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Current Applied Physics 11 (2011) 800e804

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Current Applied Physics

journal homepage: www.elsevier .com/locate/cap

Artificial lotus leaf structures made by blasting with sodium bicarbonate

Sangmin Lee, Dongseob Kim, Woonbong Hwang*

Department of Mechanical Engineering, Pohang University of Science and Technology, San 31, Pohang, Gyungbuk 790-784, Republic of Korea

a r t i c l e i n f o

Article history:Received 26 August 2010Received in revised form20 October 2010Accepted 26 November 2010Available online 10 December 2010

Keywords:SuperhydrophobicityArtificial lotus leafSodium bicarbonateNanofiberAnodic aluminum oxide

* Corresponding author. Tel.: þ82 54 279 2174; faxE-mail address: whwang@postech.ac.kr (W. Hwan

1567-1739/$ e see front matter � 2010 Elsevier B.V.doi:10.1016/j.cap.2010.11.075

a b s t r a c t

Superhydrophobic surfaces have superior hydrophobicity and micro-nano hierarchical structures likethose on the lotus leaf. The conventional methods used for the fabrication of microstructures involvedelicate and time-consuming processes. We report here a simple and cost-effective method for fabri-cating artificial lotus-leaf-like structures with uniform superhydrophobicity; this method is based ona blasting process with sodium bicarbonate. Sodium bicarbonate, a water-soluble material, can be easilycleaned off an Al surface by the addition of oxalic acid to the anodizing solution. Therefore, microparticlesdo not need to be removed in a separate process. The resulting hierarchical structures are suitable fordiverse applications, including microfluidic devices for biological studies and industrial self-cleaningproducts for automobiles, ships and houses.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

Surfaces with a contact angle (CA) of greater than 150� aregenerally known as superhydrophobic surfaces. Superhydrophobicsurfaces have been widely studied, since the self-cleaning featuresof the lotus leaf surface were first discovered. The lotus leaf hasa superhydrophobic surface on which water drops roll off easily.Superhydrophobic surfaces can be obtainedmainly in twoways; byfabricating a rough surface and by modifying a surface usingmaterials with low surface free energy [1e5]. Hydrophobic prop-erties are usually enhanced by surface roughness, especially byfractal structures created using techniques such as the self-assembly of a monolayer [6], photolithography [7,8], plasma poly-merization [9], UV illumination [10], electrospinning [11], ionirradiation [12,13], template methods [14e19], and chemicaldeposition [20e22]. Superhydrophobic surfaces with low CAhysteresis, defined as the difference between the advancing and thereceding CAs, are water repellent, self-cleaning, and have low dragfor fluid flow. In a recent paper, Bharat has reported that hierar-chical surfaces exhibit a higher CA and lower CA hysteresis thansurfaces with nanostructures [23]. These research findings high-lighted the necessity to fabricate the hierarchical surfaces witha low CA hysteresis for various applications, including self-cleaningwindows, and exterior paints for buildings and ships.

: þ82 54 279 5899.g).

All rights reserved.

Previous researches have shown that superhydrophobicsurfaces can be prepared by forming hierarchical micrometer- andnanometer-sized structures, like those on the lotus leaf. Lei et al.fabricated a micro-nanoscale hierarchical template, which can beused to replicate lotus-leaf-like structures by pressing 20-mm glassspheres into an Al foil and anodizing the dimpled Al foil [14]. On theother hand, Kim et al. prepared dual-scaled structures by blasting50-mm sand particles onto an Al plate and anodizing the sand-blasted plate [18].

Unfortunately, these methods involve delicate and time-consuming processes, like the removal of microparticles from thenanoporous template in fabrication of superhydrophobic surfacesby replication. Furthermore, the resulting superhydrophobic surfaceis limited in size (e.g., the wafer level). It is, therefore, important todevelop a simple and cost-effective method for fabricating uniformsuperhydrophobic surfaces with hierarchical structures similar tothose of the lotus leaf.

Anodic Al oxide (AAO) has recently been proposed as a suitablematerial for use in nanotechnology. In particular, porous-type AAOhas an attractive feature that its pore dimensions (pore diameterand length) and pore density can be easily controlled by varyingthe anodizing conditions [24e27]. The pore radius and interporedistance depend on the duration of immersion in an electrolytesolution and the anodic voltage. Superhydrophobic surfaceshave been fabricated by replication of a porous AAO template [28].

We report here the practical and effective fabrication of micro-nano hierarchical structures, similar to those of the lotus leaf,having uniform superhydrophobicity; the method involves blasting

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an Al foil (99.5%) with sodium bicarbonate and anodizing thedimpled Al foil. Since sodium bicarbonate is water-soluble, it can beeasily removed from the Al surface by the addition of oxalic acid tothe anodizing solution. Therefore, microparticles do not need to beremoved in a separate process. Further, sodium bicarbonate isa low-cost material. The proposed method has major advantages ofconvenience and low-cost over conventional methods. The result-ing micro-nanostructures have the potential to be deployed indiverse applications such as microfluidic devices for biologicalstudies and industrial self-cleaning products for automobiles, ships,and houses.

