Mechanically Robust Superamphiphobic Aluminum Surface with ...

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Mechanically Robust Superamphiphobic Aluminum Surface with Nanopore-Embedded Microtexture Sumit Barthwal,† Young Su Kim,‡ and Si-Hyung Lim*,§ †

Department of Bio and Nano Chemistry, Kookmin University, Seoul 136-702, South Korea Department of Mechanics and Design, Kookmin University, Seoul 136-702, South Korea § School of Mechanical Systems Engineering, Kookmin University, Seoul 136-702, South Korea ‡

ABSTRACT: A simple fabrication technique was developed for preparing a mechanically robust superamphiphobic surface on an aluminum (Al) plate. Dual geometric architectures with micro- and nanoscale structures were formed on the surface of the Al plate by a combination of simple chemical etching and anodization. This proposed methodology involves (1) fabrication of irregular microscale plateaus on the surface of the Al plate, (2) formation of nanopores, and (3) fluorination. Wettability measurements indicated that the fabricated Al surface became super-repellent toward a broad range of liquids with surface tension in the range 27.5−72 mN/m. By varying the anodization time, we measured and compared the effects of morphological change on the wettability. The adhesion property and mechanical durability of the fabricated superamphiphobic Al surface were evaluated by the Scotch tape and hardness tests, respectively. The results showed that the fabricated Al surface retained mechanical robustness because the down-directed surface made by nanopores on the microtextured surface was durable enough even after high force was applied. Almost no damage of the film was observed, and the surface still exhibited superamphiphobicity after the tests. The fabricated superamphiphobic surface also remained stable after long-term storage. The simple and time-saving fabrication technique can be extended to any large-area three-dimensional surface, making it potentially suitable for large-scale industrial fabrications of mechanically robust superamphiphobic surfaces.

1. INTRODUCTION Many naturally occurring surfaces such as lotus leaves,1 legs of the water striders,2 cicada wings,3 and others4−6 show unique wettability properties. They are called superhydrophobic surfaces, characterized by high water contact angles (CA > 150°) and low sliding angles (SAs). Inspired by nature, researchers have developed artificial surfaces with superhydrophobic properties using a variety of methods including etching techniques,7−10 sol−gel process,11 electrospinning,12,13 deposition of nanoparticles on smooth or rough substrates,14 growth of nanotubes,15 electrochemical anodization,16 and laser fabrication.17 Superamphiphobic surfaces are those that are super-repellent to both water and oil, exhibiting CAs above 150° for water as well as various oils. Superamphiphobic surfaces have attracted increased attention owing to their wide range of applications in self-cleaning,18 drag-reduction,19 corrosion resistance,20 antifouling,21 and anti-icing.22 However, such superamphiphobic surfaces with low surface energy are difficult to fabricate when compared to superhydrophobic surfaces. To design superamphiphobic surfaces that resist wetting by low-surface-tension liquids such as hexadecane (surface tension of 27.5 mN/m at 20 °C), both surface energy and surface structure must be controlled. Recently, several groups have attempted different techniques to artificially recreate superamphiphobic surfaces, and some promising results have been reported.23−30 According © 2013 American Chemical Society

to these results, artificial superamphiphobic surfaces could be fabricated by controlling both the surface energy and surface morphology, which are the two key surface parameters that govern the wettability of solid surfaces.31−33 This could be accomplished by creating hierarchical micro−nanostructures on the surface and chemically modifying the surface with lowsurface-energy materials. As an important engineering material, aluminum is widely used in many modern industrial applications. Al is naturally abundant and finds widespread applications, especially in the aerospace and automotive industries and household goods because of its high strength, excellent heat and electrical conductivities, and low weight. Therefore, it is of great importance to fabricate superamphiphobic Al surfaces with superior water and oil repellent properties. To date, a variety of synthetic techniques such as chemical etching,34−37 sol−gel method,38,39 surface coating method,40,41 phase separation,42 anion exchange method,43 and electrochemical anodization44 have been developed to fabricate artificial superhydrophobic surfaces on Al plates. Previously, some researchers have successfully fabricated superhydrophobic Al surfaces by applying simple etching or Received: July 10, 2013 Revised: August 24, 2013 Published: August 28, 2013 11966

