A Study On The Forms And Self-cleaning Mechanism Of The Scales On Butterfly Wing Surface

Wettability is one of the important characteristics of solid surface,which is determined by surface chemical composition and morphological micro-structure.The super-hydrophobic surface is a surface on which the contact angle of water is above 150°.The fabrication of super-hydrophobic surface plays a remarkably important role and has wide application potential in basic researches such as surface chemistry,material chemistry,as well as industrial and agricultural production.In order to defense the disturbances of adverse factors such as wind,rain,fog,dew,dust,the anti-adhesion and dewetting ability were formed on the wing surface of butterfly through long-term evolution and natural selection.Self-cleaning function displays on the wing surface of butterfly.The wing scale surface of butterfly has periodic substrate structure and peculiar wettability,so is regarded as an ideal modeling surface for qualitative and quantitative studies on wetting phenomena.In this research,using stereoscopic micro vision analysis system,AxioVision,MIAPS image analysis software,video-based contact angle meter and scanning electron microscope,the forms and self-cleaning mechanism of the non-smooth surface on butterfly wing scale were studied.Micro- and ultra- investigations were conducted on the rough surface structures of 8 families,36 genera and 49 species butterflies.The indices of micro-class scale unit such as size,density were classified,the nano-class structures of scale surface were characterized qualitatively and quantitatively.Wing surface is made up by micro-class scales arranging like overlapping tiles.The scale surface is composed of nano-class vertical gibbosities and horizontal links.The length of butterfly wing scale is 65¹50μm,width 35¹05μm,distance 48¹70μm,thickness 0.5².4μm.The height of vertical gibbosity on wing scale is 200⁹50 nm,width 200⁸80 nm,distance 1.06².95μm.No obvious rules are found for the scale sizes of butterflies in various genera and species.On micro-class,the forms of wing scale can be divided into four types,including angustifoliate shape,latifoliate shape,round-leaved shape,and spindle-like shape.On nano-class,the ultra-forms of wing scale can be divided into seven types,including arch bridge shape,chessboard shape,sieve pore shape,double-row sieve pore shape,coral-like shape,pinnule-like shape,and multi-row sieve pore shape.A systematic investigation on wettability of butterfly wing surface was conducted,including the effects of methanol and distilled water on surface wettability of butterfly wing surface,and the response of contact angle to change of methanol concentration on the wing scale of Polygonia c-aureum,the corresponding mechanism was analyzed.The static contact angles were measured on the wing surface of 8 families,36 genera and 49 species butterflies.The results showed that the butterfly wing surface is hydrophobic with contact angles 134.0°¹63.5°.Super-hydrophobicity (contact angle>150°)was found on the wing surfaces of 11 species butterflies(22.4% of all species).Due to the presence of rough scale,the hydrophobicity of butterfly wing surface was improved,the contacts angles increased by 8.2%⁴1.0%.Methanol wetting test was conducted on the wing surfaces of 7 families,24 genera and 33 species butterflies.The wing surfaces of most butterflies(31 species)is hydrophobic and can resist the wetting of methanol with relatively lower concentration(55%).When methanol concentration reaches 70%,spreadingwetting occurs on the wing scale surfaces of Mimathyma schrenckii and Parnassius glacialis.When methanol concentration reaches 95%,spreadingwetting occurs on the wing scale surfaces of all the 33 butterfly species,the contact angle of droplet on wing scale surface decresed instantly to 0°.The wing surfaces of 5 butterfly species(Lycaeides argyrognomon,Polygonia c-aureum,Vanessa indica,Papilio maackii and Gonepteryx rhamni)are super-hydrophobic(contact angle>150°);when methanol concentration is 55%,the contact angle is larger than 100°; meanwhile their wing surfaces can resist the spreadingwetting of methanol with relatively higher concentration(80%). Two methods,first-incline-then-drip method(FITD method)and first-drip-then-incline method(FDTI method),were adopted to measure the sliding angles(contact angle hysteresis)of water droplet on butterfly wing surfaces of 6 families,24 genera and 29 species butterflies.The butterfly wing surfaces has highly anisotropic characteristic ( forward sliding angles<3°,backward sliding angles>65°).For the same butterfly species,the sliding angles by first-incline-then-drip method are significantly smaller than those by first-drip-then-incline method in the same direction.In FITD method, forward SAs0.05).SA calculation formula were derived from Wenzel model and Cassie model,the reason for different sliding angles from the two measurement methods was analyzed.The results show that nano structure and micro/nano coupling structure play an important role in anisotropism on super-hydrophobic surface.Pollution test by CaCO₃ was conducted on the wing surfaces of 8 families,33 genera and 44 species butterflies,then distilled water was used to clean the polluted surface.The polluted area,the removed area,and the removal rate were calculated,and the self-cleaning rules were analyzed.In the 44 butterfly species,the removal rater of Lycaenidae's Lycaeides argyrognomon is the highest(69.72%),that of Papilionidae's Papilio machaon is the lowest(9.43%),the average removal rate is 40.03%. The hydrophobicity mechanism of butterfly wing surface was analyzed via Young equation , Wenzel equation , and Cassie equation . The super-hydrophobicity and self-cleaning of butterfly wing surface are due to the co-effects of micro-class scale structure and nano-class vertical gibbosities structure.In micro-dimension the smaller the scale width and the bigger the scale distance,in nano-dimension the smaller the vertical gibbosity height and the smaller the width and the bigger the distance,then the stronger the hydrophobicity on butterfly wing surface.On the basis of Cassie equation,a wettability model of micro/nano structure coupling effect was established:Silver film was coated on the wing surfaces of Nymphalidae butterfly by vacuum vapor plating.With the increase of silver film thickness on wing scale surface,the silver particles move to scale surface and form silver coating.The contact angles on butterfly wing surfaces coated by siliver were measured.With the increase of silver film thickness,the contact angles decrease gradually from 144.8°to 129.3°for Mimathyma nycteis,and from 140.7°to 126.1°for Speyeria aglaja.The hydrophobicity decreased.Nano-scale silver film was coated on the wing surfaces of Mimathyma nycteis and Speyeria aglaja by vacuum high-pressure vapor plating.The contact angles on surface of silver film increased from pure silver's 63°to 143.2°and 139.2°,respectively.The surface of silver film displayed high hydrophobicity.The presence of surface rough structure makes metal silver turn from hydrophilic to hydrophobic state.