2. Experimental details

2.1. Preparation of bicarbonate-blasted porous alumina templateand replica

The fabrication process of superhydrophobic hierarchicalstructures is shown schematically in Fig. 1(a). First, microstructureswere fabricated by blasting an Al foil with sodium bicarbonateusing compressed air. More specifically, we used compressed air ata pressure of 6 kgf cm�2 and the resulting sodium bicarbonateparticles were 250e300 mesh size (i.e., 25�30 mm diameter).Bicarbonate-blasted AAOwas fabricated through the anodization of

Fig. 1. (a) Steps in the fabrication of superhydrophobic hierarchical structures: first,preparation of sodiumblasted porous alumina template; and then PTFE replication andalumina etching in AZ 300 MIF at room temperature. SEM images of (b) a sodium-blasted porous alumina PTFE replica with uniform superhydrophobicity.

an industrial Al foil (99.5%, 60mm� 40mm� 0.3mm), using oxalicacid. Sodium bicarbonate can be easily removed by the addition ofoxalic acid to the anodizing solution, because it is soluble in water.The bicarbonate-blasted aluminum sheet was used as the anode,and a flat aluminum sheet as the cathode. The electrodes wereabout 5 cm apart. Different bicarbonate-blasted Al foils wereanodized for 0, 2, 3 and 4 min in a 0.3 M oxalic acid solution ata constant voltage of 40 V, using a computer-interfaced powersupply (Digital Electronics Co., DRP-92001DUS). Hwang et al., haveexperimentally proven that the length of pores is proportional tothe anodizing time [27]. They showed that the relationshipbetween the pore length and the anodizing time is represented by:

l ¼ �147:75þ 125:53t; (1)

where l (nm) is the length of the pore and t (min) is the anodizingtime. During the anodization process, the solution was maintainedat a temperature of 15 �C by a circulator (Lab. Companion,RW-0525G).

Superhydrophobic surfaces utilizing an AAO template havepreviously been fabricated by anodizing pure aluminum (99.999%)[14]. Since it is not necessary to achieve a regular pore arrangementin the AAO template, we fabricated an AAO template using indus-trial aluminum foil (99.5%). A replication template with largenanoholes was fabricated by immersing the AAO template ina 0.1 M phosphoric acid solution at a temperature of 25 �C for60 min. The nanoporous AAO prepared by this process was used asthe replication template. In addition, a polymer replica wasprepared by rubbing a PTFE solution (Polytetrafluoroethylene,Teflon� AF 601S2, 6 wt%, Dupont�) onto the replication template.The PTFE solution filled the nanoholes in the AAO template.A conventional vacuum process was then used to remove the airremaining in the pores that had formed in the porous aluminatemplate. During the curing process, at room temperature for oneday, the solvent was evaporated to leave a PTFE thin film. Finally,themicro-nano hierarchical structures were obtained after removalof the AAO template with an AZ 300 MIF developer at roomtemperature. Fig. 1(b) shows typical scanning electron microscopy(SEM) images of the bicarbonate-blasted porous alumina PTFEreplica, which provide sufficient evidence that the nanofibers hadbeen successfully replicated on the microstructures.

2.2. Surface characterization

We measured the CA of the fabricated superhydrophobicsurfaces using a Drop Shape Analysis system (DSA-100, Kruss Co.);the dynamic advancing CAs were recorded as the surface waspushed toward the water droplet, and receding CAs were recordedas the surface was pulled from thewater droplet [11]. A 3-ml dropletof distilled water was used for this purpose. The values reported areaverages of at least ten measurements made on different areas ofeach specimen at room temperature. SEM images were obtainedwith a JEOL JSM-7401F FE-SEM (field-emission scanning electronmicroscope, NCNT) at an accelerating voltage of 5 kV.

3. Results and discussion

We looked at the surface morphologies of the sodiumblastedaluminum and the sodiumblasted porous alumina template usingSEM, as shown in Fig. 2. Fig. 2(a) shows the microscale unevennessof the sodiumblasted aluminum surface. Fig. 2(b) shows the surfaceof the sodiumblasted porous alumina, which had undergone bothsodiumblasted and anodic oxidation. The depth is about 350 nm(see the inset of Fig. 2(c)), corresponding to an anodizing time of4 min calculated from equation (1). In the Al foil anodized for 0, 2,

Fig. 2. Field-emission SEM top images of sandblasted aluminum and sandblasted porous alumina. (a) sodiumblasted surface, (b) sodiumblasted porous alumina surface,(c) magnified image of sodiumblasted porous alumina surface with the anodizing time of 4 min Inset show the cross-sectional image of the sodiumblasted porous alumina surfacewith sodiumblasted porous surface with the anodizing time of 4 min. (d) Pore size distribution of the sodiumblasted porous surface with the anodizing time of 4 min. The meanpore diameter is about 40 nm (Standard deviation w20%).

