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ricated biomimetic surfaces were characterized with scanning electron microscopy (SEM), water contact

cal micro/nano-structures. It can exhibit excellent superhydrophobicity that the contact angle is as high

ples oated toed intic, whicdent te

and antifouling properties [68]. Micron-sized grooved scalesgrowing on shark skin, which are called dermal denticles, are inter-locked to form a natural non-smooth surface; and the grooves be-tween adjacent riblets on the scales are directed almost parallel tothe longitudinal body axis of the shark. It has been reported thatthe grooved scales can reduce vortice formation or lift the vorticeoff the surface, so resulting in water moving easily over the skin

microscale and nanoscale hierarchical structures, providing super-hydrophobicity, self cleaning, low adhesion and drag reduction[1719]. Model proposed to interpret superhydrophobic phenom-ena was published by Cassie and Baxter [20], as well as Wenzel[21,22]. In the past decades, designing articial superhydrophobicsurfaces has become one of the top issues due to their potentialapplications in different realms, and numerous techniques havebeen developed to mimic lotus effect, including electrospinning[23], plasma treatment [24,25], chemical vapor deposition [26],molding [27] and phase separation [28,29]. However, there is still

Corresponding author. Fax: +86 20 87113949.

Journal of Colloid and Interface Science 388 (2012) 235242

Contents lists available at

n

r .coE-mail addresses: liuyunhong_9@163.com (Y. Liu), gjli@scut.edu.cn (G. Li).ready popular in the eld of materials science and engineering.There are many examples of biomimetic design originated fromthe investigation and copy of the special properties and mecha-nisms of natural plants and animals [3,4].

Such an example is the shark skin effect, which is dened as amechanism of wall friction reduction of a uid resulted from a rib-let structured surface similar to that of shark skin [5]. Shark skinhas been widely studied for decades due to its drag reduction

the overlapping, ridged platelet structures of shark scales. It candisplay excellent microbe resistant properties, which is veryencouraging results to date [14,15].

Another well-known example is to design and fabricate biomi-metic surface possessing Lotus Effect, which is dened as theself-cleaning properties (phenomenon) and highly superhydropho-bic surface like a lotus leaf [16]. It has been reported that the sur-face of a lotus leaf is covered with wax and has an intrinsic1. Introduction

Nature provides abundant examand surfaces which can be investigprinciple and subsequently developapplications [1]. The term biomimetnature as an impulse for an indepen0021-9797/$ - see front matter 2012 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.jcis.2012.08.033as 160 and maintain its robustness of the superhydrophobicity during the droplet impact process at arelatively high Weber number. The mechanism of the micromorphology evolution and microstructuralchanges on the biomimetic shark-skin surface was also discussed here in the process of ame treatment.This method is expected to be developed into a novel and feasible biomimetic surface manufacturingtechnique.

2012 Elsevier Inc. All rights reserved.

f structures, materialsunderstand the basic

o fascinating technicalh means learning fromchnical design [2] is al-

surface [911]. Besides, the rough texture formed by dermal denti-cles can reduce the adhesion area available to aquatic organismsand keep the surface clean. It is exciting that the principle has beenadapted to aeroplane surfaces and achieved fuel-saving by about1.5% [12]. Speedo invented the full-body swimsuit called Fast-skin for elite swimming, which mimicked the shark-skin V-shaperidges [13]. Sharklet is another commercial product inspired byShark skinMicroreplicationLotus effect

angle measurements and liquid drop impact experiments. The results show that the fabricated dual-bio-mimetic surface possesses both the vivid shark-skin surface morphology and the lotus leaf-like hierarchi-A new method for producing Lotus Effe

Yunhong Liu, Guangji Li School of Materials Science and Engineering, South China University of Technology, Gua

a r t i c l e i n f o

Article history:Received 14 April 2012Accepted 14 August 2012Available online 27 August 2012

