min, as shown in Fig. 9. After the surface liquid is solidified, the filaments on the surface of the coagulated material are slowly clamped by using tweezers. As a result, it is found that there is extremely slight adhesion between the sample surface and the coagulated material processed by the composite method, and the solid object is almost suspended on the surface. The surface of the sample did not move or move upward (Fig. 9a). On the other hand, the smooth surface of the substrate adhered closely to the solid object and moved upward with the tweezers, indicating that the solidified polyethylene adheres to the smooth substrate surface (Fig. 9b4) and is further subjected to a certain amount of force After shaking, the two remained firmly stuck together without detachment (Fig. 9b5 and b6). Through the comparative test, it
Fig. 6. Shapes of water droplet at diverse titling angle on the different surface. (a) The coupling processing surface positioned at 0–4.8°, (b) The Ni coating surface positioned at 90° and 180°, (c) The substrate surface with the titling angles of 90° and 180°.
can stick to the surface even if the substrate is inverted up to 90° or 180°. With the tilting angles gradually increased, the water droplet has no moving tendency. But a little droop and distortion are happened effected by gravity. These phenomena indicate that the adhesion between the water droplet and the surface are very strong. 3.4. Anti–adhesion It is well known that superhydrophobic surface always accompanies the adhesion study. The adhesion property of the superhydrophobic surface was measured and the result was showed in Fig. 7. It is clearly found that a sequence of photographs of water droplet have been approached onto the superhydrophobic surface. The test temperature is room temperature (20 °C) while the relative humidity is about 20% and the volume of water is 4 μl. In the initial state, the water droplet was hung on micro–syringe needle (Fig. 7a), and lifted out the prepared surface with a translation stage. The deformation was happening while the water droplet contacts with surface (Fig. 7b and c). With the surface continuously moving closer, the water droplet was seriously condensed and the morphology of the water droplet deforms greatly. When the stage gradually approaches microsyringe, the water droplet is pushed up and moved along the side of microsyringe tip, then the water droplet gradually changed from sphere to ellipsoid on the compression, which
Fig. 7. Sequential photographs of water droplet before and after it was made to contact with the superhydrophobic surface. (a) Initial state, (b) Slight contact, (c and d) Tight contact, (e and f) Depart from the surface, (g) Final state. 277
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Fig. 8. Sequence adhesion photos of water droplet on the different surface, (a) Smooth substrate, (b) Ni coating, (c) Coupling processing.
3.5. Self–cleaning behavior
is found that the composite method also has excellent low adhesion property to the polyethylene material, which provides a reference value for the application of the aluminum material surface in the mold and other fields.
Self–cleaning is exploited in many plants and animals surface because it can help to carry away contaminants. The property is beneficial to practical applications. The self–cleaning behavior of the superhydrophobic surface is investigated by applying dusts as contaminant in
Fig. 9. Polyethylene material anti–adhesion test (a) composite processing surface; (b) polished substrate. 278
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Fig. 10. Self–cleaning behavior of the coupling processing surface: (a) Bare surface, (b)–(e) Contaminated surface with water droplet on them, (f) Contaminated surface after water droplet tumble.
doi.org/10.1016/j.optlastec.2018.12.045.
Fig. 10. The sample was provided to the cleaning test (Fig. 10a). The dusts distributed on the surface and the water droplet was dripped on the contaminated surface, shown in Fig. 10b. The water droplet was cleaning up the dusts along its rolling trace (Fig. 10c–e). With the water droplet dripping down, a similar self–cleaning phenomenon could be observed until all the covered dusts were cleaned out (Fig. 10f). Obviously, the superhydrophobic surface had the superior self–cleaning behavior, and it was a practical application of anti–adhesion.