dx.doi.org/10.1021/la402600h | Langmuir 2013, 29, 11966−11974

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Scheme 1. Schematic Diagram: (a) Up-Directed Nanostructures and (b) Down-Directed Nanostructures (Nanopores) Fabricated on a Microtextured Surface

electrochemical anodization methods individually. Guo et al.34 fabricated a superhydrophobic Al surface by etching with sodium hydroxide solution for several hours followed by spin coating a layer of poly(dimethysiloxane) vinyl terminated (PDMSVT). Similarly, Shen et al.35 reported the fabrication of a microstructured superhydrophobic Al surface by simple etching with a mixture of hydrochloric acid, water, and hydrofluoric acid, followed by decoration with a fluoroalkylsilane monolayer. More recently, Zheng’s group reported an anodization process to fabricate nanostructured alumina films in oxalic-acid electrolytes.44 Although the above cases reported obtaining micro- or nanostructured superhydrophobic Al surfaces with water CAs between 155 and 160°, these microor nanostructured surfaces may not be adequate to resist oils with low surface energy. However, to the best of our knowledge, there are only a few reports on the fabrication of superamphiphobic Al surfaces. Tsujii et al.45 prepared a superamphiphobic Al surface via anodic oxidation in aqueous sulfuric acid for 3 h. They showed only the CA of rapeseed oil, 150°, on the treated surface. However, their fabrication method may not be suitable for resisting low-surface-energy oils such as olive oil (surface tension of 32 mN/m at 20 °C) and hexadecane (surface tension of 27.5 mN/m at 20 °C). Meng et al.25 reported the fabrication of superamphiphobic surfaces on common engineering metals (zinc, aluminum, iron, and nickel) by a one-step electrochemical process using perfluorocarboxylic acid solutions at room temperature. This method is simple and can be applied to different metal surfaces, but it is a time-dependent treatment and the time required lengthens the total process. For example, after 18 days of immersion in an ethanol solution of nonadecafluorodecanoic acid (CF3(CF2)8COOH), the resulting Al plate exhibits superamphiphobicity. The lack of mechanical stability is a common problem for most of the fabricated superamphiphobic surfaces, which restricts their prospects in industrial applications. Recently, Ruhe et al.46 generated superhydrophobic silicon surfaces by etching and subsequent coating with a fluoropolymer (PFA). They compared the mechanical stability of microscale, nanoscale, and both micro- and nanoscale silicon surfaces. Similarly, Lee et al.47 fabricated copper oxide nanowires on copper substrates by an oxidation−reduction process and checked the mechanical stability on the surfaces. However, these studies showed that the nanostructures were damaged or worn off from the contact surface at higher load. This can generally happen because of the fragile nature of up-directed nanostructures. At a higher applied load, the up-directed nanostructures are directly subjected to a large amount of force. They are not able to endure the applied force and get damaged

as a result. Therefore, it is desirable to develop a simple, economical, time-saving, and mechanically stable strategy for the fabrication of superamphiphobic surfaces. In our study, we developed a simple and efficient fabrication technique for preparing mechanically robust superamphiphobic surfaces and examined the effect of topographic modulation on the surface wettability. The first step of the process involved simple etching of an Al substrate using an acidic solution, resulting in the formation of a microtextured surface composed of irregular, rectangle-shaped plateaus. The second step was to generate porous nanostructures on the obtained microstructured Al surface by anodization. Finally, the resulting 2fold, micro−nanoscale textured structure was fluorinated or chemically modified with perfluorooctyltrichlorosilane to obtain a superamphiphobic surface. Our methodologies for fabricating dual micro−nanoscale textured surfaces provide good wetting resistance for lowsurface-energy liquids. To resolve the poor mechanical stability problem of the fabricated superamphiphobic surface, we created down-directed nanostructuresnanoporeson the microstructured Al surface. Up-directed nanostructures can get damaged at higher applied load owing to their fragile nature, while the fabricated surface with down-directed nanostructures remained almost stable at higher applied load, and almost no damage was found on the fabricated superamphiphobic surface (Scheme 1). The entire procedure was very simple to operate, and no special technique or sophisticated equipment was required. Moreover, the procedure was time-saving (completed in approximately 1 h) and inexpensive. These nanoporeembedded, microtextured surfaces can have a variety of potential industrial applications such as self-cleaning, dragreduction, antisticking, corrosion resistance, antifouling, and anti-icing.