Keywords:Biomimetic surface

a b s t r a c t

Nature has long been an imthe remarkable propertiessuperhydrophobic dual-biolotus leaf-like hierarchicalmicrostructure was rst fawas treated by ame to fo

Journal of Colloid a

www.elseviell rights reserved. on a biomimetic shark skin

ou 510640, China

rtant source of inspiration for mankind to develop articial ways to mimiciological systems. In this work, a new method was explored to fabricate aetic surface comprising both the shark-skin surface morphology and the

ro/nano-structures. The biomimetic surface possessing shark-skin patternated by microreplication of shark-skin surface based on PDMS; and then ithierarchical micro/nano-structures that can produce lotus effect. The fab-

SciVerse ScienceDirect

d Interface Science

m/locate / jc is

LENOVO S-400Resaltado

understood, it can be predicted that a biomimetic multi-functionalsurface bearing the characteristics of both drag reduction and

2.2. Fabrication of the surfaces with special micro/nano-structures

2.2.1. Microreplication of shark-skin surfaceThe microstructure of the shark-skin surface can be replicated

to the surface of PDMS sheet by PDMS replica molding process.Due to its excellent processability, ease molding, wide spectrumof physical and mechanical properties, as well as excellent dimen-sional stability when curing, PDMS elastomer that can be cross-linked via the addition mechanism is chosen as the moldmaterial to create topographical patterning of micron-scale fea-tures. The specic process involves the following steps. First ofall, the treated shark skin was taken out from a refrigerator and leftat room temperature for several hours, and then the shark skin was

Interface Science 388 (2012) 235242anti-bioadhesion may be produced by combining the directionalmicroscale pattern of shark-skin surface with the nanoscalestructure observed on the lotus leaf. And such a biomimetic surfacecan be used to optimize the surface design of underwater vehiclesand uid transportation pipelines, thus enhancing efciency orreducing energy consumption; and to inhibit bioadhesion to thesurface of underwater facilities in harsh water environments.Therefore, it is necessary and signicant to develop a new methodfor achieving the particular combination of shark skin effect andlotus effect.

In this work, we developed an original and efcient method tofabricate superhydrophobic dual-biomimetic surface comprisingboth the vivid shark-skin surface morphology and the lotusleaf-like hierarchical structures. In view of the fact that poly-dimethylsiloxane (PDMS) is a malleable material for developingtopographies and its low surface energy is a key property forachieving a superhydrophobic surface state, PDMS containingnano-silica was chosen as a substrate material, the biomimeticshark-skin surface having micron-sized pattern structure was rstfabricated by microreplication of shark-skin surface; and then, itwas treated by ame to form hierarchical micro/nano-structuresthat can produce lotus effect, thereby constructing a dual-biomi-metic surface. Scanning electron microscopy (SEM) was used to ob-serve the surface morphology of the samples in the form of a PDMSsheet prepared via different routes, including microreplication,ame-treatment and microreplication followed by ame-treat-ment, respectively. Furthermore, their surface properties werecharacterized with water contact angle measurements and liquiddrop impact experiments. The results were compared and analyzedusing a at PDMS sheet with a smooth surface as a control sample.The mechanism of the micromorphology evolution and micro-structural changes on the dual-biomimetic surfaces in the processof ame treatment was also discussed within the paper.

2. Experimental

2.1. Materials

Fresh shark skin from a great white shark (Carcharodon carcha-rias), which is one of the fastest swimming sharks, was purchasedfrom a sherman. The subcutaneous fat was removed from freshshark skin rst. The shark skin was then washed several times withdeionized water, carefully attened, cut into the required shapeand dried. The treated shark skin was stored in a refrigerator beforeuse. A two-component room temperature vulcanizable liquid sili-cone rubber including precursor of PDMS and curing agent, whichwere purchased from Zhejiang Runhe Silicone New Material Co.,a long way to go for meeting the requirements for practicalapplications.

