Fabrication of Hierarchically Micro- and Nano-structured Mold Surfaces Using Laser Ablation for Mass Production of Superhydrophobic Surfaces

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<ul><li><p>Fabrication of Hierarchically Micro- and Nano-structured Mold Surfaces</p><p>Using Laser Ablation for Mass Production of Superhydrophobic Surfaces</p><p>Jiwhan Noh1;2, Jae-Hoon Lee1, Suckjoo Na2, Hyuneui Lim1, and Dae-Hwan Jung1</p><p>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</p><p>Received April 6, 2010; accepted July 13, 2010; published online October 20, 2010</p><p>Many studies have examined the formation of surfaces with mixed patterns of micro- and nano-sized lotus leaves that have hydrophobic</p><p>properties. In this study, micro- and nano-shapes such as lotus leaves were fabricated on a metal mold surface using laser ablation and ripple</p><p>formation. A microstructure on the mold surface was replicated onto poly(dimethylsiloxane) (PDMS) using the polymer casting method to</p><p>manufacture low-cost hydrophobic surfaces. A PDMS surface with micro- and nano-structures that were the inverse image of a lotus leaf showed</p><p>hydrophobic characteristics (water contact angle: 157). From these results, we deduced that portions of the microstructures were wet and that airgaps existed between the microstructures and the water drops. In this paper we suggest the possibility of the mass production of hydrophobic</p><p>plastic surfaces and the development of a methodology for the hydrophobic texturing of various polymer surfaces, using the polymer casting</p><p>method with laser-processed molds. # 2010 The Japan Society of Applied Physics</p><p>DOI: 10.1143/JJAP.49.106502</p><p>1. Introduction</p><p>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 eect.1)</p><p>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)</p><p>Studies of the formation of micro- and nano-structures withsilica particles in a carbon nanotube (CNT) have beenundertaken.4) Some studies have used a peruorinatedpolymer monolayer on porous silica as a hydrophobicsurfaces.5) It has been shown that plasma surface modica-tion can change the hydrophobic characteristics.6,7) Thesemethods improved superhydrophobic properties by formingirregular micro- and nano-patterns on surfaces. However, itis dicult to control hydrophobic characteristics by formingthese irregular patterns.In order to resolve the above-mentioned demerits, studies</p><p>are 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 simple</p><p>alternative 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</p><p>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 superhydrophobic</p><p>surfaces 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 dicult 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 leaf</p><p>microstructures 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.</p><p>2. Theoretical Basis</p><p>2.1 Wetting theory</p><p>In a simple case, the wetting phenomenon of the idealat surface (i.e., one that is chemically homogeneous) iscommonly described by the contact angle y given byYoungs equation as the following equation:11)</p><p>cos y sv ls</p><p> lv; 1</p><p>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 at, chemically homogeneous surfaces. HoweverE-mail address: dhjung@kimm.re.kr</p><p>Japanese Journal of Applied Physics 49 (2010) 106502 REGULAR PAPER</p><p>106502-1 # 2010 The Japan Society of Applied Physics</p></li><li><p>if only such at surfaces are used, it is very dicultto 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 rst approach to the hydrophobic phenomenon on</p><p>rough surfaces was performed by Wenzel,12) who assumedthat liquid lls the grooves of a rough surface. In Wenzelsmodel, 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 modied by the quantity dF, which isgiven as</p><p>dF rsl sv dx lv dx cos w; 2where r is the roughness factor, dened as the ratio of theactual area of a rough surface to the geometrically projectedarea. The equilibrium is given by the minimum F. dF 0 isthe result of the following equation:</p><p>cos w r cos y rsv ls</p><p> lv: 3</p><p>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 surfaces 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 a</p><p>rough 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 liquidsurface interfaceis actually an interface consisting of two phases, namely, aliquidsolid interface and a liquidvapor interface. When thearea fraction f1 with a contact angle 1 and the area fractionf2 with a contact angle 2 can be expressed using Youngsequation, the resulting contact angle c of the rough surfacecan be derived using</p><p>cos c f1 cos 1 f2 cos 2: 4If a water drop is on a chemically homogeneous and</p><p>rough surface ( f2 1 f1, 2 0), the eq. (4) can bederived as</p><p>cos c f1 cos 1 1 f1: 5The main dierence between Wenzels theory and Cassie</p><p>and Baxters 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 dierence between the two theories is that there is amarked change in the contact angle for a rough surface wheny (the contact angle of the smooth surface) is about 90</p><p>. IfWenzels equation is used, the dierence in the contactangle between a smooth surface and a rough surface is small,while if Cassie and Baxters equation is used, that dierenceis large.</p><p>2.2 Laser ablation processing and ripple formation</p><p>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 uence below 105W/cm2. This heat transfer mecha-nism is used as a means of surface hardening for heattreatment. When the laser uence ranges from 105 to 106</p><p>W/cm2, melting processing occurs on the metal surface.This melting process can be used as a form of weldingprocessing. When the laser uence ranges from 106 to109W/cm2, the temperature of the metal surface exceeds themelting temperature and reaches the metals 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 laseruence exceeds 109W/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. The</p><p>ultrashort laser is a laser with a pulse duration of severalfemtoseconds or picoseconds that can be used to minimizethe heat-aected zone. When a nanosecond laser is used forlaser ablation processing, it creates a larger heat-aectedzone. 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-aected zone, since this zone can bereduced by its shorter pulse duration.1416) 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 dicult to use thefemtosecond laser in industrial applications. The picosecondlaser can overcome such disadvantages, although it stillgenerates a larger heat aection 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 heataection zone in this paper.The other process for fabricating nanostructures shaped like</p><p>lotus 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 uenceis slightly above the ablation threshold uence 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 reected from materials. If thistheory is applied, the following equation can be derived:</p><p>d n1 sin ; 6</p><p>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 1), and is the incident angle of the input laser beam.1719)</p><p>3. Experimental Procedure</p><p>The picosecond laser used in the experiments was a diode-pumped, mode-locked Nd:YVO4 laser with a pulse width of</p><p>Jpn. J. Appl. Phys. 49 (2010) 106502 J. Noh et al.</p><p>106502-2 # 2010 The Japan Society of Applied Physics</p></li><li><p>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 laserbeams path. The horizontally polarized beam was focusedby a telecentric scan lens having a maximum scan area of64 64mm2 (Sill Optics S4LFT4100/075). The beam wasmanipulated over the sample by a two-mirror galvoscanner(SCANLAB HurrySCAN 14). The eective focal length ofthe telecentric lens was 105.9mm. 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 uence 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 a</p><p>uniform 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 surface</p><p>have 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 solidication process was carried outfor 5 h. The PDMS was then separated from the mold.</p><p>4. Results and Discussion</p><p>4.1 Fabrication of lotus-leaf-like micro and nano</p><p>structures on the mold surface</p><p>Figures 1(a) and 1(b) show the micro- and nano-structures ofa lotus leaf. The lotus leafs 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 le...</p></li></ul>


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