Fabrication of Hierarchically Micro- and Nano-structured Mold Surfaces

Using Laser Ablation for Mass Production of Superhydrophobic Surfaces

Jiwhan Noh1;2, Jae-Hoon Lee1, Suckjoo Na2, Hyuneui Lim1, and Dae-Hwan Jung1�

1Korea Institute of Machinery and Materials (KIMM), 104 Sinseongno, Yuseong-gu, Daejeon 305-343, Republic of Korea2Korea Advanced Institute of Science and Technology (KAIST), 371-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea

Received April 6, 2010; accepted July 13, 2010; published online October 20, 2010

Many studies have examined the formation of surfaces with mixed patterns of micro- and nano-sized lotus leaves that have hydrophobic

properties. In this study, micro- and nano-shapes such as lotus leaves were fabricated on a metal mold surface using laser ablation and ripple

formation. A microstructure on the mold surface was replicated onto poly(dimethylsiloxane) (PDMS) using the polymer casting method to

manufacture low-cost hydrophobic surfaces. A PDMS surface with micro- and nano-structures that were the inverse image of a lotus leaf showed

hydrophobic characteristics (water contact angle: 157�). From these results, we deduced that portions of the microstructures were wet and that air

gaps existed between the microstructures and the water drops. In this paper we suggest the possibility of the mass production of hydrophobic

plastic surfaces and the development of a methodology for the hydrophobic texturing of various polymer surfaces, using the polymer casting

method with laser-processed molds. # 2010 The Japan Society of Applied Physics

DOI: 10.1143/JJAP.49.106502

1. Introduction

With the continued development of microprocessing tech-nology, various surfaces that mimic natural characteristicsare being realized. In particular, many researchers are tryingto reproduce lotus leaf patterns, which have excellentsuperhydrophobic characteristics, i.e., a 150� or larger watercontact angle with a low surface hysteresis. On super-hydrophobic surfaces, water drops roll at a small slidingangle. Owing to this particular feature, lotus leaves have aself-cleaning effect.1)

Many previous studies focused on the formation ofsuperhydrophobic surfaces using chemical processes. Re-cently, however, many researchers have attempted to realizesuch surfaces by forming micro- and nano-structuresinspired by lotus leaves in accordance with the ongoingdevelopment of micro-nano-processing technologies.2,3)

Studies of the formation of micro- and nano-structures withsilica particles in a carbon nanotube (CNT) have beenundertaken.4) Some studies have used a perfluorinatedpolymer monolayer on porous silica as a hydrophobicsurfaces.5) It has been shown that plasma surface modifica-tion can change the hydrophobic characteristics.6,7) Thesemethods improved superhydrophobic properties by formingirregular micro- and nano-patterns on surfaces. However, itis difficult to control hydrophobic characteristics by formingthese irregular patterns.

In order to resolve the above-mentioned demerits, studiesare being conducted to create reproductive superhydropho-bic surfaces by forming regular microstructures throughphotolithography.8,9) However, photolithography is compli-cated by the limitations of available materials and theenvironmental problems related to wet processing. Further-more, three-dimensional curved surfaces with micropatternscannot be formed by photolithography.

Laser ablation process has been used as a simplealternative for forming lotus-leaf-shaped micro patterns. Itcan be implemented with only one piece of equipment in asingle, environmentally-friendly dry process. In addition, itcan be used for the patterning on three-dimensional curvedsurfaces using a dynamic focusing unit. Its main advantage

is that its process materials can be selected more freelythan those in the case of other processes. However, thedisadvantage of laser ablation is that it produces a larger linewidth than photolithography.