Based on the understanding of the multi-level structures ofmultifunctional biological surfaces, the future research into bioin-spired multi-functional surfaces can focus on the combination ofvarious biomimetic structures by incorporating multiple technolo-gies of forming surface topology so as to make the prepared surfaceexhibit excellent comprehensive properties close to the real bio-logical surfaces as far as possible. The articial surfaces inspiredby the shark skin or the lotus leaf have showed unique propertiesand broad application prospects. Although the relationshipbetween the nano- and microscale topographies and the surfaceproperties of real shark skin and lotus leaf has not been fully

236 Y. Liu, G. Li / Journal of Colloid andLtd., was used as received. The low viscosity precursor is composedof vinyl-polydimethylsiloxane (PDMS), which contains 25 wt.% offumed silica (300 m2/g) ller treated with Si(Me)2-O-oligomers.carefully tted on a plate as a microreplication template. The platewith shark skin pattern was made into a mold for casting as shownin Fig. 1, which was designated as Mold-A. Secondly, a mixture ofprecursor and curing agent (10:1 by weight) was poured intoMold-A and cured for 20 min at 90 C so as to transfer the surfacemicrostructures of the shark skin to the PDMS surface contactingwith the shark skin; and then the cured PDMS sheet was separatedfrom Mold-A, thus obtaining the PDMS template for microreplica-tion. The prole of the PDMS template was a counter-shape of theshark-skin pattern. Thirdly, the as-prepared PDMS template wasused to manufacture a cavity block or negative mold in the sameway as described in the rst step, which was designated asMold-B. Through the same microreplication process as the preced-ing step, the desired shark-skin replica (SSR), PDMS sheet with themicrostructure of shark-skin surface, was fabricated.

2.2.2. Preparation of hierarchical structured surfacesThe prepared shark-skin replica and the at PDMS sheet with-

out microstructure (F-PDMS) were used as substrates. The hierar-chical morphological structures on the surfaces of both sampleswere prepared via ame treatment. Alcohol lamp was used in theame treatment due to its consistent ame suitable for laboratorypreparation. The surface of a substrate was close to the outer edgeof the ame from an alcohol lamp and moved back and forth at apredetermined speed. After being treated for prescribed time, thesamples were properly preserved for surface characterization andmeasurements.

2.3. Surface characterization

2.3.1. Observation of surface morphologyThe surface morphology of the prepared PDMS sheets was ob-

served by a scanning electron microscope (S-3700N, Hitachi, Japan)and a eld emission scanning electron microscope (LEO 1530 VP,Oberkochem, Germany). The sample surfaces to be observed weregold-coated using a sputter coater beforehand.Fig. 1. Schematic diagram of the microreplication process of shark-skin surface.

LENOVO S-400Resaltado

LENOVO S-400Resaltado

2.3.2. Measurements of contact angle and sliding angleContact angle (CA) and sliding angle (SA) were measured with a

Contact Angle Goniometer (DSA 100, Krss GmbH, Germany) atambient temperature. Each sample was measured for three timesat three random locations and the average values of contact angleand sliding angle were calculated.

to obtain the information on the drop impact dynamics.

3. Results and discussion

3.1. Analyses of surface morphology

Fig. 2 illustrates the SEM images of the surfaces of the real sharkskin (Mold-A) and the PDMS sheet samples prepared by the mic-roreplication technique. Compared with the real shark skin in

Fig. 2. The SEM images of the shark-skin surface and the surfaces of PDMS sheets prepared via microreplication.

Y. Liu, G. Li / Journal of Colloid and Interface Science 388 (2012) 235242 2372.3.3. Liquid drop impact experimentIn order to evaluate the robustness of various superhydrophobic

solid surfaces, a very helpful method is to investigate the liquiddrop impact dynamics onto the surfaces via the water drop impactexperiment.

