Effects of Methanol on Wettability of the Non-Smooth Surface on Butterfly Wing

Available online at www.sciencedirect.com中ScienceDirectJournal of Bionic Engineering 5 (2008) 127-133Effects of Methanol on Wettability of the Non-SmoothSurface on Butterfly WingYan Fang'2, Gang Sun'", Qian Cong , Guang-hua Chen', Lu-quan Ren'1. Key Laboratory of Terrain-Machine Bionics Engineering (Ministry of Education, China),Jilin University, Changchun 130022, P R. China2. School of Life Science, Changchun Normal University, Changchun 130032, P. R. ChinaAbstractThe contact angles of dstilled water and methanol solution on the wings of bttrlies were determined by a visual contactangle measuring system. The scale structures of the wings were observed using scanning electron microscopy, The influence ofthe scale micro- and ultra-structure on the wettability was investigaied. Results show that the contact angle of ditilled water onthe wing surfaces varies from 134.0* to 159.2. High hydrophobicity is found in six species with contact angles greater than150*. The wing surfaces of some species are not only hydrophobic but also resist the wetting by methanol solution with 55%concentration. Only two species in Parnassius can not resist the wetting because the micro-structure (spindle-like shape) andultra-structure (pinnule-like shape) of the wing scales are remarkably different fom that of other species. The concentration ofmethanol solution for the occurrence of spreading/wetting on the wing surfaces of different species varies from 70% to 95%.After wetting by methanol solution for 10 min, the ditlld water contact angle on the wing surface increases by 0.8*-2.1",showing the promotion of capacity against wetting by ditilled water.Keywords: non-smooth surface, bttfly wing, scale, wettability, micro/ultra-stuctureCopyright。2008, Jlin University. Published by Science Press and Elsevier Limited. All rights reseved.chemical composition to regulate surface free energy.1 IntroductionTo get larger CA, the surface micro-morphology of theWettability is one of the important properties of amaterial have to be changed!8-10.solid surface, which depends on several factors, such asRecently, many researchers have paid attention tosurface free energy, roughness, morphology, the mannerthe study of bydrophobic materials that can be found inof surface preparation, surface cleanliness, and chemicalbiology. Through evolution and natural selection, manycomposition!" . The hydrophobic (water-epellence),creatures have formed various non-smooth surfaceoil-repellent, and self-cleaning characteristics of a solidstructures to adapt to the environment. When pollutantssurface have found wide applications in industrial, agsuch as dust, rain or snow drop on the surface, it canricultural, domestic and military fields, such as proofingself-clean, while it needs several times of effort to cleanagainst snow， water and fog, pollution guarding,an artificial surface of the same area. The leaf surfaces ofanti-oxidation, aerobat, submarine, radar, etc. The hy- hundreds of different plant species have been studied todrophobicity of a surface can be promoted by increasingsee the effect of roughness on hydrophobicity'".the surface area and surface roughness. Superhydro-Among them are leaves of water-repellent plants such asphobic or superhydrophilic surfaces can be constructedNelumbo nucifera (lotus) and Colocasia esculenta,by using materials of low or high surface free energywhich_ have high CAs with water and show strongrespectivelyS-7. On smooth surfaces, the contact angleself-cI中国煤化工e“ut-usefcr(CA) can be only increased to 120 by changing Edwir;YC N M H Ge and hysteresis ofCorresponding author: Qian CongE-mail: congqian@jlu.edu.cn128Jourmal of Bionic Engineeing (2008) Vol.5 No.2lotus-effect plant surfaceshs. Wagner et al. examinedexperimental and theoretical basis for fabricating nano-the wing surfaces of ninety-nine insect species by highhydrophobic materials, lubricating and self-cleaningresolution scanning electron microscopy, in order tomaterials.identify