2. EXPERIMENTAL DETAILS Aluminum plates (thickness: 0.81 mm; composition: Al 95.8−98.6%, Mg 0.8−1.2%, Si 0.4−0.8%, Cr 0.04−0.35%, Cu 0.15−0.4%, Fe 0.7% max, Zn 0.25% max, Mn 0.15% max, Ti 0.15% max), sulfuric acid (H2SO4), hydrochloric acid (HCl), and 1H,1H,2H,2H-perfluorooctyltrichlorosilane (PFOTS) were purchased from Alfa Aesar Inc. All other chemicals were analytical grade and used without further purification. In a typical process, the Al plate that was cut into small pieces was ultrasonically cleaned in acetone and deionized water (DI) for 3 min each and then dried in a stream of nitrogen. The cleaned Al plates were then etched with acid solution of DI water and HCl (volume ratio: 2:1) for 3 min at room temperature to fabricate the microstructured surface. After etching, the samples were washed in DI water and dried at 120 °C for 10 min. The microstructured Al plate was anodized in a 1 M H2SO4 solution, under a constant voltage of 25 V at a temperature of 10 11967


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Scheme 2. Schematic Diagram of the Steps Involved in Fabrication of Superamphiphobic Micro- and Nanostructures on an Al Substrate

°C for 10 min. After the anodization, the Al plate was rinsed with DI water and dried in air. Finally, the as-prepared Al plate was dipped in 0.5% PFOTS in n-hexane for 8 min, washed in hexane, and heated on a hot plate at 100 °C for 30 min (Scheme 2). Characterization. The morphology of all the samples prepared in the present study was examined using field-emission scanning electron microscopy (FESEM; JSM-740 1F, JEOL, Japan). The CA and SA were measured with 5 μL droplets of water and various oils using a CA measurement system (Phoenix 300 Touch, SEO Co. Ltd., South Korea). The average CA and SA values were obtained by measuring each sample at a minimum of five different positions at room temperature. The CA value was calculated using the tangent line method. The optical images of the droplets were obtained using a digital camera (Sony Inc., Japan). The hardness test of the fabricated surfaces was performed by means of a microtribometry system (RB Model 102-PB, R&B Inc., South Korea).

SEM images of the M-structured surface obtained by etching in acid solution for 3 min. It could be clearly seen that the entire Al surface was rough and composed of irregular “protrusions” after etching with the acid solution. The high-resolution FESEM image shown in Figure 1b confirms that the etched aluminum surface consisted of irregular protuberances that appeared like building blocks composed of rectangular plateaus. The plateaus were ∼1−2 μm in size and were consistently distributed throughout the surface. Further, from the AFM image, analysis of the microstructured Al surfaces shows that the height/roughness of the plateau surface is more than 1 μm. The concept leading to this type of microstructures was established approximately a half century ago, and several special etchants have been developed since then to detect dislocations in crystals. A large number of dislocation defects exist in common crystalline metals. These dislocation sites have relatively higher energy, and thus when attacked by chemical etchants, they would be dissolved first. When a crystal face is exposed to a dislocation etchant, pits are often formed on the surface. The etch pit formation is generally thought to result from the preferential nucleation of unit pits one atom deep at a dislocation and the movement of the monatomic steps across the surface. The relative rates of these two processes determine the shape of the etch pits.35,48 Figure 1c and d shows the SEM images of Al surfaces obtained by etching in an acidic solution for 6 min. The variations in microstructure between the samples etched for 3 and 6 min confirmed that microstructured Al surfaces with different surface roughness could be achieved by varying the etching time. Figure 2a and b shows the SEM images of the self-ordered Nsurface obtained by anodizing the Al surface in the presence of 1 M H2SO4 at an applied voltage of 25 V for 10 min. As seen from the high-resolution SEM image shown in Figure 2b, nanopores with diameters of 25−30 nm were distributed uniformly throughout the surface. By combining the abovementioned fabrication steps (etching followed by anodization), MN-structured surfaces were fabricated, the results for which are shown in Figure 2c and d. Uniform, well-distributed nanopores were formed on the surface of plateaus after the Al surface was etched for 3 min, followed by anodization in the presence of a 1 M H2SO4 solution at a constant voltage of 25 V for 10 min. This honeycomb-like surface with a pore diameter ranging from 15 to 20 nm provided a good micro−nanosurface, making it a superamphiphobic surface. 3.2. Contact Angle Measurement. The CA measurements were performed to analyze the surface wettability