The experimental procedure generally consists in releasing awater droplet with a millimeter-level diameter from a micro-syringe at different heights to vary its impact velocity onto the so-lid surface. The falling liquid drops can be accelerated by gravity,thus hitting the solid surface at a certain impact velocity. The ver-tical impact velocity of the droplet released at the height h abovethe surface, V, can be calculated according to the followingequation:

V 2gh

p1

where g is the acceleration of gravity (g 9.8 m/s2). The physicalparameters of the water droplet are as follows: droplet radius r isapproximately equal to 1.1 mm with 6% deviation, surface tensionr is 0.0728 N/m and density q 1000 kg/m3 at 20 C. The process ofdrop impact was recorded by a high-speed camera (pco.dimaxHD, CooKe) with a recording rate of 1500 fps (frames per second)Fig. 3. The crosslinking mechanism ofFig. 2a, the SSR sample in Fig. 2c possesses the almost same surfacemicrostructure as that of the dermal denticles on shark skin.

The crosslinking mechanism of additive cure type PDMS isillustrated in Fig. 3 [30]. It involves the addition of a silicon hydride(SiH) to an unsaturated double bond in the presence of a noblemetal catalyst such as platinum. Since the Mold-B and SSR are bothmade of PDMS, a technical key for preparing the Mold-B is to adjustthe ratio of the PDMS precursor to its curing agent and to controlreaction conditions, so as to make the reactive groups (vinyl groupsor SiH bonds) on the surface of the Mold-B exhausted as far aspossible. When the Mold-B with no reactive groups on its surfaceis used to prepare SSR via the microreplication process describedabove, the addition curing reaction only occurs within the PDMSmaterial that makes up SSR and makes SSR exhibit good elasticity,thus preventing the interface reaction between the Mold-B and theSSR. For this reason, the SSR can be easily separated from the Mold-B with keeping the high-precision surface patterns.

It indicates that the surface morphology of the dermal denticleson shark skin can be replicated with silicone rubber by the way ofmicroreplication. In this process the properties of PDMS as a mold-ing material plays an important role. The precursor of PDMS hasexcellent owability due to its low viscosity, which makes the pre-cursor ll into the interstice of the mold; and the subsequent cross-linking process can transform the owable precursor into a solidadditive cure type silicone rubber.

Inte238 Y. Liu, G. Li / Journal of Colloid andPDMS sheet so as to maintain a conformal contact with the moldcavity and faithfully replicate its ne structure. The relativelylow surface free energy of PDMS itself (csv = 21.6 dynes/cm2) [31]and the elasticity, and extremely chemical inertness of the curedPDMS will enable the prepared PDMS sheet to demold easily.Therefore, the silicone rubber replica molding process is a low-costand reliable way for microstructure replication. The other poly-mers can also be used in such a microreplication with appropriatemodications.

The further research into the surface morphology of the SSRsamples treated by ame was conducted by SEM. The results areshown in Fig. 4. It can be seen from the SEM images (I) and (II)in Fig. 4a that the ne surface proles of the SSR sample can be re-tained upon treated by ame. Moreover, it is very interesting thatin Fig. 4a, the high-magnication SEM images (III) and (IV) of theame-treated SSR surface display a unique hierarchical roughstructure comprising a sub-microstructure and a nano-structure,in which the uniformly distributive micron-level aggregates(300500 nm) constitute the sub-microstructure, and this is com-posed of nano-silica particles (2050 nm). Accordingly, it can besaid that the unique hierarchical micro/nano-structures observedon the ame-treated SSR surface bear much resemblance to thoseof natural lotus leaves. What is consistent with the above observa-tion and analysis is that as shown in Fig. 4b, the ame-treated SSRsurface can exhibit considerable superhydrophobicity, or water onthis surface forms a spherical droplet. These results implies thatthe dual-biomimetic surface possessing the surface prole of sharkskin and the hierarchical micro/nano-structures similar to those ofa lotus leaf can be successfully fabricated through the microrepli-cation of shark skin followed by ame treatment.

