The Optical Response Effect Of Co-plating Butterfly Wing Scales Controlled By Magnetic Field

In order to adapt to the surroundings and escape from predators, manyspecies living in nature own pretty wings showing different vivid colors formating, camouflage and even species recognition through million years ofevolution. The mechanisms for colors on the wings include pigments andthree-dimensional (3D) periodic sub-micrometer-level hierarchical structures,which are referred to as “chemical color” and “physical color” respectively.The striking colors on the wings of butterflies are typically physical colors,which are also called structural colors and combination of several opticaleffects including thin-film interference, multilayer interference, diffractiongrating effect, photonic crystals and light scattering. These structural colorsgive the butterfly wing scales the characteristics of iridescence, which meansthat the wings of these butterflies with structural colors can appear in differentcolors from various angles or directions.Generally, the famous iridescence caused by the microstructures of wingscales from Morpho butterflies has attracted attention and interests of a greatnumber of researchers for a long time, including measuring the refractiveindexes of original organisms, exploring the mechanisms and simulating themfor these optical phenomena, and searching how to prepare the3Dsub-micrometer structures with artificial materials by template method.However, few scientists so far have focused on the applications of thestructural colors and iridescence exhibited on the butterfly wing scales insome real fields even though the3D microstructures with well controlled hierarchical morphologies down to the sub-micrometer scale on the wingscales have attracted considerable attention for their broad applications owingto their unique optical, electronic, magnetic, thermal, and catalytic properties,which can be modulated by their intrinsic microstructures.Herein, it is a new idea that we can control the single wing scale statusincluding angles, directions and even the movements through externalmagnetic field when the single scale has been coated with magnetic metalsuch as Cobalt, which may make the iridescence on wing scales be applied tosome real domains. First, the wings of E.mulciber and Sasakia charonda arechosen as templates. After a series of chemical treatments the pigments andprotein particles on the wings, which make the wings hydrophobic, arecleaned. Second, the active molecular groups of EDTA are planted on thesurface of wing scales to chelate the metallic ions of Au3+that is subsequentlyreduced with the aqueous solution of NaBH4to gain Au atom activated wings.Last, a thin layer of magnetic metal Co down to nanometer scale is coated onthe wing scales through chemical situ synthesis in the CoSO4electrolessplating solution. The metallic wing scales preserve the3D periodicsub-micrometer-level hierarchical structures from the original ones perfectlyand inherit the structural colors and iridescence as to some extent, whichmeans that the metallic wing scales possess the characteristics of bothmagnetism and iridescence.Single magnetic wing scale plated with metal Co is easy to magnetizeand demagnetize because of its weak coercivity, which indicates that themetallic scale could be controlled easily by the external magnetic field. Theresponse of single scale coated with Co to magnetic field is synchronous withthe building of external magnetic field. When exposed to magnetic field, thesingle metallic scale can respond to magnetic field immediately standing onthe Si wafer to keep its angle and direction in accord with the externalmagnetic field. While retreating the magnetic field, the metallic scale thengets back to its initial status lying on the surface of Si wafer. Betweenstanding and lying, the single magnetic scale would appear in different colors as well as show different reflection spectra. Except for the synchronization,the response process could be repeated many times. Based on the iridescence,synchronization and repeatability, the single metallic wing scale is the keypoint in designing the absolutely new device—the magnetism-light-switch. Inaddition, the single wing scale plated with Co can change its positioncontinuously in consistence with the moving external magnetic field, whichcan be used as the mechanism of color variation controlled by magnetic fieldif a lot of scales can array like the way on the original wing surface. Althoughthese devices are still in their designing stage, they have tremendous potentialapplication value in color-change, light-electricity-switch, information storage,paint, camouflage, light absorption. What we have done opens a window tothe structural colors and iridescence existing in the nature widely.