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4. Conclusion In this study, we have fabricated a superhydrophobic surface with anti–adhesion on the aluminum alloy by coupling electro–brush plating with laser processing. There were many uniform grooves, micro–protrusions and micro–grains covered on the surface. The complex micro–structures played an important role on superhydrophobic surface research. The sample surface showed excellent superhydrophobic properties. The contact angle was 156.3° and the sliding angle was 4.8°. Through the adhesion test, coupling processing surface had excellent anti–adhesion and the water droplet was easily desorbed from the surface. The water droplet did not adhere to the surface even if it was pushed onto the surface. The adhesion to the polyethylene material was further tested, the polyethylene material was easily desorbed from the surface. Through the self–cleaning behavior test, with the water droplet dripping down, all the covered dusts were cleaned out. We also found that the surface had superior self–cleaning property. It indicates that the water droplet contact with air and plenty of air immerse into the interspaces of rough surface, which can prevent the water droplet immersing into the inner of the surface. The study found that the air was captured by the remained protrusions structures, which leaded to anti–adhesion property. The superhydrophobic surface with anti–adhesion had potential applications including fluid transfer, stain–resistant and anti–fouling surfaces. Acknowledgements The authors are grateful to the National Natural Science Foundation of China (Grant No. 51505039), the Natural Science Foundation of Science and Technology Department of Jilin Province (Grant No. 20180101322JC) and the “111” Project of China (Grant No. D17017) for support of this work. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// 279
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Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Two–step fabrication of superhydrophobic surfaces with anti–adhesion a,⁎
b
a
a
Jing Li , Feng Du , Yanhui Zhao , Shicai Zhao , Huadong Yu a b
T
a
College of Mechanical and Electric Engineering, Changchun University of Science and Technology, Changchun 130022, PR China Army Academy of Amored Forces, Changchun 130117, PR China
H I GH L IG H T S
superhydrophobic surfaces were fabricated by coupling processing. • The contact angle was as high as 156.3° and the sliding angle was 4.8°. • The superhydrophobic surface exhibited an excellent low adhesion property. • The • The superhydrophobic surface had excellent self–cleaning property.
A R T I C LE I N FO
A B S T R A C T
Keywords: Electro–brush plating Laser processing Superhydrophobicity Anti–adhesion
A self–cleaning property with anti–adhesion and superhydrophobicity was fabricated on aluminum alloy surface via a facile and low–cost method coupling electro–brush plating and laser processing. The surface morphology, chemical composition, wettability, anti–adhesion and self–cleaning effect were analyzed by multiple ways. The results indicated that an array of micro–scale grooves and typical hierarchical protrusion structure were covered on the surface, and the edge of the groove was covered by micro–scale sputtering particles. The special composite structures played an important role in amplifying the hydrophobic property of surface. The surface showed excellent superhydrophobic properties. The contact angle was as high as 156.3° and the sliding angle was 4.8°. The water droplet has approached onto the superhydrophobic surface and completely bounced back to the tip of the needle on the coupling surface. The adhesion between the coupling surface and the water droplet is very weakly, the surface has excellent anti–adhesion property. Through the comparative test, the composite method also has excellent low adhesion property to the polyethylene material. In this case, the low adhesion led to a rolling and self–cleaning behavior. The approach presented in this study provides a facile, low–cost and efficient method to fabricate superhydrophobic surfaces on all types of metallic materials for commercial applications.
1. Introduction
observed on surfaces of some plants and animals in nature, such as rice leaves [18], lotus leaves [19], rose petals [20–23] and butterfly wings [24], which exhibited excellent hydrophobicity. Their hydrophobicity was determined by the cooperation of geometric microstructure and chemical composition of surfaces. Therefore, those natural surface structures, as mimetic models, could be used to design and fabricated on the engineering materials surfaces. So far, a large number of approaches have been successfully used to develop hydrophobic surface, including chemical etching [25,26], solution immersion [27], interference lithography [28–31], high speed wire electrical discharge machining processing (HS–WEDM) [32] and laser fabrication [33–35]. Up to now, the various methods have been reported for fabricating superhydrophobic surface on aluminum and its alloys material. The preparation method mainly comes down to two types: one of the ways is
As the continuous exploration of the special morphologies and properties of some animal skins and plant leaf surfaces in nature, new hydrophobic surfaces are explored through imitation of the observed surface effects. The hydrophobic [1,2] theory has been analyzed by many people, and hydrophobic surface has been investigated by numerous researchers because of the potential in self–cleaning [3–6], anti–adhesion [7–10], anti–icing [11–13], water repellency [14], drag reduction in micro–fluidics [15], prevention of corrosion [16], etc. A surface with static water contact angle (CA) more than 150° and contact angle hysteresis less than 10° was defined as superhydrophobic surface [17]. The preparation of the superhydrophobic surface has very important significance and value. The hydrophobicity has been originally ⁎
Corresponding author. E-mail address: [email protected] (J. Li).
https://doi.org/10.1016/j.optlastec.2018.12.045 Received 4 June 2018; Received in revised form 12 November 2018; Accepted 22 December 2018 0030-3992/ © 2018 Elsevier Ltd. All rights reserved.