the relationship between the wing microstruc-2 Materials and methodstures and wettability with water, and their behavior un-der the influence of contamination!4.2.1 MaterialsRecent studies have been carried out to characterizeButtrfly specimens, belonging to seven families,the leaf surfaces on the micro- and nano-scales whiletwenty-five genera, thirty-three species, were collectedseparating out the effects of the micro- and nanobumps,from June to August 2005 in the suburbs of Changchun,and the wax (crystals that are a mixture of large hydro-Jjilin and Dalian cities, all locating in Northeast China.carbon molecules, measuring about 1 nm in diameter) ofThe specimens were identified by an insect taxonomisthydrophobic leaves on the hydrophobicityl5.bo. Wagnerwith systematic taxonomy23.24. All the reagents were ofet al. made a quantitative assessment of the structuralanalytical reagents (AR) grade and used as receivedbasis of water repellence in natural and technical sur-without further purification. Methanol was purchasedfacesh7. Based on the researches on lotus leaf, Feng et al.from Wulian Chem. Co. (China). The ditilld waterpointed out that self-cleaning is atributed to both mi-used was obtained through a distillation system.cro-structure convex on rough surface and epicuticularwaxesh8. Cong et al., Fang et al, and Chen et al. ex-2.2 Experimental instrument equipment andamined the micro- and ultra-structures of btterfly wingmeasurement methodsscales and drew the conclusion that the high hydropho-A video-based contact angle meter (OCA20,bicity of the butterfly wing (static CA 136.3° to 156.6)DataPhysics Instruments GmbH, Germany) was used tois atrbuted to the co-effects of micro-class scale andconduct wettability experiment and measure the CAssubmicro-class vertical gibbosities on the wing sur(distilled water and methanol) on the discal cell of thefacell 221. By learning from what is found in nature, wewing surfaces of all thity-three species by the sessilecan tailor surfaces to mimic those properties. Creatingdrop method. The 0CA20's measurement range ofCA isroughness on materials such as polymers and studying0° to 180* with, accuracy of +0.1', the resolution oftheir surface properties will lead to successful imple-surface tension is +0.01 mN.m . The volume of thementation in applications where water-repellence isdroplet is 5 μL. For each species, ten duplicate samplesimportant. Up to now, the rules and mechanisms ofwere measured, and the average value was adopted. Thewetting of butterfly wing scales by water and organictemperature was held constant at (25+1) °C in all ex-solvent have been studied very limitedly. Methanol,periments. In the experiments of the effect of methanolwhose suface tension is less than that of water, is ansolution on wing surface wettability, the starting con-important organic solvent. This research is aimed atcentration was zero with increment of 5% until the wingexploring the effects of this organic solvent on wet-surface was completely wetted; the maximum concen-tability of buttrfly wing surface, and the wettingtration was 95%. In another experiment, the wing sur-mechanism of the non-smooth structure of butterflyface was wetted first with 60% methanol solution forwings. Methanol and distilled water were used to wet10 minutes, then the CA of distilled water on the wingthe wing surfaces of bttrflies living in the Northeastwere measured. The data processing and analysis wasChina. A scanning electron microscope and aconducted with SPSS 13.0.video-based contact angle measuring system were usedAll buttrfly specimens were flattened and tento observe the micro- and ultra-structure of non-smoothsamples were selected for each species. The wingssurface and to examine qualitatively and quantitativelywere中国煤化工including face-sidethe wettability and hydrophobicity of the non- smoothandYHC N MH Gg veins. Then, thesurface of a buttrfly wing. The results may provide anving samples were crosscut and afixed to aluminumFang et al: Efects of Methanol on Wetability of the Non Smooth