ial

oSanlogy

wie mhe

structure, themechanical durability of superhydrophobic aluminumhydroxide surfacewas improved. The

ontactof less

making an aluminum surface superhydrophobic has many appli-cation possibilities. However, only a few methods for fabricatingsuperhydrophobic aluminum surfaces have been reported. Qianet al. reported a dislocation-selective chemical etching technique

l etching inducesum substrate andmethod consumesthe alumina layer

n method for analuminum hydroxide layer that uses alkali surface modication[24,25]. The fabricated aluminum hydroxide layer is composed ofgibbsite (g-Al(OH)3) and has a ake-like nanostructure. By meansof this method, uniformly distributed nanostructures on analuminum surface can be obtained quickly and simply. Theydemonstrated that the wettability of the fabricated surface issuperhydrophilic.

Herein, we report uniformly superhydrophobic surface wherenano-scale structures were fabricated by alkali surfacemodication

* Corresponding author. Department of Mechanical Engineering, PohangUniversity of Science and Technology, San 31, Pohang, Kyungbuk 790-784, Republicof Korea. Tel.: 82 54 279 2174; fax: 82 54 279 5899.

Contents lists available at

Current Appl

.e

Current Applied Physics 13 (2013) 762e767E-mail address: whwang@postech.ac.kr (W. Hwang).a nanometer-sized structure superimposed over a micrometer-structure, similar to those of the lotus leaf. Moreover, many recentstudies have reported that a hierarchical roughness ensures super-hydrophobicity even after the surfaces are worn away [15e17].

Aluminum is widely used in various industrial elds as a basicmaterial for numerous mechanical components. For this reason,

anodization of aluminum. However, chemicaserious problems such as damage to the alumina non-uniformly etched site. The anodizationa large amount of electricity and causes defects infrom a failure to control the current.

Recently, Seo et al. reported a fabricatioknown as superhydrophobic surfaces. These surfaces have attractedmuch interest in industry, because many studies have shown thatthese surfaces have properties such as self-cleaning [1e3], anti-corrosion [4e6], anti-frosting [7e9], uid drag reduction [10e12],non-sticky to oil and inks [13,14]. A superhydrophobic surface can beattained by forming a hierarchical roughness structure, that is,

On the other hand, anodization combined with a low-temperatureplasma treatment was used to form hierarchical structures onaluminum surfaces [20]. Wu et al. realized a superhydrophobicaluminum surface by forming alumina nanowire forests via higheld anodization [21]. Kim et al. [22] and Jeong et al. [23] reporteda self-aggregation phenomenon for alumina nanowires by theKeywords:SuperhydrophobicityAluminum hydroxideSandblastingHierarchical structure

1. Introduction

Surfaces with a very high water cand a low contact angle hysteresis1567-1739/$ e see front matter 2012 Elsevier B.V.http://dx.doi.org/10.1016/j.cap.2012.11.021resulting hierarchical structures are suitable for diverse applications of aluminum in various industrialareas, including self-cleaning, anti-frosting, and microuidic devices for rigorous environments.

2012 Elsevier B.V. All rights reserved.

angle larger than 150

than 10 are generally

for superhydrophobic aluminum surfaces with contact angleslarger than 150 [18]. Guo et al. fabricated a superhydrophobicsurface using chemical etching by immersing aluminum in sodiumhydroxide (NaOH) and then decorating with peruorononane [19].Accepted 28 November 2012Available online 17 December 2012 structured aluminum hydroxide layer. The superhydrophobic surfaces fabricated by both methods

exhibited a high water contact angle and very low contact angle hysteresis. By forming the hierarchical27 November 2012 hierarchical structures combining both microstructure via sandblasting techniques and the nano-A simple fabrication method for mechansuperhydrophobic surface by hierarchicstructures