Another problem with the use of superhydrophobicsurfaces is cost. Conventional methods including photo-lithography are costly. However if mold surfaces withsuperhydrophobic patterns can be formed, the resultingsuperhydrophobic surfaces can be used to manufactureproducts inexpensively by injection molding, hot embossing,nanoimprinting lithography, or polymer casting. Lee andKwon10) conducted a study of the low-cost manufacturing ofsuperhydrophobic surfaces using a mold that replicated thepatterns of natural lotus leaves. They made a nickel moldof a lotus leaf by nickel electroforming and applied UV-nanoimprinting lithography. However, this method posestwo problems: 1) it is difficult to transcribe nanopatternsfrom soft lotus leaves to a nickel mold using nickelelectroforming; and 2) the nickel mold itself is less suitablefor the mass production of superhydrophobic surfaces thanmolds made of other metals.

In this study, laser ablation was used to form lotus leafmicrostructures on a mold surface. Micro- and nano-hierarchical structures of lotus leaf patterns were formedon a NAK80 mold material using laser ablation. With thefabricated mold, micro- and nano-structures transferred veryeasily to the poly(dimethylsiloxane) (PDMS) surface. Andthe superhydrophobicity of the microstructured PDMSsurface was elucidated with the wetting theory.

2. Theoretical Basis

2.1 Wetting theory

In a simple case, the wetting phenomenon of the idealflat surface (i.e., one that is chemically homogeneous) iscommonly described by the contact angle �y given byYoung’s equation as the following equation:11)

cos �y ¼�sv � � ls

� lv

; ð1Þ

where �sv, � ls, and � lv are the surface tension or surfaceenergies of the solid/vapor, liquid/solid, and liquid/vaporphase interface, respectively. This equation can only beapplied to flat, chemically homogeneous surfaces. However�E-mail address: dhjung@kimm.re.kr

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if only such flat surfaces are used, it is very difficultto achieve a contact angle exceeding 130�. Therefore, thisstudy was conducted to develop a hydrophobic surface witha contact angle of over 150�, using micro- and nano-patternsshaped like lotus petals.

The first approach to the hydrophobic phenomenon onrough surfaces was performed by Wenzel,12) who assumedthat liquid fills the grooves of a rough surface. In Wenzel’smodel, the apparent contact angle �w of the rough surfacecan be evaluated by considering a small displacement of thecontact line parallel to the surface. Then the surface energyper unit contact line is modified by the quantity dF, which isgiven as

dF ¼ rð�sl � �svÞ dxþ � lv dx cos �w; ð2Þ

where r is the roughness factor, defined as the ratio of theactual area of a rough surface to the geometrically projectedarea. The equilibrium is given by the minimum F. dF ¼ 0 isthe result of the following equation:

cos �w ¼ r cos �y ¼ r�sv � � ls

� lv

: ð3Þ

Now assume that the contact angle is more than 90� on asmooth surface. If a micro- or nano-pattern is fabricated onthat smooth surface, the surface’s hydrophobic characteristicwill be enhanced. The same analogy applies to hydrophilicsurfaces. For example, there is a surface whose contact angleis less than 90� on a smooth surface. If a micro- or nano-pattern is fabricated on that smooth surface, its hydrophiliccharacteristic will be enhanced.

The other approach to the wetting phenomenon for arough surface involves the theory proposed by Cassie andBaxter.13) They assume that water forms a composite surfaceon a rough surface, that is, there is air between the water andthe rough surface. In this case, the liquid–surface interfaceis actually an interface consisting of two phases, namely, aliquid–solid interface and a liquid–vapor interface. When thearea fraction f1 with a contact angle �1 and the area fractionf2 with a contact angle �2 can be expressed using Young’sequation, the resulting contact angle �c of the rough surfacecan be derived using

cos �c ¼ f1 cos �1 þ f2 cos �2: ð4Þ

If a water drop is on a chemically homogeneous andrough surface ( f2 ¼ 1� f1, �2 ¼ 0), the eq. (4) can bederived as

cos �c ¼ f1 cos �1 þ ð1� f1Þ: ð5Þ

The main difference between Wenzel’s theory and Cassieand Baxter’s theory is terms of the air gap between themicro-nano-structure and the water drop. Wenzel assumesthat water covers the entire the micro and nano surfaces;conversely, Cassie and Baxter assume that an air gap existsbetween the micro-nano-structure and the water drop. Theother difference between the two theories is that there is amarked change in the contact angle for a rough surface when�y (the contact angle of the smooth surface) is about 90�. IfWenzel’s equation is used, the difference in the contactangle between a smooth surface and a rough surface is small,while if Cassie and Baxter’s equation is used, that differenceis large.