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fabricating rough structure on low energy surface, the other way is fabricating rough structures on high energy surface and using chemical modification. The chemical modification could improve processing costs or other problem of usage. So the second method has more difficulty than the first one. Liu et al. [36] fabricated lotus–leaf–like superhydrophobic surface on carbon steel substrate via two–layer nano–composite electro–brush plating and subsequent surface modification with low free energy materials and the contact angle is up to 155.5°, the sliding angle is 5°. Zhong et al. [37] used a microstructure array template to transform a polydimethylsiloxane surface into a superhydrophobic surface with high transparency and stable mechanical properties using a femtosecond laser. By adjusting the size and separation of the microstructure, the hydrophobicity and transparency of the surface could be controlled. Jagdheesh et al. [38] fabricated micro channels and pillars by a nanosecond laser source on thin aluminum surface. They found that the frequency power, density and pulse width could significantly influence the new aluminum oxide surfaces formed after laser processing. Liu et al. [34] reviewed facile methods to fabricate large–area biomimetic hierarchical structures via laser processing, a diversity of large area hierarchical structures can be obtained and show rich biomimetic functions such as superhydrophobicity. Gu et al. [39] obtained a superhydrophobic surface by repeatedly immersing the copper sheet in a 0.05 M silver nitrate aqueous solution and then modifying it in an ethanol solution of n-dodecanethiol. In this study, the superhydrophobic surface was successfully fabricated on aluminum alloy. An array of micro–scale grooves and typical hierarchical protrusions structure were fabricated on the surface coupling electro–brush plating and laser processing. The wettability of the surface had been changed from hydrophilicity to superhydrophobicity, and the superhydrophobic surface has excellent low–adhesion and self–cleaning property.
Fig. 1. Schematic views of formation for the superhydrophobic surface.
were placed in air for many days. The processing flowchart is shown in Fig. 1. 2.3. Characterization
2. Experimental
The morphologies of the aluminum alloys were observed by scanning electron microscope (COXEM, EM–30 Korea) and laser scanning confocal microscopy (LSCM, LSM 700, ZEISS, Germany). The composition of the prepared surfaces was analyzed using X–ray diffraction (XRD, D/max–2500PC, Rigaku, Japan). The wettability of various surfaces were measured by statics contact angle and slide angle on the contact angle meter (OCA15 Pro, Dataphysics, Germany) with a water drop volume 4 μl at room temperature. All water contact angles and slide angle were measured at 4–6 different points and were averaged.
2.1. Materials 20 mm × 20 mm × 2 mm 7075 aluminum alloy (major elements, wt %: Si 0.4, Fe 0.50, Cu 1.2–2.0, Mn 0.30, Mg 2.1–2.9, Cr 0.18–0.28, Zn 5.1–6.1, Ti 0.20 and remaining element Al) purchased from Shanghai Ao feng metal product co, ltd. China. The Ni coatings were prepared by an improved electro–brush plating technique. The used the plating solution mainly consists of nickel sulfate hexahydrate (265 g/L), purchase from Beijing Zhen hai Institute of chemistry China, ammonium citrate (100 g/L), ammonium acetate (23 g/L), ammonium hydroxide (25–28%, 100–200 mL/L) purchase from Qidong Ming into chemical co, ltd. China, the pH value is 7.3–7.8.