3. RESULTS AND DISCUSSION 3.1. Fabrication. To understand the significance of the micro−nanosurface on the fabrication of a superamphiphobic Al surface, we compared the morphology and wettability of the micro−nanostructured Al surface (MN-structured surface) with that of a microstructured Al surface (M-structured surface), nanostructured Al surface (N-structured surface), and smooth bare Al surface (S-surface). Figure 1a and b shows the top-view

Figure 1. SEM images of the aluminum surface after etching in acid solution for (a, b) 3 min and (c, d) 6 min; parts b and d are corresponding magnified images of images a and c, respectively. 11968


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energy and morphology, the superamphiphobic property can be achieved on a solid surface. Recently, it has been demonstrated that superamphiphobicity can also be achieved by creating special features such as reentrant geometries and overhanging structures. Tuteja et al.49 designed superoleophobic surfaces by generating re-entrant structures on electrospun polyhedral oligomeric silsesquioxane (POSS) fiber surfaces. Cao et al.50 fabricated an “overhanging” structure. Such structures are capable of preventing liquids from penetrating the cavities as a consequence of capillary forces, making a composite interface of air, liquid, and solid. Such a state is often referred to as the Cassie−Baxter state. In our research, by employing simple etching in an acidic solution and anodization followed by fluorination, we fabricated superamphiphobic surfaces. Figure 4 shows the surface wettability of the as-prepared and PFOTS-modified surfaces by different liquids. Upon fluorina-

Figure 2. SEM images of the aluminum surface obtained by (a, b) anodizing Al for 10 min (N-surface) and (c, d) etching Al with acid solution for 3 min followed by anodization for 10 min (MN-structured surface); parts b and d are corresponding magnified images of images a and c, respectively.

properties of the as-prepared Al plate in the presence of water and various oils. Figure 3 shows the surface and contact angle

Figure 4. Static contact angle as a function of liquid surface energy measured on various fabricated Al surfaces. Liquids used were water (72 mN/m), glycerol (63.6 mN/m), ethylene glycol (48 mN/m), olive oil (32 mN/m), and hexadecane (27.5 mN/m).

tion, all (MN-, M-, and N-) structured surfaces, except the Ssurface, displayed superior hydrophobic properties (CAs > 150°) but different oleophobicity. The CAs on the MNstructured surface for glycerol, ethylene glycol, polyalkylene glycole (PAG oil), olive oil, canola oil, soybean oil, and hexadecane were 159, 158, 155, 153, 152, 151, and 150°, respectively, showing that the MN-structured surface had improved superoleophobicity compared to the M-structured surface and N-structured surface. The combination of both microscale and nanoscale surface features introduced the special geometry required for this wetting resistance. After treatment with perfluorocarboxylic acid, the fabricated Al surface created a sufficiently low surface energy owing to its high content of −CF3 and −CF2 groups to enhance the super-repellency toward different oils. In the case of the M- and N-structured surfaces, the pure rectangular plateaus or nanopore structure lacked special geometry structures on the surfaces, and thus, low-surface-energy liquids could easily penetrate and wet the surfaces. Figure 5 shows the optical image of droplets of different liquids on the MN-structured surface. It could be observed that water, glycerol, ethylene glycol, and olive oil exhibited a spherical shape on the surface.

Figure 3. Contact angle profile of (a) water and (b) olive oil on an untreated bare aluminum surface. Contact angle profile of (c) water and (d) olive oil on an aluminum surface modified with PFOTS.

profiles of the Al plate before and after surface modification. The untreated Al plate was found to be amphiphilic with CA < 90°. Before surface modification, the CAs with respect to water and olive oil were measured to be 75 and 25°, respectively; after modifying the surface with PFOTS, the amphiphobic properties improved with an increase in CA (110 and 74°, respectively). However, after application of the PFOTS coating, no significant change in the surface structure was observed. In order to obtain the superamphiphobic property, the CA should be higher than 150° for both water and oil. As mentioned earlier, by controlling two key surface parameters, namely, the surface 11969


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Figure 5. (a) Optical image and (b) cross-sectional view of different liquid droplets on a MN-structured surface.

Figure 6. Sliding angles (SAs) of different liquids on a MN-structured surface.