Fig. 4. (a) The SEM images (shown at four magnications (I)(IV)) of the ame-treated Srface Science 388 (2012) 235242It should be noted that the duration of ame treatment is animportant factor affecting and controlling the hierarchical micro/nano-structures of SSR surface. To clarify the effect of ame treat-ment on the surface morphology of SSR, a few of SSR samples weretreated by ame for different preset time, respectively; and thenthe ame-treated surfaces were observed with SEM. The SEMimages are presented in Fig. 5.

It can be clearly observed from Fig. 5 that the surface morphol-ogy and roughness of the used SSR samples change dramatically asthe ame-treatment time increases. On the untreated surface thereis no discernable feature. Once the surface is treated by ame for510 s, some relatively regular and discrete embossment-likewrinkled surface structures along different directions begin toappear as shown in Fig. 5b and c; and the number of the regularwrinkled surface structures on the treated surface increases withan extension of the ame-treatment time. However, on the surfacetreated by ame for 30 s the ridges and troughs of the wrinkledsurface structures disappear instead of a uniformly rough micro/nano-structures as shown in Fig. 5d.

The mechanism of the above-described micromorphology evo-lution and microstructural changes can be explained according tothe principle of ame treatment and the analysis of the changein the physical properties of surface material during this process.Due to the action of the heat from ame, the ame treatment ofsurface conducted in air will lead to the surface oxidation of trea-ted object in essence, especially for the surface of organic material.On the other hand, there may exist the thermal stress of surfacegenerated by the non-uniform heating in the initial stage of ametreatment. Based on this principle, it can be speculated that whilethe surface of PDMS sheet is subjected to ame treatment, the ef-

SR sample and (b) image of a water droplet sitting on the ame-treated SSR surface.

InteY. Liu, G. Li / Journal of Colloid andfect of the surface thermal stress and the oxidative activation ofame will induce the micro-deformation of the surface layer;and simultaneously, the non-uniform heating in the initial stageof ame treatment makes the polysiloxane located in the high-temperature region of the surface layer break down and partiallydegrade into inorganic SiO2 networks. Consequently, the relativelyregular embossment-like wrinkled structures form on the surfaceof PDMS sheet. With the continuance of the ame treatment, thehydrophobic nano-silica particles previously embedded in thePDMS matrix are gradually exposed to generate ower-like nano-scale aggregates made from nano-silica. Finally, a uniformly roughsurface possessing the unique hierarchical micro/nano-structuresforms with increasing nano-silica aggregates on the samplesurface.

3.2. Wettability of various surfaces

It has been found that the surface wettability or hydrophobicitystrongly depends on its topology besides its chemical nature. Thus,the water CA values of the different microstructured surfaces ofPDMS sheets were measured to evaluate their hydrophobicity.The tested PDMS sheet samples included (a) F-PDMS sample withsmooth surface, (b) SSR sample with vivid shark-skin morphology,

Fig. 5. The comparison of the SEM images of the SSR surfaces treated by ame for [(a) arface Science 388 (2012) 235242 239(c) the ame-treated F-PDMS sample with nano-structuredsurface, and (d) the ame-treated SSR sample with a hierarchicalmicro/nano-structured surface. The results are illustrated inFig. 6 and the data on the water contact angle and sliding angleon the various surfaces are summarized in Table 1. The CA onthe smooth surface of F-PDMS as a control sample is 103(Fig. 6a), showing the hydrophobicity of PDMS as a low-surface-en-ergy material. Compared to it, various microstructures fabricatedon the sample surface can make the CA increase signicantly, asshown in Fig. 6bd. On the microstructured surface of SSR, theCA is 120, which indicates that the SSR surface possesses strongerhydrophobicity than F-PDMS surface, but it is still not superhydro-phobic. However, on the nano-structured surface of the ame-trea-ted F-PDMS sample and the hierarchical micro/nano-structuredsurface of the ame-treated SSR sample, the measured CA valuesreach 153 and 160, respectively. In other words, the CA of boththe surfaces increases by 50 and 40, respectively, as comparedwith that on the untreated surface of the corresponding sample.This implies that the surfaces of both samples (c) and (d) aresuperhydrophobic and the reason for it is that on the surfaces thereform the hierarchical micro/nano-structures produced by ametreatment. And according to the experimental observation, thewater droplet on the superhydrophobic surfaces is unstable and

nd (b)] 0 s; [(c) and (d)] 10 s; [(e) and (f)] 20 s; and [(g) and (h)] 30 s, respectively.

contact the very top of the surface and does not penetrate the inter-

icated via different routes and the corresponding schematic diagrams.