2.2 Laser ablation processing and ripple formation

Laser ablation processing is a form of elimination processingby which a laser beam interacts with a material. In the caseof metal, when a laser beam irradiates the metal surface, heattransfer occurs between the laser and metal surface withlaser fluence below 105 W/cm2. This heat transfer mecha-nism is used as a means of surface hardening for heattreatment. When the laser fluence ranges from 105 to 106

W/cm2, melting processing occurs on the metal surface.This melting process can be used as a form of weldingprocessing. When the laser fluence ranges from 106 to109 W/cm2, the temperature of the metal surface exceeds themelting temperature and reaches the metal’s boiling point.In this state, metal particles evaporate, resulting in theirelimination. This process, known as laser ablation, is usedfor laser surface structuring, cutting, or drilling. If the laserfluence exceeds 109 W/cm2, plasma is generated because theevaporating particles absorb the input laser energy. Amongthe several laser application methods currently available,laser ablation was used in this study to fabricate micro- andnano-structures that mimic the lotus leaf.

Recently, ultrashort lasers have been developed. Theultrashort laser is a laser with a pulse duration of severalfemtoseconds or picoseconds that can be used to minimizethe heat-affected zone. When a nanosecond laser is used forlaser ablation processing, it creates a larger heat-affectedzone. If this zone could be reduced, it would indicate that asmaller structure could be fabricated. Many researchers use afemtosecond laser instead of a picosecond laser in orderto minimize the heat-affected zone, since this zone can bereduced by its shorter pulse duration.14–16) However, thefemtosecond laser has certain disadvantages, such as its lowaverage power, low repetition rate, tendency to cause airbreakdowns, and high price. As such, it is difficult to use thefemtosecond laser in industrial applications. The picosecondlaser can overcome such disadvantages, although it stillgenerates a larger heat affection zone than the femtosecondlaser. Therefore, a picosecond laser was used for laserablation processing, and the laser processing parameterswere carefully controlled in order to minimized the heataffection zone in this paper.

The other process for fabricating nanostructures shaped likelotus leaves is ‘‘ripple formation’’ or ‘‘laser-induced periodicsurface structure’’ (LIPSS). The ripple formation process is alaser processing method that can produce a nanostructure thatis smaller than the focused spot size. When the laser fluenceis slightly above the ablation threshold fluence of the surface,a nanoripple is created. The mechanism of ripple formationhas been explained by the interference between an inputlaser beam and a laser beam reflected from materials. If thistheory is applied, the following equation can be derived:

d ¼�

nð1þ sin �Þ; ð6Þ

where d is the ripple line distance, � is the wavelength of theinput laser beam, n is the refractive index (in the case of air,n ¼ 1), and � is the incident angle of the input laser beam.17–19)

3. Experimental Procedure

The picosecond laser used in the experiments was a diode-pumped, mode-locked Nd:YVO4 laser with a pulse width of

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106502-2 # 2010 The Japan Society of Applied Physics

