Research Of Moth Wings’ Surface Superhydrophobicity And Fabrication Of Biomimetic Surfaces

The organisms in nature optimize a variety of shape, configuration, structure andmaterials over evolutionary for millions of years and show a wide range of functionalcharacteristics. It makes them possessing optimum adaptability and coordination to survivethe bad environment. Because moths have long lived in a moist environment, thehydrophobicity and self-cleaning characteristics on the moth wing surface are developedthrough evolutionary processes to resist the impact of adverse external factors. The excellentfunction has a very broad application prospects in the field of military, industry andagricultural production, biomedical engineering and daily lives. In recent years, mimicingthe shape and structure of organism surfaces, the materials with special wetting functionhave been prepared. It has become today’s multi-disciplinary research hotspot, theemergence of a number of physical and chemical methods have successed in preparation ofbiomimetic surfaces.In this paper, the morphology, structure and chemical composition of moth wingsurface were researched, and their influence on the wettability of surface were investigated.According to this, the related wetting model was made. Furthermore, mimicking the mothwing’s surface, several hydrophobic materials were fabricated by different means at thenanometer scale, micron-scale and micro-nano combined scale. Cassie model, Wenzel modeland Herminghaus model as theoretical analysis explained the mechanism with specialwetting function. The details were as following:(1)The surface shape and structure of typical moth wing(33species，9families) werestudied by means of a stereoscopic microscope, a scanning electronic microscope and anatomic force microscope. The observations showed that there were micron scales arranged like domino on the surface of the moth wings. The arrangement direction was consistentwith the wing veins. There was a certain angle between scale and elytron of moth wing. Thescales were in two shapes, namely angustifoliate shape and latifoliate shape. The wholesurface of scale was structured by submicron-class grooves and nanometer-class verticalgibbosities. The contigeous contiguous gibbosities were connected by nanometer-classparabola-shaped ribwork. Trapeziform gibbosities and parabola-shape ribworks formedthrough holes. The through holes were in three shapes, namely nest eye shape, pane shapeand sieve pore shape. The shape and structure parameters of scales between different specieshave no obvious law.The biomaterial of moth wing scales and elytron were studied byfourier transform infrared spectroscopy(FT-IR). It indicated that the primary components ofscale and elytron were similar and the components of them were mostly protein, lipids andchitin.(2)The wetting property of the moth wings’ surface was detected by the interfacecontact angle measurement. The observation showed that the contact angle for wings withscales was in the range from130.7～152.9°, while that for those without scales was from85.2～128.4°. It indicated that the surfaces of the wings with scales were more hydrophobic.According as Cassie model, the equation of wettability on the moth wing surface wasestablished and the hydrophobic mechanism was analyzed. It was concluded that thehydrophobicity of the moth wings was induced by the multivariate coupling of the shape,structures and biomaterial of the scales. There were significant difference between theforward, reverse and lateral roll angle on moth wing surface. It indicated that the wettingproperties on the surface of the moth wing were anisotropic. The forward scrolling angle wasless than10°. It made the moth wing surface having a self-cleaning function.(3)Mimicking the nanoscale hole-like structure of scale on moth wings, highly orderedanodic aluminum oxide (AAO) surface was fabricated by a two-step anodization approach.The pore diameter of as-prepared AAO was about30nm and the thickness was about3μm.Then Ag nanorods grew on AAO surface via a galvanic reduction approach. The length ofAg nanorods was in the range of50~70nm and the diameter was about20nm. So the rough AAO-Ag multilayer was fabricated. Furthermore, the surface free energy of AAO-Agmultilayer was reduced by modifying with perfluorodecanethiol(PDT). The modifiedAAO-Ag multilayer was superhydrophobic and the static contact angle reached as high as168°. The results showed that the co-coupling of the rough structure and low surface freeenergy induced the superhydrophobic performance of the AAO-Ag multilayer surface.(4)Mimicking micron scale shape of moth wings’ surface, Al sheet was used assubstrate, the striped and grid-shaped morphologies with different gradients was processedby a laser etching method. The etching spacing is70μm,110μm,160μm,210μm and260μm.After laser etching, the surface of rough structure was modified by the hydrophobicsubstance PDT for3days. The contact angles of sample on surface were measured in eachtreated stages. Before the laser etching，it showed that the contact angle on sample surfaceapproximately52.5°, so the samples were hydrophilic. After laser etching，the contact angleson all sample surfaces were less than30°, changing more hydrophilic. Then the samplesetched were soaked into the solution of PDT for3days, with the storage time, the wettingproperties of the sample surface changed significantly. On the first day, the contact angle ofsample surface was approximately40~65°and still hydrophilic. After45days, the contactangle of grid-type Al surface was in the range101.5~130.7°, and that of the stripe-shaped Alsurface was93.2~110.2°.And the etching space had a significant effect on the surface contactangle. The greater the etching space was, the smaller the contact angle was. After90days,the static contact angle of both the striped and grid-shaped samples were more than140°and close superhydrophobic.(5)Inspired by the multivariate coupling inducing the hydrophobicity of moth wingsurface, AAO was used as substrate, multilevel and rough biomimetic materials werefabricated by laser etching combined with chemical immersion method. The as-preparedsamples were modified in by PDT. The contact angles of sample on surface were measured.For striped samples, the larger the etching space was, the smaller the contact angle of samplesurface was. However, the immersion time in ammoniacal silver solution had no obviouseffect to the contact angle. For grid-shaped samples, Both etching space and immersion time are less obvious influence to the contact angle of sample surface. The contact angle of thegrid-like sample surface was higher and the contact state was more stable than that of thestriped sample.The superhydrophobic and high adhesion material prepared in this experiment hasfunction similar to micro-robot. It can be used for transfer of precious and minim liquidslossless, coatings in aerospace field and a wide range of applications. In this work, thepreparation methods and theoretical analysis of hydrophobic surface has made someprogress, some valuable conclusions was summarized. The materials and methods can offernew ideas and experimental reference in the design and preparation of biomimetic materials.The analytical methods in this work can provide the theoretical basis for the mechanismanalysis to other wetting surface.