Interface Science 388 (2012) 235242can roll back and forth with no visible distortion, exhibiting ultra-low adhesion to the superhydrophobic surfaces.

In addition, as shown in Table 1, the water droplet on the F-PDMS surface does not move when the slide angle is smaller than90; and the water droplet on the SSR surface can stick to it eventhough the surface is turned upside down. However, the SA forwater droplet on the superhydrophobic surfaces having nano-structures or hierarchical micro/nano-structures merely reaches alimiting value of 1. This result suggests that the lotus leaf-likehierarchical structured surfaces can be created through a simple

Fig. 6. The CA of water droplet on the surfaces of PDMS sheets fabr

Table 1Water contact angle and sliding angle measured on the various surfaces.

Sample CA () SA

F-PDMS 103 90SSR 120 Stick to the surfaceThe ame-treated F-PDMS 153

topology is applied to practical dynamic water environment. Thetransition making a non-stick droplet sticky contradicts what weexpect from a superhydrophobic surface. As the Wenzel statemay not be suitable for many industrial applications involving dragreduction or self-cleaning, it is of great importance to predict thetransition between Cassie and Wenzel state.

The transition between different wetting states on the solid sur-face can be evaluated by analyzing the dynamic responses duringthe liquid drop impact, which is inuenced by several parameterssuch as droplet size and impact velocity. These effects can be de-scribed in terms of dimensionless number, the Weber numberWe [38,39], which is dened as the ratio of kinetic energy to sur-face energy, characterizing the deformability of the droplet. TheWeber numberWe can be calculated using the following equation:

We qV2rr

3

here r is the radius of the liquid drop, V is the impact velocity, q isthe liquid density, and r is the surface tension. Obviously, the

Weber number, We, can be altered or adjusted by simply changingthe impact velocity of the liquid drop, V, for the given liquid. In thiswork, water was used in liquid drop impact experiment.

To gure out how the impact velocity inuences the transitionfrom the composite solidairliquid interface to the homogeneoussolidliquid interface during the water droplet impact, we per-formed bouncing droplet experiments on various surfaces byreleasing droplets at different heights. Fig. 8 shows the snapshotsof water droplet with 1.1 mm radius on various surfaces at differ-ent time intervals, including the smooth surface of F-PDMS, themicrostructured surface of a SSR sample and the hierarchical mi-cro/nano-structured surface of the ame-treated SSR sample.

As observed from snapshots in Fig. 8, the water droplet impact-ing upon the F-PDMS surface and the SSR surface do not bounce offeven though the impact velocity applied is up to 0.9 m/s, whichmeans that the wetting states on the smooth PDMS surface andthe microstructured surface similar to shark skin are in the Wenzelstate. It is different in the case of the hierarchical micro/nano-structured surface exhibiting superhydrophobicity. Upon falling

Y. Liu, G. Li / Journal of Colloid and Interface Science 388 (2012) 235242 241Fig. 8. The snapshots of a droplet with 1.1 mm radius hitting various surfaces.

on the hierarchical micro/nano-structured ame-treated SSR sur-face at the impact velocity of 0.5 m/s and 0.9 m/s, respectively,whose corresponding Weber numbers are 3.8 and 12.2, respec-tively, the water droplet rst deforms followed by spreading and

developed into a novel and feasible biomimetic surface manufac-turing technique, that can create multifunctional biomimeticstructured surfaces providing a better performance on self-clean-

242 Y. Liu, G. Li / Journal of Colloid and Interface Science 388 (2012) 235242retracting, and nally rebounds off the surface within 10.98 msand 10.54 ms. The Cassie model is usually used to explain this phe-nomenon. In the Cassie model, air can be trapped underneath thedroplet and in the interstices of the microstructures, so that thedroplet bounces off and cannot wet the surface.