Surface on Buttrly Wingsubstrates with double-sided adhesive tape, air-dried,stantly to 0. When the concentration gets to 95%，and gold coated (approximately 20 nm). An ion sputterspreading/wetting occurs on the wing surface of all thecoater (SBC-12, Beijing Instrument Research andspecies tested.Manufacture Center of CAS, China) and a scanningThe wing surfaces of thirty-one butterfly specieselectron microscope (S-570, HITACHI, Japan) wereare not only hydrophobic but can also resist wetting byused for preparation, observation and photography.methanol solution with relatively lower concentration(55%). The minimum concentration of methanol for3 Results and discussionspreading/wetting to occur on wing surface is 70%. Thewing surface of six butterfly species (Lycaeides argy~3.1 Wetting the surface of butterfly wing scales withrognomon, Argyronome laodice, Vanessa cardui,distilled waterThe CAs of ditilled water on the wing surfaces ofGonepteryx mahaguru, Minois dryas, and Satyriumthe thirty-three species are listed in Table I. It is seeneximium) can resist spreading/wetting with methanolthat the measured CAs are between 134.0 and 159.2,solution when the methanol concentration is lower thanwhich indicates that the wing surfaces of all tested spe-95%. On the wing surface of five butterfly speciescies are highly hydrophobic. The wing surfaces of six(Lycaeidesargyrognomon, Polygonia c-aureum,species (Lycaeides argyrognomon, Polygonia c-aureum,Vanessa indica, Papilio maackii, and Gonepteryx ma-Vanessa indica, Papilio maackii, Aporia crataegi,haguru), the CA of distildl water is greater than 150°Gonepteryx mahaguru) are super-hydrophobic, whoseand that of the methanol solution is greater than 100when methanol concentration is 55%, and the minimumCAs are measured over 150*.concentration for spreading/wetting with methanol is3.2 The wettability of methanol solution on surface80% to 95% (Table 1). Those five butterfly species areof butterfly wingboth super- hydrophobic and resistant to wetting withThe surface tension of distilled water is 72.75+0.10methanol solution with relatively higher concentrationmN:m~' , and that of methanol is 22.70+0.10 mN.m^ '.(<80%). Only two butterfly species, belonging to Par-Wetting buttrfly wing surface with methanol solution,nassius, cannot resist wetting by 55% methanol solution.the surface tension decreases but the morphologicalhe micro-structure and ultra-structure of Parnassiuspatterm does not change, so the wing surface is easier tobutterfly wing surface scale is spindle-like shape andget wetted. Wettability experiments and CA measure-pinnl-like shape, respectively (Fig. la and Fig. Ib),ment of methanol solution at different concentrations onwhich are remarkably different from the micro-structurewing surfaces were conducted with all butterly species.(rectangle-like shape) and ultra-structure (grid-likeThe starting concentration of methanol solution was zeroshape) of other species, such as Vanessa (Fig.2a and Fig.with an increment of 5%. As the concentration was in- 2b). So the capacity of Parnassius to resist methanolcreased three wetting patterns were observed. First,wetting is obviously lower than that of other buttrlywhen concentration of methanol solution is lower thangeneral2n. The hydrophobicity and repellence of50%，the CAs of the wing surface of all species aremethanol on the butterfly wing surface are due to thegreater than 90*. If the wing suface is inclined slightlypeculiar structure of the scales.