3. Results and discussion 3.1. Morphology It is well known that hydrophobic surfaces are usually associated with geometrical micro–structure and surface roughness. The different images of surfaces were observed with different processing methods from Fig. 2. The morphology of the prepared surface is shown in the SEM images. It can be seen that the smooth substrate surface is very flat, which doesn’t have other raise or the appearance of micro–scale structures (Fig. 2a). Fig. 2b showed the micro–topography of the Ni coating prepared by electro–brush plating technique with the plating voltage of 14 V. Many micro–scale protrusions and pits are uniformly distributed on the surface. The high–resolution shows that the size of pit is 5–20 μm. The diameter in top of the protrusion is 25–40 μm. The micro–groove structures are distributed on the Ni coatings, the surface becomes a composite structure with grooves and micro–protrusions. The protrusion structures are distributed between adjacent grooves. As shown in Fig. 2c, the distance between adjacent grooves is generally in the range 90–100 μm, the width of one groove is 45–55 μm, and the width of one bare protrusion is about 45–60 μm. This image also shows that the feature of coating do not effect directly by laser processing.
2.2. Fabrication First, aluminum alloy surfaces were polished by No. 800–2000 SiC paper to obtain a relatively smooth substrate, and subsequently cleaned with acetone, alcohol and de–ionized water, and then dried in air. Second, the micro–scale asperities and pits are uniformly distributed on surface with a special DC power (NBD–150, Jinlin, China). And the hierarchical composite structures which were consisted of micro–protrusions and submicro–grains were formed on the sample surfaces by electric brush–plating technique. When preparing coatings, graphite anode should move back and forth along a direction relative to the substrate. The working parameters were determined: plating voltage is 14 V and its chosen is according to plating solution concentration, the speed of plating is 60–130 mm/s, working time is 10 min. The Ni coating surface was dried completely at 150 °C for 2 h. Finally, the sample was machined at distance 100 μm between the adjacent grooves centrality by laser (HBS–GQ–20, Beijing, China), the speed of laser scanning is 500 mm/s and scans twice in a row. The prepared surfaces 274
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Fig. 2. SEM micrographs of different surfaces, (a) Smooth substrate, (b) Ni coating surface, (c) Coupling processing surface; Three–dimensional LSCM images of different surfaces, (d) Smooth substrate, (e) Ni coating surface, (f) Coupling processing surface.
Many spurting particles are located on the edge of grooves, made the surface much rougher. In laser processing, the surface of the material would be generated high temperature instantly. Then part of the surface could appear melting phenomenon around laser beam point with the moving of the laser beam, the melting area is going to be cooled far from the center of laser. So the groove structures are formed by laser beam. These micro–structures can capture a large amount of air and significantly reduce the actual contact area between water droplet and the surface, which can prevent the wetting between water droplet and solid surface. The morphology of the surface was fabricated by different processing. To more intuitively show the morphology of the surface composite structure, the images were characterized by LSCM, as shown in Fig. 2(d–f). It can be seen that the surface is very smooth, just have many slight polishing traces (Fig. 2a). Fig. 2b shows that many protrusions are uniform and dense distribution on the surface, the height is about 15 μm. There are pit structures between different protrusions. Fig. 2c clearly reveals the groove structures and protrusion structures on the surface, and the depth of groove is 6–10 μm.
Fig. 3. Comparison of the XRD patterns obtained for: (a) Coupling processing surface, (b) Ni coating surface, (c) Substrate surface.
diffraction angles are all fit well with the standard pattern of polycrystalline Ni (ICSD PDF NO: 70–1849). As inferred from the strength of the diffraction peak, the laser processing made the crystal grow better along (200) crystal face.
