Beetles, butterflies and birds are masters in fantastic bright colours and remarkable metallic reflections. Similar effects are now becoming possible in man-made materials.
Many insects and birds have surprisingly sharp and bright colours and some even have metallic looking appearances. The deep blue Morpho butterfly sends sharp flashes when it flaps its wings – probably to attract the attention of potential mates. Many of the scarab beetles look like jewels with metallic look and intense colours.
The appearance is strange since one normally expects such small insects to camouflage themselves in order not to catch the attention of hunting predators. But in the dark tropical forests the reflections actually confuse the hunters and give the beetles a competitive advantage. Peacocks are known for their impressive tail feathers with structural colours. Here the biological explanation is suggested to be that female peacocks select the males with the biggest and most impractical feathers – if they can survive with this disadvantage they must have good genes.
Traditional colours as we know them from paint and crayons come from pigments that absorb some of the colours. When we see a yellow colour it is because the pigments absorb the red, blue and green colour and therefore only reflect some of the light. The so-called structural colours work differently, which makes it possible to reflect the sharp bright colours and the metal look. The colours come from delicate micro- and nano-structures that reflect the light but also change it using physical effects like interference and diffraction.
One type of structure is a thin layer of transparent material that reflects light from both top and bottom surfaces. Light reflected from lower surfaces interfere with light reflected from the higher surface and this causes some of the colours to be boosted. The thickness of the layer determines the colour.
Thin layers give colours in the blue end of spectrum and by increasing the layer thickness the rest of the spectral colours can be made. Such layers can be produced industrially on metallic surfaces like building tiles and cutlery. Apart from the bright appearance the structural coloured surfaces have other advantages. Often it is possible to achieve more durable and long lasting colours that do not fade. The thin layers applied to stainless steel are chromium-oxides, which is very hard and therefore also have good wear-properties.
The structural colours in thin layers can also be made using polymers. Research results from Sheffield university shows that all the spectral colours can be made by changing the proportions between the to components in a di-block copolymer.
The alignment in thin layers is achieved by sliding the copolymer between two pieces of glass. However this type of material is still at the research state and is not commercially available yet.
The metallic appearance can be achieved from non-metallic materials like polymers by using several transparent layers of two different materials. Since a certain thickness boost a single colour a range of thicknesses will reflect the whole spectrum, which causes the metallic look. The metallic looking film from 3M Coorporation has about 200 layers of two different polymers.
Another nanostructure that gives structural colours is found in the so-called cholesteric liquid crystals. Here are microscopic bi-refringent fibres arranged in spiral structures, which also causes the light to interfere and produce colours. An additional effect in this type of reflection is the iridescence, which means that the colour changes depending on the viewing angle.
Within scientific research there is more and more attention on structural colours as a means to achieve not only colours and metallic appearances but also a broad range visually aesthetic properties.
About Torben Lenau
Torben is an Associate Professor, PhD at DTU, Denmark. He has many years of experience in working with both new and old materials and how to include them in the design process.
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About the Hello Materials exhibition
Experience fascinating examples of present and future materials and gain an insight into what they will mean to society and the individual. Visit the exhibition between the 2nd of April and the 10th of June 2012.
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