(<3"), the solution droplet flows away rapidly. ThisFor some butterfly species, the CAs of distilledshows the resistant capacity of butterly wing twater on the wing surfaces differ remarkably, while theremethanol wetting. Second, when the concentration ofare no significant differences among the minimum wet-methanol gets to 55%, wetting by droplet occurs onting concentrations of methanol and the correspondingthe wing surface of twenty-three species, the CA de-CAs, as well as the minimum snreading/wetting concen-creases gradually with time. Third, when the concentra-tratior中国煤化工vater CAs on wing .tion gets to 85%, spreading/wetting occurs on the wingsurfacYHC N M H G Gonepteryx maha-surface of twenty-three species, the CA declines in-guru are 146.0 and 153.1* respectively, but the minimum130Journal of Bionic Engineeing (2008) Vol.5 No.2Table 1 CAs of ditilled water and methanol solution on btterfly wing surface, and the minimumconcentrations for wetting and spreading wetting of methanol solutionDistilledThe minimum cocomrtio for mocha The mininum cocentraon Dillel waer CA afer weting byButtrefly speciesnol solution wetting on wing surface, for methanol solution spread-water CAmethanol, and the increase of CAand the CAing wetting on wing surfaccPygus malvase146.2°55%93.3"80%148.1"1.9*Everes angiades147.7*60%90.4*90%149.3"1.6*159.2*I07.1"95%160.5*1.3*Angyronome laodice146.0* .65%96.3"147.9"1.9"Brenthis ino147.7"93.8*75%149.2"1.Childrena zenobia146.0"55% .115.7*147.6"1.6Mimathyma schrenckil146.5'100.570%147.8" .Nephangynnis anadyomene149.9"115.8"151.2"1.3Nepis rivularis147.3"116.7"85%148.8"1.5Polygonia calbum1443" .109.7"145.7"1.4"Polygonia c-aureum'152.4"105.3*154.5"2.1"Vanessa cardui140.3*100.4"142.2"19"Kanessa indica'152.3"104.6"1.4136.2"50%101.8*137.9*1.7"Parasius glacialis143.2"108.6"144.I'0.9"Papllio bianor138.5"5%106.8"Papilio macki158.0"109.4"1.8Papilio machaon134.0*105.4"Papilio xuthus138.2°100.6"140.2"2.0"Pieris brassicae140.8*97.7*142.3"1.S"Aporia crataegi150.4"99.5152.2"1.8"Colas erate143.8*90.S5"145.3"1.5"Gonepteryx mahaguru'153.1"105.6'2.1'Pieris rapae146.5*104.5"1.8*Pieris meleteI 36.3"106.9*1.1Pontia daplidice9.3"16.1"141.1"142.6"112.0*143.9"13"149.6"Melanangia halimede136.5'93.4"138.2"Minois drnyas143.8". 116.4*1.5*Oenieis sp.135.9"137.6*1.7Apatura laverna142.3'"95.6"0.8Satyrium eximium46.5"104.1'* Distilled water CA>150*; and methan| solution CA>100 when methanI concenration is 55%s; and the minimum concenration for spreading wetingof methanol solution is 80% to 95%.中国煤化工(a)(b)100018 20KVap......百uMYHCN MH Gm-Fig. 1 The micro-structure (a) and ultra-structure (b) of wing scale surface of Parnassius glacialis.Fang et al: Effects of Methanol on Wettability of the Non-Smooth Surface on Butterfly Wing131.1DB8SR0)9D118099191996099091m189909 20KV Rk"t5:duD 188858 20KV文白:bek"3: Bu.Fig. 2 The micro-structure (a) and ulta-structure (b) of wing scale surface of Vanessa indica.spreading/wetting concentrations of the solution are bothwing surface against wetting by methanol solution flls95%. On the other hand, for some other butterfly species, significantly, and finally spreading/wetting occurs as thethe distilled water CAs on wing surface are relativelyconcentration reaches 95%. Fig. 4 shows photographs oflarger, while they can be wetted by methanol solutionthe contact conditions of distilled water and methanolwith relatively lower concentrations. For example, thesolution (10%, 30%, 55%, 70% and 90%) wetting ondistilled water CA on the wing surface of Mimathymawing surface of Vanessa indica.schrencki is 146.5, but the wing surface can be wetted3.3 Effect of prior methanol wetting on distilledwith 70% methanol solution.water wettability of butterfy wing surfaceTo further explore the effects of methanol solutionAfter the wing surfaces were wetted with 60%with dfferent concentrations on wettability of bttrflymethanol for 10 minutes, the CAS of distilled water onwing surface, Vanessa indica was selected to measurethe surfaces were measured. Compared with that withoutthe CAs on the wing surface by increasing methanolprio-wetting by methanol solution, the CAs of distilledconcentration from zero (isilled water) to 95%, Fig. 3water on the wing surfaces increase by 0.8* to2.1°.showing the results. With ditlld water the CA isLillifors test of normality was conducted for the152.3, as the methanol concentration increases the CAtwo data groups (distilled water CAs and distilled waterdecreases. When the methanol concentration increasesCAs after wetting by methanol), the P values are 0.518to 60%, the CA drops steeply (inflexion). With furtherand 0.469 