3.2. Chemical composition The XRD pattern of substrate surface, Ni coating surface and coupling processing surface are shown in Fig. 3. It is obvious that the diffraction peak characteristic of smooth substrate surface is different from the other two surfaces. The diffraction peaks of smooth surface diffraction angle 2θ are 38.441°, 44.737°, 65.025°, 78.141° and 82.343°, respectively. All the diffraction peaks are fit well with those of the standard pattern of polycrystalline Al (ICSD PDF No: 89–0437) of smooth surface. The diffraction angle on Ni coating surface are 44.500°, 51.802°, 76.421° and on coupling processing surface are 44.480°, 51.859°, 76.282°, respectively. The diffraction angles and the diffraction peaks are changed on the sample surfaces. This result indicates that the electro–brush plating processing has completely changed the position of the diffraction peak, and then new phase appeared. By comparing the diffraction peak about Ni coating surface and coupling processing surface, it can be found that the laser processing on the Ni coating has not changed the crystal texture and other production phases, but the diffraction peak intensity enhanced obviously. The
3.3. Wettability Wettability is a very important property of the surface, static contact angle (CA) and sliding angle (SA) are used to describe it. The images show the wettability of these surfaces fabricated by different methods in Fig. 4. The CA measurement showed that the contact angle was about 78° on the bare aluminum alloy surface, water droplet spread on its surface and quickly penetrated into sample. The surface revealed a hydrophilic property (Fig. 4a). The CA was obviously improved after electro–brush plating and was up to 143°. The structures of close arrays micro–papillae endow the coating surface with sufficient roughness for hydrophobicity. The rough structures effectively reduced the actual contact area between the solid surface and water droplet. The surface has transformed from hydrophilicity to hydrophobicity (Fig. 4b). Due to 275
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Fig. 5. Contact angles of coupling surface with different stored times.
Here, f is the area fraction of solid surfaces in contact with water, the value of θ0 (78°) and θr (156.3°) are the CA on the smooth surface and coupling processing surface, and r (0.989) is the roughness of sample surface. From the equation, it is easy to calculate the fraction of the trapped air, which is about 93% on the superhydrophobic surface. It indicates that the water droplet contact with air and plenty of air immerse into the interspaces of rough surface, which can prevent the water droplet immersing into the inner of the surface. This result demonstrates that the rough structure with abundant air fraction is the major factor to achieve superhydrophobicity. We also discussed the exchange of the energy, the energy equation is:
Fig. 4. Physical map of the sample and corresponding contact angle profiles: (a) Smooth substrate, (b) Ni coating and (c) Coupling processing.
the roughness increasing, the Ni coating became more hydrophobic. But the air–bubble extrusion phenomenon almost cannot be observed between the water droplet and the surface. The water droplet nearly kept Cassie state on the surface. After further testing the wettability of the coupling processing surface, the result showed that the biggest CA has reached 156.3° on the surface along laser–made groove direction (Fig. 4c). The profile of water droplet was approaching perfect spherical at the moment, and the CA was almost not changed after the water droplet kept on the surface for 5 min. The wettability of the surface is stable. The increasement of the roughness strongly amplified the hydrophobicity of the surface. The surface oxidation has been accelerated after laser processing, it made the surface form a dense oxide films, which can reduce the surface energy. And the special groove surface made the water droplet contact with more air cushion. The air cushion prevented the penetration of water molecules. The surface had better hydrophobicity compared with other surfaces. The durability and stability are very important factors for surface applications. In this study, the change tendency of CA was investigated on the coupling processing surface (Fig. 5). The surfaces were dried completely at 150 °C for 2 h and stored at room temperature for 1–60 days. The CA of surface was about 20° in the first day, showed an obvious hydrophilicity. The CA was found to enhance from 20° to around 90° when the stored time was 10 days, the state has changed from hydrophilicity to hydrophobicity. And the CA was reached 147° ± 4.2° when the surface stored about 20 days. After 30 days, the CA rose to 155.8° ± 0.5°. However, the surface which was placed in air for a long time had not significantly change tendency for the CA, which were stained over 155° after 30–60 days. Obviously, the prepared superhydrophobic surfaces have good durability and stability in air. In order to explain the wettability on the surface better, the contact angle can be determined through the Cassie equation:
cos θr = −1 + f (1 + rcos θ0)
G = π 1/3 (3V )2/3 (1 − cos θr )2/3 (2 + cos θr )1/3γLV
(2)