To further understand the wettability of the interface between the surface and a water droplet, a contrast experiment for measuring the contact angle on a smooth surface was performed. The contact angle in terms of the Cassie and Baxter equation is described as follows:51 cos θ* = fs (cos θ + 1) − 1

Table 1. Comparison of Sliding Angles (SAs) of Various Liquids on As-Prepared and PFOTS-Modified Al Surfaces (N.S.: No Sliding) liquid

(1)

water glycerol ethylene glycol olive oil hexadecane

where θ* (163°) is the apparent contact angle for a droplet on a rough surface, θ (110°) is the equilibrium contact angle obtained on an ideally flat surface of the same chemical composition, and fs is the fraction of the solid surface in contact with the liquid. According to the above-mentioned Cassie and Baxter equation, fs of the fabricated surface was calculated to be about 0.06. Thus, the area fraction of the air trapped within the interstices of the fabricated surface was 0.94. Similarly, fs of olive oil was estimated to be about 0.08. The much lower surface fraction revealed that the MN-structured surface produced a large amount of air trapped between the microand nanostructures. This greatly increased the air−liquid interface, prevented the penetration of liquid droplets into the cavities, and played an important role in enhancing the superamphiphobic nature of the surface. In addition to the larger contact angles,the MN-structured surface also showed lower SAs with different liquids. Liquids with sliding angles below 6° easily rolled off the surface, as shown in Figure 6. Similar super-repellency was also observed for alkanes such as hexadecane, whose CA and SA were 150° and about 20°, respectively. Table 1 also provides a comparison of SAs for various liquids on different fabricated surfaces. Furthermore, we also studied the effect of anodization time on the morphology of the Al plate and their corresponding wetting properties. For this analysis, the microstructured Al surface (obtained by etching for 3 min in an acidic solution) was anodized for a prolonged period of 30 min in 1 M H2SO4 at a constant voltage of 25 V. Figure 7 shows the SEM images of the Al surface thus obtained by prolonging the anodization duration to 30 min. It could be observed that the irregular microscale plateaus disappeared and the surface was densely covered with nanowires. These nanowires originated from the nanopores owing to the pore-widening phenomenon. As the anodization time was increased, the walls between two adjacent nanopores became thinner and finally collapsed into the nanowires.52

surface tension (mN/m) (at 20 °C)

MN surface

M surface

N surface

S surface

72.0 63.6 48.0

2° 3° 4.3°

6° 10° 16°

21° 25° N.S.

N.S. N.S. N.S.

32.0 27.5

5.3° 20.6°

35° N.S.

N.S. N.S.

N.S. N.S.

Figure 7. SEM image showing surface morphologies of Al surfaces obtained by etching in acid solution for 3 min, followed by anodization for 30 min. Image b is the corresponding magnified image of image a.

We also measured the static CA for the nanowire surface with different liquids. The CAs with water, glycerol, ethylene glycol, olive oil, and hexadecane were measured to be 158, 157, 152, 149, and 145°, respectively. Figure 8 shows a graph comparing the static CAs on the MN-structured surface (fabricated by etching the Al plate for 3 min, followed by anodization for 10 min) and nanowire morphology (obtained by etching the Al plate 3 min in acidic solution, followed by anodization for 30 min). It could be seen that the CAs for the nanowire morphology were lower compared to those for the MN-structured surface, again confirming that the dual structure (microstructure together with nanostructure, or micro− nanostructure) was crucial to establishing superamphiphobic properties toward different liquids. From this detailed analysis, we could arrive at the conclusion that the micro−nanosurfaces with rectangular plateau structures 11970


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indicated no damage or detachment of the film after the peeling tests. Figure 10 shows the CA measured after each peeling

Figure 8. Graph showing the contact angles of different liquids measured on the MN-structured surface (black bars) and nanowire morphology (gray bars).

played a significant role in attaining the superwater/oilrepellency. 3.3. Mechanical Stability. In order to extend the practical applicability of fabricated superamphiphobic surfaces, they must exhibit adhesion properties and high mechanical stability. Generally, a fabricated surface with high roughness has poor adhesion property, which limits its use in commercial applications. There are few reports focusing on this aspect regarding superhydrophobic and superamphiphobic materials.47,53−55 In our study, we used a Scotch tape test and the hardness test to check the mechanical stability of the fabricated MN-structured surface. 3.3.1. Adhesion Property Test. The Scotch tape test, based on the ASTM D3359-02 standard, was applied to examine the stability and adhesion of the fabricated superamphiphobic Al surface. The Scotch tape was pressed against the fabricated Al sample and then peeled off. Figure 9 reveals the optical images of the Al surface before and after the tape test. We repeated the peeling test up to 10 times. Figure 9b−d shows the optical images of the Al surface after different peeling attempts. Compared with the original film surface, the images clearly