Handong Cho a, Dongseob Kimb, Changwoo Lee a, WaDepartment of Mechanical Engineering, Pohang University of Science and Technology,bGraduate School of Engineering Masterships, Pohang University of Science and Techno

a r t i c l e i n f o

Article history:Received 9 August 2012Received in revised form

a b s t r a c t

Superhydrophobic surfacesfabricated by alkali surfacmechanical durability of t

journal homepage: wwwAll rights reserved.cally robustaluminum hydroxide

onbong Hwang a,b,*

31, Pohang, Kyungbuk 790-784, Republic of Korea, San 31, Pohang, Kyungbuk 790-784, Republic of Korea

th uniformly superhydrophobic surface where nano-scale structures wereodication method and self-assembled monolayer coating. To enhancesuperhydrophobicity, we propose the fabrication process for dual-scale

SciVerse ScienceDirect

ied Physics

lsevier .com/locate/cap

method and self-assembled monolayer coating. And, we proposethe fabrication process for dual-scale hierarchical structurescombining both microstructure via sandblasting techniques andthe nanostructured aluminum hydroxide layer to enhancemechanical durability of the superhydrophobicity.

This process has the following key advantages: industrycompatibility, robustness, and a uniform superhydrophobic surface.We particularly investigated the wetting characteristics bymeasuring the water contact angle and the improvement in themechanical robustness by an abrasion test.

2. Experimental details

The fabrication process for superhydrophobic nanostructured

normal aluminum specimen exhibited slight hydrophilicity. Fora droplet in contact with a rough surface, the contact angle wasexplained by the Wenzel state [26]. According to the Wenzel state,the contact angle of the aluminum surface became more hydro-philic because of the increase in its roughness after sandblasting.

H. Cho et al. / Current Applied Physics 13 (2013) 762e767 763and hierarchical aluminum hydroxide surfaces are shown sche-matically in Fig. 1. Industrial grade aluminum sheets (99.5%,50 mm 40 mm 1 mm) were used in all of the experiments. Theformation of nanostructured aluminum hydroxide was carried outin a 0.05 M NaOH solution at 80 C for 5 min. After this, the spec-imenwas immediately immersed in 100 C deionized water for thesubsequent stabilization process. A microroughness structure wasprepared by sandblasting the aluminum sheet with sand particles.The size of the sand particle was 500 mesh, and the particles wereejected from a nozzle using compressed air at a pressure of6 kgf cm2. After sandblasting, the aluminum sheet was cleanedwith deionized water. The hierarchical surface was fabricated byforming aluminum hydroxide layer on the microroughenedaluminum specimen. Finally, superhydrophobic nanostructuredand hierarchical surfaces were obtained after self-assembledmonolayer coating with heptadecauoro-1,1,2,2-tetrahydrodecyl-trichlorosilane (HDFS, Gelest) was applied on the specimen. Thespecimen was dipped in a mixture of n-hexane and HDFS (volu-metric ratio 1000:1) for 10 min. They were then washed withdistilled water and dried in the oven (105 C) for 1 h.

The static contact angle, contact angle hysteresis, and slidingangle on the fabricated surface were measured using a drop shapeanalysis system (DSA-100, Kruss). The contact angle hysteresis wasobtained by measuring the advancing contact angle and recedingcontact angle at the maximum sliding angle by which the dropwould move by gravity. A 5-ml droplet of distilled water was usedfor this purpose. The contact angle results were averaged over atleast ten measurements on different areas of each specimen atroom temperature. Scanning electron microscopy (SEM; JSM-7401FFE-SEM, JEOL) images were obtained to investigate the surfacemorphology.

In order to evaluate the robustness of the superhydrophobicsurface, abrasion test illustrated in Fig. 2 was performed. Thefabricated superhydrophobic surface was tested facing an abrasiveFig. 1. Steps in the fabrication of superhydrophobic nanostrlm (1 micron grade Imperial lapping lm, 3 M). Applyingweights to the specimen, the surface was moved in one directionwith 5 mm/s at a stroke of 15 cm. The preload was applied on eachspecimen and increased up to 1000 g. The static contact angles andhysteresis changes of the superhydrophobic surfaceweremeasuredafter abrasion test.

3. Results and discussion

The surface morphologies of several specimens were examinedusing SEM, as shown in Fig. 3. Fig. 3(a) shows the surface ofindustrial grade normal aluminum. Fig. 3(b) shows that thealuminum surface had microscale unevenness and its morphol-ogies changed signicantly after sandblasting. Fig. 3(c) shows thatthe surface of the aluminum was covered with a ake-likealuminum hydroxide layer. The thickness of this aluminumhydroxide layer was around 500 nm with a nanoscale surfacemorphology. Fig. 3(d) reveals that the hierarchical structureincluded the microscale structure formed by sandblasting andnanoscale aluminum hydroxide structures over the micro-roughened structure.