In this equation, G is the needed energy of water at the steady state, V (4 μl) is the volume of water, γLV is the surface tension between liquid and air. The liquid is water and tension is 72. Therefore, the energy changed from the initial state to the eventual state (θ) is a monotonic increasing quantity of θ, which is 0–180°, at a given water volume V. If there has an energy barrier for the transition between homogeneous and heterogeneous wetting, this energy barrier should be bigger than either of them. So the energy which added to the water droplet can counteract this energy difference and has a plus energy. Taking the θr volume (78°, 143° and 156.3°) into this equation, the G are 0.061614 J, 0.086900 J and 0.0875865 J, respectively. The results indicated that the superhydrophobic surface needs more energy to maintain balance. The energy difference is 0.025286 J between 78° and 143°. The energy difference is 0.0006865 J between 143° and 156.3°. The energy differences between smooth surfaces and rough surfaces have been counteracted and the surfaces changed from hydrophilicity to hydrophobicity. The microstructure is the main structure of the hydrophobic surface. We investigated the wettability of the superhydrophobic surface by measured its sliding angle (SA). The surface shows an excellent rollability with a sliding angle less than 4.8°. The roll–off process of water droplet is shown in Fig. 6a. The water droplet is sliding along the arrows direction. The water droplet is in instable state when it lands on the surface. It became instable with the increasing of tilting angle. Finally, it immediately slides across the surface when the angle reach about 4.8°, the sliding speeds is about 11.56 mm/s. The wetting condition had not been obviously changed after rolling. The superhydrophobic surface has excellent water–repellent performance. In the test, we can find that the smooth substrate and Ni coating surfaces have strong adhesion. The profiles of water droplet with different tilting angles on the surface are shown in Fig. 6(b and c). The water droplet
(1) 276
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was very difficult to drop on the surface in all state (Fig. 7d). When the surface was separated from the water droplet, the water droplet returns to the head of microsyringe with a slowly speed. But the water droplet doesn’t continue to move with the surface moving, it still stay on the head part of microsyringe. The water droplet did not adhere to the surface even if it was pushed onto the surface. There was no wet trace on the surface (Fig. 7e–g). But this phenomenon did not happen on the other two surfaces. The result shows that the adhesion between the coupling processing surface and the water droplet is very weakly, the surface has excellent anti–adhesion property. The experiment is designed to further explain this anti–adhesion property on different surfaces, and the results are shown in Fig. 8. In the typical experiment, water droplet is placed on three kinds of surfaces for 5 min. The maximal volume for the water droplet hanged on the microsyringe is about 10 μl. The volume of water on the surface and pinhead are 4 μl. The water droplet was moved along the arrow’s direction on the surface. By comparing the three groups of images, we can find that the water droplet on the smooth substrate and Ni coating surface can be absorbed by the surface and away from the pinhead, only the water droplet on the Ni coating surface was more elongated while it left the pinhead. This indicates that the surface has stronger adhesion (Fig. 8b). When the surface was kept away from the water droplet, the water droplet completely bounced back to the tip of the needle (Fig. 8c), this phenomenon is a typical Cassie state. The phenomenon indicated that the superhydrophobic surface had obvious low adhesion. Obviously, the experimentally prepared aluminum–based surface has a very low adsorption force on water, and the water droplet is in a highly mobile state on the surface, demonstrating that the surface has low adhesion to water. In order to better explain the low adhesion properties of the surface, the adhesion of the surface to the polyethylene material was further tested. After the plastic adhesive stick containing polyethylene melted into a liquid state under high temperature conditions, it was quickly dripped onto the surface of the prepared sample and the smooth surface, and then cooled and solidified for 3 min, as shown in Fig. 9. After the surface liquid is solidified, the filaments on the surface of the coagulated material are slowly clamped by using tweezers. As a result, it is found that there is extremely slight adhesion between the sample surface and the coagulated material processed by the composite method, and the solid object is almost suspended on the surface. The surface of the sample did not move or move upward (Fig. 9a). On the other hand, the smooth surface of the substrate adhered closely to the solid object and moved upward with the tweezers, indicating that the solidified polyethylene adheres to the smooth substrate surface (Fig. 9b4) and is further subjected to a certain amount of force After shaking, the two remained firmly stuck together without detachment (Fig. 9b5 and b6). Through the comparative test, it
Fig. 6. Shapes of water droplet at diverse titling angle on the different surface. (a) The coupling processing surface positioned at 0–4.8°, (b) The Ni coating surface positioned at 90° and 180°, (c) The substrate surface with the titling angles of 90° and 180°.