Figure 10. Graph showing CA measured with respect to peeling attempts on a fabricated Al surface. Insets: photographs of tape test and image of Scotch tape surface after test, indicating no detachment of film.

attempt, which also confirmed that the fabricated film had a good adhesion property and retained its superamphiphobicity after the Scotch tape test. 3.3.2. Hardness Test. The mechanical stability of the surface was evaluated by the microhardness test. As shown in Figure 11, a flat polydimethylsiloxane (PDMS) stamp (diameter: 0.8 cm) was fixed to the holder as the counterpart material. The testing sample was then moved toward the PDMS stamp and pressed against the stamp up to a specific load value. Afterward, the sample was detached from the PDMS. The measured CAs

Figure 9. Optical microscope images of fabricated Al surfaces: (a) bare Al surface; (b) after one peeling test; (c) after five peeling tests; (d) after performing the tape test 10 times. Scale bars: 100 μm.

Figure 11. Schematic diagram of the hardness test setup. 11971


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surface. By combining simple chemical etching and anodization, we developed a two-step technique to fabricate a dual morphology on Al plates. We could achieve super-repellency against a broad range of liquids with surface tension in the range 27.5−72 mN/m. Our morphology-dependent wettability results showed that, in addition to surface energy and roughness, designing the surface morphology with dual micro−nanoscale texture (MN-structured surface) plays an important role in establishing superamphiphobicity. Owing to the special down-directed nanostructures (nanopores) on the microstructured Al surface, the fabricated MN-structured surface showed good mechanical stability at different loads. The up-directed nanostructures on the microstructured surface are normally fragile in nature and can get damaged when subjected to high applied force because of their fragile nature. In contrast, the fabricated surfaces with down-directed nanostructures remained almost stable because of their unique morphology. Thus, at even high applied force, almost no damage was found on these fabricated surfaces compared to updirected nanostructures. This method could also be applied to any curved surface to produce large-area superamphiphobic surfaces. Our fabrication process is simple, time saving, and cost-effective and provides a new approach for designing mechanically robust, super-repellent surfaces. Hence, this study is expected to create new opportunities for the production of large-scale three-dimensional superamphiphobic engineering materials for many industrial applications such as dragreduction, antisticking, corrosion resistance, antifouling, and anti-icing.

for various liquids clearly revealed that the fabricated Al plate with MN-structured surface retained very stable micro- and nanostructures after a load of up to 4 N. Figure 12 shows

Figure 12. Contact angles of various liquids on a fabricated MN-Al surface after applying different forces.

insignificant changes in the CAs of the fabricated surfaces at different applied loads. Figure 13 shows SEM images of the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +82-2-910-4672. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by grants from the Global Excellent Technology Innovation R&D Program and Human Resources Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) (No. 20114010100070), funded by the Ministry of Knowledge Economy, Republic of Korea, and the Global Scholarship Program for Foreign Graduate Students at Kookmin University, Korea.

Figure 13. SEM images of a MN-structured surface: (a) before and (b) after hardness test with a load of 4 N.

fabricated samples before and after the hardness test at 4 N loads. Compared with the image of the original film (Figure 13a), almost no damage occurred on the film surfaces and the micro−nanostructured surfaces were well preserved even after the test load was increased to 4 N. The results showed that the developed micro−nanosurfaces, in which the nanostructures were pores instead of up-directed nanostructures (nanowires, nanorods, etc.), were stable enough to sustain higher load. Therefore, we can conclude that the dual surface composed of microstructures and nanopores (MN-structured surfaces) showed better mechanical stability and good superamphiphobic property. Furthermore, to check the long-term stability of the fabricated surface, we measured the CAs after exposure in air for 2 months. The CAs of various liquids showed only a slight decrease after exposure for 60 days, indicating that the Al surfaces also retained good long-term stability.

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REFERENCES

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4. CONCLUSIONS We have demonstrated an easy-to-implement technique for rapid fabrication of a mechanically stable superamphiphobic Al 11972


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