The wettabilities of several specimens were characterized indetail using contact angle measurements, as shown in Fig. 4(a). The

Fig. 2. Schematic of abrasion test. (Inset: SEM image of abrasive lm). The abrasiontests were conducted by changing applied weights from 10 g to 1000 g.uctured and hierarchical aluminum hydroxide surfaces.

ied PH. Cho et al. / Current Appl764Under this situation, the hydrophilic nature of the aluminumhydroxide containing the hydroxyl group (eOH) [25] and theincreased roughness caused by the ake-like morphology playedimportant roles in the wettability. Therefore, both the aluminumhydroxide nanostructure and the hierarchical structure showed thesuperhydrophilic property.

Fig. 3. SEM images of (a) industrial grade normal aluminum, (b) sandblasted aluminum,sandblasted aluminum. The thickness of the aluminum hydroxide layer was about 500 nmhysics 13 (2013) 762e767After the SAM coating with HDFS, the wettabilities of thespecimens changed to hydrophobic. In the case of a rough hydro-phobic surface, a water droplet cannot penetrate into a structuralgroove because of the air pocket present between thewater and thesurface, which is described as the Cassie state [27]. With theexception of normal aluminum, the specimens had high static

(c) aluminum hydroxide layer on aluminum, and (d) aluminum hydroxide layer on.

contact angles, as shown in Fig. 4(b). Clearly, both the aluminumhydroxide nanostructure and the hierarchical structure showedsuperhydrophobic properties, with a high static contact angle of160 and low contact angle hysteresis of 2 at a low sliding angle.Thus, a superhydrophobic surface was successfully fabricated withaluminum hydroxide and HDFS coating. However, the sandblastedaluminum had a static contact angle of around 150, while a waterdrop on the surface was attached even at a sliding angle of 90. Thiswas because increased surface roughness by sandblasting rein-forced the hydrophobic characteristics but also amplied theamount of pinning defect. These led to high contact angle and

According to Xiu et al., the superhydrophobicity of a hierarchicalstructure is more robust than that of a single nanoscale structure[16]. In this study, the abrasion resistance of the super-hydrophobicities of the aluminum hydroxide nanostructure andaluminum hydroxide hierarchical structure was evaluated, and therobustness values of these superhydrophobic structures werecompared. Fig. 5(a) shows that the aluminum hydroxide nano-structures were crushed and changed into a smooth surface afterabrasion. In the case of the aluminum hydroxide hierarchicalstructure, however, only the hydroxide layer on the ridge was wornaway after the abrasion test, as shown in Fig. 5(b). The nano-

Fig. 4. (a) Water contact angles of normal aluminum (S1), sandblasted aluminum (S2), aluminum hydroxide nanostructure (S3), and aluminum hydroxide hierarchical structure(S4). (b) Water contact angles and contact angle hysteresis of these four specimens after application of HDFS coating.

H. Cho et al. / Current Applied Physics 13 (2013) 762e767 765sticky behavior at the same time [28].Whereas the contact areawasreduced in the nanostructured surfaces and minimized in thehierarchical-structured surfaces due to the presence of the airpockets inside the grooves [29], so they had high contact angle andslippery behavior.Fig. 5. (a) SEM images of nanostructures and (b) hierarchicastructures on the grooves remained unchanged because the peaksof the microstructures helped the structures retain their shapeduring the abrasion test.

Fig. 6(a) shows the effect of abrasion on the wetting proper-ties of the superhydrophobic aluminum hydroxide structures.l structures after abrasion tests with a weight of 100 g.