However, when the impact velocity reaches 1.5 m/s or the cor-responding Weber number is 34.0, the droplet rst deforms, thendevelops outward to form water ring with wavy bumps, andeventually merges together at the center to form an elongatingwater fountain. The bottom of the elongated droplet irreversiblyadheres to the solid surface, which cause the droplet not to bounceoff completely and just to be pinned on the partly wetted surfacewith maintaining small contact area after 3000 ms. The above-mentioned results indicate that a transition from the compositeinterface to the homogenous interface can occur as the kinetic en-ergy overcomes the surface energy and the liquid surface tension.For the ame-treated SSR sample, if the Weber number that de-pends on the features, size and impact velocity of liquid dropletreaches 34 or high, the Cassie state is broken and the droplet turnsto the wetting Wenzel state, in which it is pinned on the surface.

4. Conclusion

A brand new method was successfully developed using PDMScontaining nano-silica as a substrate for producing a dual-biomi-metic surface structure comprising both the shark-skin surfacemorphology and the lotus leaf-like hierarchical micro/nano-struc-tures. It involves the PDMS microreplication processes using sharkskin as a template and the subsequent ame treatment. The SEMobservations show that the biomimetic shark-skin surface fabri-cated by the way of microreplication possesses vivid shark-skinsurface morphology, or micron-sized shark-skin pattern structure;and the subsequent ame treatment makes it possess hierarchicalmicro/nano-structures with no damaging its shark-skin patternstructure, thereby constructing the dual-biomimetic surface as ex-pected. The duration of ame treatment is an important factoraffecting and controlling the hierarchical micro/nano-structuresof the treated surface. The dual-biomimetic surface or the ame-treated biomimetic shark-skin surface exhibits excellent superhy-drophobicity with a low SA. Its CA reaches 160, which increasesby 40 as compared with that of the untreated biomimetic shark-skin surface. This implies that the ame treatment producing hier-archical micro/nano-structures on PDMS surfaces is a key processto fabricate superhydrophobic surface. According to the results ofliquid drop experiment and their analyses, the robustness of thedual-biomimetic superhydrophobic surface is conrmed and thetransition from the Cassie state to the Wenzel state arises andthe phenomenon of pining a droplet on the surface occurs whenthe impact velocity exceeds a threshold velocity (V 1.5 m/s), atwhich the corresponding Weber number We reaches 34.0.

Summarily, the novel method developed in this study can notonly fabricate a superhydrophobic dual-biomimetic surface as ex-pected, but also is characterized by simplicity, high efciency andlow cost. What is more signicant, combined with the other tech-niques for fabricating biomimetic surfaces, it is expected to being, antifouling, drag reduction, antireection, and so forth, therebysatisfying the requirements for practical applications in differentelds. The further study on the drag reduction and anti-bioadhe-sion of the fabricated dual-biomimetic surface is in progress andwill be reported in our future papers.

Acknowledgments

This work was nancially supported by the National NaturalScience Foundation of China (NSFC, Grant No. 50873039) and theFoundation of Key Laboratory of Surface Functional Structure Man-ufacturing of Guangdong Higher Education Institutes, South ChinaUniversity of Technology (SFS-KF201011). The authors would liketo thank Peng Xinyan, Su Dong, and Li Shuai of South China Univer-sity of Technology. The authors also grateful acknowledge Mr. WuChaomao and his fellow workers from Yuan Ao International TradeCo., Ltd. (Hongkong) for the friendly supply of the high-speedcamera.

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