can stick to the surface even if the substrate is inverted up to 90° or 180°. With the tilting angles gradually increased, the water droplet has no moving tendency. But a little droop and distortion are happened effected by gravity. These phenomena indicate that the adhesion between the water droplet and the surface are very strong. 3.4. Anti–adhesion It is well known that superhydrophobic surface always accompanies the adhesion study. The adhesion property of the superhydrophobic surface was measured and the result was showed in Fig. 7. It is clearly found that a sequence of photographs of water droplet have been approached onto the superhydrophobic surface. The test temperature is room temperature (20 °C) while the relative humidity is about 20% and the volume of water is 4 μl. In the initial state, the water droplet was hung on micro–syringe needle (Fig. 7a), and lifted out the prepared surface with a translation stage. The deformation was happening while the water droplet contacts with surface (Fig. 7b and c). With the surface continuously moving closer, the water droplet was seriously condensed and the morphology of the water droplet deforms greatly. When the stage gradually approaches microsyringe, the water droplet is pushed up and moved along the side of microsyringe tip, then the water droplet gradually changed from sphere to ellipsoid on the compression, which
Fig. 7. Sequential photographs of water droplet before and after it was made to contact with the superhydrophobic surface. (a) Initial state, (b) Slight contact, (c and d) Tight contact, (e and f) Depart from the surface, (g) Final state. 277
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Fig. 8. Sequence adhesion photos of water droplet on the different surface, (a) Smooth substrate, (b) Ni coating, (c) Coupling processing.
3.5. Self–cleaning behavior
is found that the composite method also has excellent low adhesion property to the polyethylene material, which provides a reference value for the application of the aluminum material surface in the mold and other fields.
Self–cleaning is exploited in many plants and animals surface because it can help to carry away contaminants. The property is beneficial to practical applications. The self–cleaning behavior of the superhydrophobic surface is investigated by applying dusts as contaminant in
Fig. 9. Polyethylene material anti–adhesion test (a) composite processing surface; (b) polished substrate. 278
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Fig. 10. Self–cleaning behavior of the coupling processing surface: (a) Bare surface, (b)–(e) Contaminated surface with water droplet on them, (f) Contaminated surface after water droplet tumble.
doi.org/10.1016/j.optlastec.2018.12.045.
Fig. 10. The sample was provided to the cleaning test (Fig. 10a). The dusts distributed on the surface and the water droplet was dripped on the contaminated surface, shown in Fig. 10b. The water droplet was cleaning up the dusts along its rolling trace (Fig. 10c–e). With the water droplet dripping down, a similar self–cleaning phenomenon could be observed until all the covered dusts were cleaned out (Fig. 10f). Obviously, the superhydrophobic surface had the superior self–cleaning behavior, and it was a practical application of anti–adhesion.
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4. Conclusion In this study, we have fabricated a superhydrophobic surface with anti–adhesion on the aluminum alloy by coupling electro–brush plating with laser processing. There were many uniform grooves, micro–protrusions and micro–grains covered on the surface. The complex micro–structures played an important role on superhydrophobic surface research. The sample surface showed excellent superhydrophobic properties. The contact angle was 156.3° and the sliding angle was 4.8°. Through the adhesion test, coupling processing surface had excellent anti–adhesion and the water droplet was easily desorbed from the surface. The water droplet did not adhere to the surface even if it was pushed onto the surface. The adhesion to the polyethylene material was further tested, the polyethylene material was easily desorbed from the surface. Through the self–cleaning behavior test, with the water droplet dripping down, all the covered dusts were cleaned out. We also found that the surface had superior self–cleaning property. It indicates that the water droplet contact with air and plenty of air immerse into the interspaces of rough surface, which can prevent the water droplet immersing into the inner of the surface. The study found that the air was captured by the remained protrusions structures, which leaded to anti–adhesion property. The superhydrophobic surface with anti–adhesion had potential applications including fluid transfer, stain–resistant and anti–fouling surfaces. Acknowledgements The authors are grateful to the National Natural Science Foundation of China (Grant No. 51505039), the Natural Science Foundation of Science and Technology Department of Jilin Province (Grant No. 20180101322JC) and the “111” Project of China (Grant No. D17017) for support of this work. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// 279
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