(c) Aarea

H. Cho et al. / Current Applied Physics 13 (2013) 762e767766After the abrasion test, the static contact angle of the hierarchicalstructure changed slightly, from 159 to 151. The contact angle ofthe nanoroughness structure declined with an increase in theapplied load of the abrasion test, but the angle was also highabove 140. However, with respect to the contact angle hysteresisand the sliding angle, the difference between these values of thenanostructure and the hierarchical structure increased afterabrasion, as shown in Fig. 6(b). The contact angle hysteresis andsliding angle of the hierarchical structure were 8.8 and 9.6

respectively. These values accorded with the criteria for thesuperhydrophobicity, and therefore the superhydrophobicitycharacteristic was preserved until a weight of 100 g was applied.The nanostructure, on the contrary, lost its superhydrophobicityafter the application of a weight heavier than 10 g. The contactangle hysteresis and sliding angle of the nanostructure increasedto more than 10, a water drop nally adhered to the surface of

Fig. 6. Changes in (a) contact angle and (b) contact angle hysteresis after abrasion test.slightly. But air pockets in the groove were maintained. (d) After abrasion, the contactthe nanostructure at a sliding angle of 90 when more than 300 gwas applied.

In the case of the hierarchical structure, the microstructuresprotected the nanostructures, which formed air pockets inside thegrooves under a water droplet. These reduced the contact area andadhesiveness between the droplet and the surface, as shown inFig. 6(c). Therefore, low contact angle hysteresis and small slidingangle were maintained to some extent external load, and the Cassie

Fig. 7. Superhydrophobic aluminum letters fabricated using sandblasting and aluminum haluminum structures of any shape and size.state was stabilized by the dual scale structures [15]. On the otherhand, in the case of the nanostructure, the increase in the contactangle hysteresis originated from the destruction of the nano-structure that increased the contact area, as shown in Fig. 6(d).Meanwhile, the contact angle hysteresis values of both structuresincreased with an increase in the weights used for the abrasiontest. This was because the actual contact area between thesuperhydrophobic surface and the abrasive lm increased whenheavier weights were applied, and surfaces of the structureswere subjected to a large amount of wear. Also, some regionsbecame more hydrophilic due to the loss of hydrophobic coatingand the exposure of hydrophilic aluminum hydroxide layer andaluminum surface. Thus, mechanical durability of superhydro-phobic aluminum hydroxide surfacewas improved by forming sucha hierarchical structure.

The proposed method for fabricating a hierarchical super-

s to the hierarchical structure, the contact area between droplet and surface increasedof nanostructure increased sharply and air layers disappeared.hydrophobic aluminum hydroxide structure could be easily appliedto an aluminum structure with a complex geometry or large size.Fig. 7 shows that aluminum letters with a superhydrophobic hier-archical surface structure were immersed in colored water. Themicro- and nano-structures maintained a microscopic air layer onthe surface even when submerged in water. This super-hydrophobicity amplied the effect of surface tension, and awall ofwater formed around the specimens.

ydroxide were immersed in colored water. The proposed method can be applied to

4. Conclusions

In conclusion, we successfully fabricated a superhydrophobicsurface with uniform superhydrophobicity by nanostructuredaluminumhydroxide and SAMcoating. Andwedeveloped the simpleand cost-effectivemethod to fabricate superhydrophobic hierarchicalsurface composed of microstructures by sandblasting and nano-structured aluminumhydroxide lm. These fabricated surfaces showvery low contact angle hysteresis less than 10. The mechanicaldurability of the fabricated surfaces has been evaluated using abra-sive test and the result shows that the superhydrophobicity ofaluminum hydroxide structure becomes more robust by forminghierarchical roughened structures. The proposed fabrication processhas the advantages of simplicity, industry compatibility, and easyscale-up. We believe that proposed method would be favorable foraluminum devices having self-cleaning, anti-frosting and anti-corrosion properties in rigorous environments.

Acknowledgments

This researchwas supportedby theMinistryof Education, ScienceTechnology (MEST) andNational ResearchFoundationofKorea (NRF)through the Human Resource Training Project for Regional Innova-tion and Do-yak research program (No. 2011-0018645).

References

[1] B. Bhushan, Y.C. Jung, K. Koch, Langmuir 25 (2009) 3240e3248.[2] T.-I. Kim, D. Tahk, H.H. Lee, Langmuir 25 (2009) 6576e6579.