12 ps. The maximum repetition rate was 640 kHz and thefundamental wavelength was 1064 nm. The laser wasequipped with second- and third-harmonic generators tomake laser wavelengths of 532 and 355 nm. In the experi-ments, a laser wavelength of 355 nm was used in theultraviolet range because this allowed higher energy ab-sorption into metal. Also, an ultraviolet laser beam couldbe focused onto the smaller beam spot. A half-wave plateand a polarizer were used to control the laser power. Insteadof a mechanical shutter, an external TTL signal served as ashutter for the laser pulses. This prevented extra irradiationfrom the laser pulses onto the specimen caused by time delayof the mechanical shutter at every end point of the laserbeam’s path. The horizontally polarized beam was focusedby a telecentric scan lens having a maximum scan area of64� 64 mm2 (Sill Optics S4LFT4100/075). The beam wasmanipulated over the sample by a two-mirror galvoscanner(SCANLAB HurrySCAN 14). The effective focal length ofthe telecentric lens was 105.9 mm. The focused, optical spotsize (1=e2 of maximum intensity value) was about 35 mm.In this paper, a conical spike with a size of 10 mm wasfabricated using a 35 mm laser focused beam. If the shorterfocal length of the telecentric lens were used, it couldbe expected that a spike smaller than 10 mm would befabricated. The fluence was determined by measuring thepower at the exit of the telecentric lens and then dividing itby the repetition rate and the irradiated surface area. Thesevalues were averages, as the energy distribution of the pulseswas Gaussian using the TEM00 mode.

The processing material was NAK80 mold steel with auniform hardness of approximately 40 HRc throughout: itnever required stress relieving, even after heavy machining.It also had a uniform grain structure without pinholes,inclusion or hard spots. Table I shows the composition ofNAK80.

After the micro- and nano-structures on the mold surfacehave been fabricated, they should be replicated on thePDMS surface. First, 50 g of a silicone elastomer base (DowCorning Sylgard 184) was mixed into 5 g of a siliconeelastomer curing agent (Dow Corning Sylgard 184). Amicromixer was used to mix the two chemicals and removeair bubbles from the mixture. The mixture was poured into afabricated mold and a solidification process was carried outfor 5 h. The PDMS was then separated from the mold.

4. Results and Discussion

4.1 Fabrication of lotus-leaf-like micro and nano

structures on the mold surface

Figures 1(a) and 1(b) show the micro- and nano-structures ofa lotus leaf. The lotus leaf’s micro structure has a two-scalestructure, which refers to nanopatterns on micro-conicalstructures with a diameter of 10 mm. It is well known thatnanopatterns improve superhydrophobic characteristics.Figures 1(c) and 1(d) show the fabricated microstructurethat mimics the lotus leaf. The conical structure’s diameterwas 10 mm and there were 200 nm patterns on top of theconical structure. This is the artificial micro structure that wethink best mimics the lotus leaf. However, the fabricationtime for the artificial microstructure was too long to make aninexpensive hydrophobic surface. Therefore, such micro-and nano-structures on a mold surface were replicated on

PDMS. Even though the micro- and nano-structures ofthe PDMS surface were the inverse of the micro-nano-structures of a lotus leaf, it was observed that the watercontact angle of the inverse structure was 157�, which meansthat the inverse structure was still hydrophobic.

The laser fluence shown in Figs. 1(c) and 1(d) was25.98 mJ/cm2. The laser scanning was repeated 30 times andthe scanning speed was 0.1 m/s. The focused optical spotsize (1=e2 of the maximum intensity) was about 35 mm. Thisoptical spot size was calculated using

do ¼4

��f

dM2: ð7Þ

where do is the focused optical spot diameter, � is the laserwavelength, f is the focal length of the focusing lens, d isthe input beam diameter of the focusing lens, and M2 is thelaser propagation factor. With a focused laser beam whosediameter was 35 mm, a conical structure was fabricatedthat had a diameter of 10 mm. This 10 mm diameter micro-structure could be fabricated when controlling the laserfluence according to the ablation threshold of the materialand the multireflection phenomenon of the laser beam. Thefabrication line width was decided by the ablation thresholdof the material and the laser fluence. If the laser beam profileis the Gaussian intensity distribution, the decrease in laserfluence over the ablation threshold indicates that the width ofthe fabrication line on the material has been reduced. In thisway, it was possible to fabricate a width smaller than that ofthe optical focused diameter. This method could be usedwith an ultrashort laser (femtosecond laser or picosecondlaser). When we used this method with a nanosecond laser,

Table I. Composition of NAK80.