Fig. 3. ((a)e(d)) SEM image of the replicated hierarchical structures. (a) Sodiumblasted PTFE replica. (b) Sodiumblasted porous alumina PTFE replica (2 min anodizing time). (c)Sodiumblasted porous alumina PTFE replica (3 min anodizing time). (d) Sodiumblasted porous alumina PTFE replica (4 min anodizing time). Insets (a)e(d) show the apparentadvancing contact angle for the specimens.

S. Lee et al. / Current Applied Physics 11 (2011) 800e804802

Fig. 4. Contact angle hysteresis for each specimen. The inset shows a virtually ball-shaped water droplet on superhydrophobic hierarchical structures. As the meannanofiber length increases, the apparent advancing contact angle increases and thecontact angle hysteresis decreases.

S. Lee et al. / Current Applied Physics 11 (2011) 800e804 803

and 3 min, the pore depth is 0 nm, 100 nm and 220 nm, respec-tively. The Fig. 2(d) reveal that the mean pore diameter of thesodiumblasted porous alumina is about 40 nm, having an irregulararray and varying pore diameter because of the use of industrialAl [19], as shown in Fig. 2(c).

Fig. 3 shows SEM images of the bicarbonate-blasted porousalumina PTFE structures. More specifically, Fig. 3(a)�(d) show thereplicated micro-nano hierarchical structures for anodizing timesof 0, 2, 3, and 4 min, which led to a mean nanofiber length of 0, 100,220, and 350 nm, respectively. The micro-nano hierarchical struc-tures contained trapped air, which reduced the actual contact areabetween the surface and water droplet, so that the structuresexhibited strong superhydrophobicity. The apparent advancing CAsof the hierarchical structures were 143�, 155�, 160� and 165�

respectively, as shown in insets in Fig. 3 (a)�(d). These are theaverage values of the measured CAs.

Fig. 4 shows advancing and receding CAs on the hierarchicalstructures with a nanofiber mean length of 0, 100, 220, and 350 nm.As the nanofiber mean length increases, the apparent advancing

Fig. 5. Effect of age on contact angles of the hierarchical superhydorphobic structure(anodizing time of 4 min). The structures are unaffected by aging in air or water.

CA increases and the CA hysteresis decreases (see Fig. 4). In otherwords, as the mean length of nanofibers on the microstructureincreases, the air trapped in the hierarchical structures increases.On the contrary, the actual contact area between the replicatedsurface and water droplet is reduced. Water droplets placed onthese hierarchical structures cannot penetrate into the surface. Thehierarchical structure with a nanofiber mean length of 350 nmshows a low value of contact angle hysteresis of 3�. Bharat et al.measured static contact angle of 164� and low contact anglehysteresis of 3� on the lotus leaf [23]. Both static contact angle andcontact angle hysteresis for our hierarchical surface are very similarto those of the Lotus leaf.

The apparent advancing CA of the nanofiber structures (theanodizing time was 4 min) was measured for 3 months using 3-mlwater droplets, to investigate the effects of aging in air and water.Unlike surfaces derived from chemical deposition [29], the nano-fiber structures remained unaffected by aging in air and water, asshown in Fig. 5. This is due togeometrical structures that can containthe trapped air which reduces the actual contact area between thesurface and water droplet. Namely, the superhydrophobic propertyof the nanofiber structures was stable in air and water.

4. Conclusion

In conclusion, we have successfully fabricated micro-nanohierarchical structures with uniform superhydrophobicity, similarto those on the lotus leaf, by blasting an Al foil with sodiumbicarbonate and anodizing the dimpled Al foil. Sodium bicarbonateis a low-cost material, which is soluble in water. Therefore, theblasted particles can be easily removed from the Al surface bythe addition of oxalic acid to the anodizing solution, and micro-particles do not need to be removed in a separate process. Thisfabrication procedure is simple and cost-effective as compared toconventional methods. Furthermore, the fabricated hierarchicalstructures have high CAs, and the relatively low CA hysteresisdecreases as nanofibers become longer. It is therefore possible todesign more robust nonwetting surfaces. These artificial lotus-leaf-like structures are suitable for diverse applications includingmicrofluidic devices for biological studies and industrial self-cleaning products for automobiles, ships and houses

Acknowledgements

This work was supported by the National Research Foundationof Korea (NRF) grant funded by the Korea government (MEST) (No.2010-0018457) and performed for the HydrogenEnergy R&DCenter, one of the 21st Century Frontier R&D Program, funded bythe Ministry of Education, Science and Technology of Korea. Thisresearch was supported by the LG Yonam Foundation, Korea.

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