respectively (both > 0.05), which indicates theincrease in methanol concentration, the capacity of thetwo data groups are both consistent with normal distri-bution (Table 2).144The result of paired-samples t-test indicates sig-食12nifcant difference between the two data groups (dis-tlled water CAs and ditilled water CAs after wetting bymethanol) because of the P of 0.000 (<0.05) (Table 3).This indicates that after wetting by methanol solution thecapacity of butterfly wing surface against re-wetting bydistilled water increases significantly. The maximum0102030405060788090increase of CA (2.1) is found in Polygonia c-aureumConcentation of methanol solution (%)and中国煤化工mininum increseFig. 3 Effects of methanol solution concentration on wet-of CA[he No remarkabletability of btterly wing scale surface. Data presented are theCAs on wing surface of Vanessa indica in the process ofpatter. CN M H Gevels of family andmethanol concentration increase from zero to 95% (+SE).genus.132Jourmal of Bionic Engineering (2008) Vol.5 No.2CDFig. 4 The contact conditions of dstilledl water and methanol solution wetting on wing surface of Vanessa indica.A is dstilled water, B, C, D, E and F are 10%, 30%, 55%, 70% and 90% methanol solution, respectively.concentration reaches 95%, spreading/wetting occurs onTable 2 Results of normality testall species wing surfaces, the CAs of methanol solutionData groupP value Lilifors value)drop instantly to 0'. There are five species whose wingDisilled water CA0.518surfaces can resist the wetting by both water and high-Disilled water CA afer wetting0.469by methanolconcentration methanol solution.The surface of a butterfly wing is one of the mostTable 3 Results of significance test of differencecomplex three-dimensional periodic substrates in nature.P valueof pailired-sample sAs an ideal template for biomimetic fabrication, in re~Fvalue。0.000cent years the butterfly wing has drawn great interest dueDisiled water CA afer wettingto its excellent properties, such as iridescence, super-by mcthanolhydrophobic characteristics and quick heat dissipation,which are closely related to its surface structure. The4 Conclusionsstudies on wettability of pattermned bio-surfaces willThe wing surfaces of the thirty-three butterfly spe-provide proof for super-hydrophobization of solid sur-cies tested are highly bhydrophobic, the ditilled waterface, development of microfluidic systems, tribologicalCAs varies from 134.0* to 159.2*. Super-hydrophobicityengineering, etc. The results of this paper are expected tois found in six species with CAs over 150*. The wingbe applied in design and fabrication of novel biomimeticsurfaces of most species can resist wetting whenmaterials which can resist wetting by both water andmethanol concentration is lower then 55%, only twoorganic solvent.species in Parnassius cannot. The lower capacity ofParnassius butterfly wing surfaces to resist methanolAcknowledgementwetting is due to the special micro-structure of the scalesWe thank Dr. X. L. Li (Northeast Normnal Univer-(spindle-like shape) and ultra-structure (pinnule-likesity, P. R. China) and Mr. T. Q. Wang (Tsinghua Uni-shape) which are remarkably different from the mi-versity, P. R. China) for their cooperation throughout thecro-structure (rectangle-like shape) and ultra- structurework中国煤花宝Naional Natural(grid-like shape) of the scales of the other twenty-fourScienNo.50635030), bybutterfly genera. The minimum concentration ofthe Sp |fMYHC N M H Ge Doctral Programmethanol for spreading/wetting is 70%. When methanolof Higher Education of China (Grant No. 20040183048).Fangetal: Ef of Methanol on Wtbilily of the Non- smooh Srface on Butely wWing133References[14] Wagner T, Neinhuis C, Barthlot W. Wtabiltly and conr[u] Bhushan B. Inroducin 10 Tribology, Wiley, New York,taminability of insect wings as a function of their surfaceUSA, 2002.sculptures. Acta Zoologica (Slockholm), 1996, 77,213- -25.[2] Bhushan B. Nanotribology and Nanomechanics: An Intro. [15] Burton z, Bhushan B. 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