Al C Cu Fe Mn Ni Si

Quantity

(%)1 0.15 1 93.05 1.5 3 3

Fig. 1. (a, b) SEM images of natural lotus leaf; (d, e) SEM images of

fabricated micro- and nano-structures by laser ablation process and ripple

formation. (Laser fluence: 25.89 mJ/cm2, laser scanning repeating time:

30, scanning speed: 0.1 m/s.)

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the heat affection zone was so large that the width of thefabrication line was not reduced.

The other method used in fabricating the mold pattern wasbased on the multireflection phenomenon of a laser beam.When the laser was focused on the material, the V-shapedregion of the groove was fabricated because the intensityprofile of the laser was Gaussian. After the small V-shape ofthe groove was fabricated, the incident angle of the laserbeam on the V-shape was large. Therefore, a reflection of theincident laser occurred on the V-shaped surface. A largeamount of laser power was reflected on the V-shape andirradiated into the valley of the V-shape. The laser powerthat entered the valley ablated the material in the valley ofthe V-shape.

Superhydrophobic molds fabricated with different laserfluence are shown in Fig. 2. Figures 2(a) and 2(b) show thefabrication result, which has a random nanopatterns on theconical structure. One important advantage of laser ablationprocessing is that it allows one to change the roughnessof the fabrication area by changing the laser processingparameters. The most significant parameter of the roughnessis laser fluence. The laser fluence in Figs. 2(a) and 2(b) was12.99 mJ/cm2, the laser scanning was repeated 50 times, andthe scanning speed was 0.1 m/s. The laser fluence ofFigs. 2(c) and 2(d) was 25.98 mJ/cm2, the laser scanningwas repeated 10 times, and the scanning speed was 0.1 m/s.

The laser fluence of Figs. 2(e) and 2(f) was 25.98 mJ/cm2,the laser scanning was repeated 50 times, and the scanningspeed was 0.1 m/s. The difference between the parametersof Figs. 2(e) and 2(f) and the other result is the scan lineinterval. The line interval in Figs. 2(e) and 2(f) was 20 mm;the other was 10 mm.

If we compare Fig. 1(c) with Fig. 2(a), we find that thereare different nanostructures. Figure 1(c) has a 200 nm linepattern, while Fig. 2(a) shows a chaotic structure. InFig. 2(a), the valley of the conical structure has a linepattern while the center of the conical structure has a randomstructure. The lateral face of Fig. 2(f) has about 1 mmpatterns, which resemble stairs.

In Fig. 1(c), � was 355 nm, n was 1, and, when it wasassumed that � is 45�, then d was calculated to be 500 nm,using eq. (6). This calculation result did not coincide withthe 200 nm line pattern, which was the fabrication result.Many other researchers have shown an experimental rippleformation result that does not coincide with this interferencetheory of ripple formation. The other approach to elucidatethe mechanism of ripple formation is ‘‘self-organized patternformation from instability’’.20) This mechanism is based onthe assumption that incident ions can ionize the surfaceatoms of the target. The resulting surface charge leads tosurface layer erosion due to Coulomb repulsion. A surfacediffusion and stability analysis showed that a plane surfacecan be unstable and a new spatially periodic structure canbe fabricated. However, it is true that a theory which canexplain all the experimental results for ripple formation hasnot yet been established.

The differences in ripple structure show that ripplestructure can be controlled by changing the laser parameters,even though the mechanism of ripple formation has not yetbeen established. In Figs. 2(a) and 2(b), the line pattern ofthe valley could be seen as result of ‘‘ripple formation’’,while the chaotic structure of the center was not fabricatedby ‘‘ripple formation’’. The power was so low that a ripplewas not fabricated at the center of the conical structure. Inthis paper, we assumed that the particles created by theablation were piled up on the center of the conical structureinstead of those created by the ripple formation.