[3] K. Lee, S. Lyu, S. Lee, Y.S. Kim, W. Hwang, Appl. Surf. Sci. 256 (2010)6729e6735.

[4] B. Grignard, A. Vaillant, J. de Coninck, M. Piens, A.M. Jonas, C. Detrembleur,C. Jerome, Langmuir 27 (2010) 335e342.

[5] T. Ishizaki, Y. Masuda, M. Sakamoto, Langmuir 27 (2011) 4780e4788.[6] K. Liu, M. Zhang, J. Zhai, J. Wang, L. Jiang, Appl. Phys. Lett. 92 (2008) 183103.[7] A.J. Meuler, G.H. McKinley, R.E. Cohen, ACS Nano 4 (2010) 7048e7052.[8] K.K. Varanasi, T. Deng, J.D. Smith, M. Hsu, N. Bhate, Appl. Phys. Lett. 97 (2010)

234102e234103.[9] M.A. Sarshar, C. Swarctz, S. Hunter, J. Simpson, C.H. Choi, Colloid Polym. Sci.

(2012).[10] S. Lee, J.H. Kang, S.J. Lee, W. Hwang, Lab Chip 9 (2009) 2234e2237.[11] C.-H. Choi, C.J. Kim, Phys. Rev. Lett. 96 (2006) 066001.[12] C. Lee, C.H. Choi, C.J. Kim, Phys. Rev. Lett. 101 (2008) 064501.[13] K. Ellinas, A. Tserepi, E. Gogolides, Langmuir 27 (2011) 3960e3969.[14] S.M. Kang, S.M. Kim, H.N. Kim, M.K. Kwak, D.H. Tahk, K.Y. Suh, Soft Matter 8

(2012) 8663e8668.[15] T. Verho, C. Bower, P. Andrew, S. Franssila, O. Ikkala, R.H.A. Ras, Adv. Mater. 23

(2011) 673e678.[16] Y. Xiu, Y. Liu, D.W. Hess, C.P. Wong, Nanotechnology 21 (2010) 155705.[17] W.G. Bae, K.Y. Song, Y. Rahmawan, C.N. Chu, D. Kim, D.K. Chung, K.Y. Suh, ACS

Appl. Mater. Interfaces 4 (2012) 3685e3691.[18] B. Qian, Z. Shen, Langmuir 21 (2005) 9007e9009.[19] Z. Guo, F. Zhou, J. Hao, W. Liu, J. Am. Chem. Soc. 127 (2005) 15670e15671.[20] H. Wang, D. Dai, X. Wu, Appl. Surf. Sci. 254 (2008) 5599e5601.[21] W. Wu, X. Wang, D. Wang, M. Chen, F. Zhou, W. Liu, Q. Xue, Chem. Commun.

(2009) 1043e1045.[22] D.-H. Kim, Y. Kim, B.M. Kim, J.S. Ko, C.-R. Cho, J.-M. Kim, J. Micromech.

Microeng. 21 (2011) 045003.[23] C. Jeong, C.-H. Choi, ACS Appl. Mater. Interfaces 4 (2012) 842e848.[24] Y.I. Seo, Y.J. Lee, D.-G. Kim, K.H. Lee, Y.D. Kim, Appl. Surf. Sci. 256 (2010)

4434e4437.[25] Y.I. Seo, D.-G. Kim, Y.D. Kim, B.J. Lee, K.H. Lee, Scr. Mater. 59 (2008) 889e892.[26] D. Kim, S. Lee, W. Hwang, Curr. Appl. Phys. 12 (2012) 219e224.[27] B. Bhushan, Y.C. Jung, K. Koch, Philos. Trans. R. Soc. A 367 (2009) 1631e1672.[28] K. Koch, B. Bhushan, Y.C. Jung, W. Barthlott, Soft Matter 5 (2009) 1386e1393.[29] D. Qur, A. Lafuma, J. Bico, Nanotechnology 14 (2003) 1109e1112.

H. Cho et al. / Current Applied Physics 13 (2013) 762e767 767

A simple fabrication method for mechanically robust superhydrophobic surface by hierarchical aluminum hydroxide structures1. Introduction2. Experimental details3. Results and discussion4. ConclusionsAcknowledgmentsReferences