Ripples (nanostructures) can improve the hydrophobiccharacteristics of a surface. In the case of photolithography,the processing of a microstructure and the processing of ananostructure are separated, entailing a long processing timeand a high processing cost. However, in the case of laserablation processing, the microstructure and nanostructure arefabricated at the same time. Because laser ablation is a typeof explosion process, the fabricated surface displays higherroughness than during photolithography processing. Thisdisadvantage, i.e., high roughness, of the laser ablationprocess becomes an advantage in hydrophobic applicationsbecause the nanostructure on the microstructure improvesthe hydrophobic characteristic.

4.2 Polymer casting and hydrophobic characteristics

Figure 3 shows the replicated results. Figures 3(a) and 3(b)show the replicated results in Figs. 1(c) and 1(d); Figs. 3(c)and 3(d) show the replicated results of Figs. 2(a) and 2(b);Figs. 3(e) and 3(f) show the replicated results of Figs. 2(c)and 2(d); and Figs. 3(g) and 3(h) show the replicated results

Fig. 2. SEM images of a mold surface fabricated by laser ablation

processing: (a, b) laser fluence: 12.99 mJ/cm2, laser scanning repeating

time: 50, scanning speed: 0.1 m/s, (c, d) laser fluence: 25.89 mJ/cm2,

laser scanning repeating time: 10, scanning speed: 0.1 m/s, (e, f) laser

fluence: 25.89 mJ/cm2, laser scanning repeating time: 50, scanning

speed: 0.1 m/s (line pitch: 20 um). Each image in the right-hand column is

a magnified image of the one to its left.

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of Figs. 2(e) and 2(f). Figures 3(b), 3(d), and 3(f) show the200 nm replication pattern on the PDMS surface. The topof the spike on the PDMS has a 200 nm pattern whichis replicated from the 200 nm pattern of the mold made by‘‘ripple formation’’. These 200 nm patterns improve thehydrophobic characteristics. Figure 4 shows the results ofthe contact angle experiment. Figure 4(a) shows the contactangles on the smooth PDMS surface. Figures 4(b)–4(e)show the contact angles on the surfaces of Figs. 3(a), 3(c),3(e), and 3(g), respectively. The contact angles shown inFigs. 3(a)–3(e) are 91, 157, 151, 110, and 149�, respectively.These contact angles were measured without any surfacechemical treatment. This means that such a treatment can beomitted when choosing the proper material for the hydro-phobic surface. Of course, if a chemical treatment wereadded, the hydrophobic characteristics would be enhanced.The use (or non-use) of chemical treatment can be decideddepending to the application.

The contact angle of PDMS microstructures is applied tothe Wenzel equation and Cassie equation. If a micropatternis made on a smooth surface of the PDMS, the contact anglewill increase according to Wenzel’s equation. In Wenzel’sequation, r is the ratio of an actual area of a rough surfaceto a geometric projected area. The confocal microscopemeasurement in Fig. 3(a) shows the height of the spike as5 mm and the interval of the spike as 10 mm. If it is assumedthat the shape of the spike is a quadrangular pyramid,Figs. 3(a) and 3(c) can be simplified to Fig. 5. The triangulararea in Fig. 5 is 35.35 mm2 (1=2� 10 mm� 7:07 mm). Theactual area of the rough surface is 141.4 mm2 because thequadrangular pyramid consists of four triangles. The areaof the projected area is 100 mm2, when r is 1.414 and �w iscalculated as 91.41�, according to Wenzel’s equation. Thiscalculated value (91.41�) is different from the experimentalvalue (157�). Therefore, Wenzel’s model cannot be appliedto this experimental result.

It can also be inferred that air exists between the water andthe PDMS microstructure. In order to apply this experimen-tal result to Cassie and Baxter’s equation f should becalculated. f is the fraction of the solid surface upon whichthe water drop sits, and 1� f is the fraction below the drop.In the case of a conical spike structure with a super-hydrophobic surface, it is difficult to measure f . In thisstudy, f is calculated from the experimental results. InFigs. 4(b) and 4(a), �c is 157� and � is 91�. f is calculated as0.081, according to Cassie and Baxter’s equation.

In addition, it can be estimated that 8.1% of the triangulararea of the quadrangular pyramid is wetted by the waterdrop; thus, 91.9% of the triangular area of the quadrangularpyramid is not wetted by the water drop. In Fig. 5, it can beinferred that w (the width of the wetting triangle) is 2.39 mm

Fig. 3. SEM images of the replicated PDMS surface: (a) replica from

Fig. 1(b); (c) replica from Fig. 2(a); (e) replica from Fig. 2(c); and (g)

replica from Fig. 2(e). Each image in the right hand column is a magnified

image of the one to its left.

Fig. 4. (Color online) Photograph of contact angle. 10 ml water drops

on: (a) a flat PDMS surface, (b) Fig. 3(a) surface, (c) Fig. 3(c) surface,

(d) Fig. 3(e) surface, and (e) Fig. 3(g) surface, The contact angles

(� ¼ average� standard deviation) are: (a) 91� � 1:5�, (b) 157� � 3�,

(c) 151� � 3:5�, (d) 110� � 2:5�, and (e) 149� � 3�.

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and h (the height of the wetting triangle) is 1.69 mm. In thesame way, f in Fig. 4(c) is 0.127.

It was estimated that 12.7% of the quadrangular pyramid’striangle area was wetted by the water drop, and that 87.3%of the quadrangular pyramid’s triangle area was not wettedby the water drop. In Fig. 5, it can be inferred that w (thewidth of the wetting triangle) is 2.99 mm and that h (theheight of the wetting triangle) is 2.11 mm.

Figure 4(d) shows that the contact angle is relativelysmall, because the depth of ablation is small. This resultshows that the contact angle can be controlled by changingthe depth of ablation, which in turn can be controlled by thelaser scan number.

Figures 3(g) and 4(e) show that a 20 mm interval structurecan enhance the hydrophobic characteristics. Of course, thecontact angle of a 10 mm interval structure is higher than thatof a 20 mm interval structure. However, the structure ofFig. 3(g) has an advantage in terms of external force. InFigs. 3(a) and 3(c), the pin-type is easily destroyed byexternal force. The depressed wall structure of Fig. 3(g) ismore resistant than the pin-type structure to external force.The ability of the hydrophobic surface to resist externalforce is very important because hydrophobic surfaces can beused in harsh environments. Therefore, a structure such asthat shown in Fig. 3(g) is necessary for applications wherestrong resistance to external force is required.

5. Conclusions

In this study, micro- and nano-patterns shaped like to lotusleaves were fabricated on a metal mold surface directlyusing the laser ablation process. Nanostructures (200 nm)were fabricated on a micro structure (10 mm) using a rippleformation process during laser ablation. The hydrophobicmicro structure of the mold was replicated onto PDMS by a

polymer casting method. The contact angle of the water onthe replicated PDMS surface was 157�, which could providesuperhydrophobic characteristics. It was deduced that thehydrophobic surface prepared in this study was in agreementwith Cassie and Baxter’s theory rather than with Wenzel’stheory. The wetting area and non-wetting area of the samplewere 8.1 and 91.9%, respectively. These results are verysimilar to those of a natural lotus leaf. This fabricationmethod for superhydrophobic surfaces advances the possi-bility of mass production of various polymers for hydro-phobic surfaces. Products with a superhydrophobic surfacecould be used for various applications such as gene delivery,wetting liquid transfer,21) non-micro fluidic channels,22) andplastic containers that can be washed without detergent.

Acknowledgments

This work was supported in part by the Korean Ministry ofKnowledge Economy as part of the ‘‘Technology Develop-ment of Plastic Injection Molding for SuperhydrophobicSurface by Nano-on-Micro Patterns’’ project.

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Fig. 5. (Color online) Schematic drawings of images shown in

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Jpn. J. Appl. Phys. 49 (2010) 106502 J. Noh et al.

106502-6 # 2010 The Japan Society of Applied Physics