I know not everyone is a huge fan of spiders, but I think this guy is pretty cute: Photo credit Jean and Fred, Flickr. [CC-BY 2.0] He’s a Peacock Spider (Maratus spp.) and his way of impressing the ladies is to do a little dance, showing off his brightly coloured backside: As he moves, notice how the colours change all the way from purple to blue through to yellow and red. In fact, this little spider can show off all colours of the rainbow, depending on which way you look at him. Iridescence in nature is not uncommon – think of butterflies, peacocks and pigeons – but usually it’s over a very limited range of colours, like a blue that shifts from bluey-green to purple. It turns out this little spider is incredibly unique because he’s able to display every colour in the visible spectrum. So, how do these spiders make their rainbows? In a recent paper published in the journal Nature, researchers set out to answer just that. They used a range of imaging and probing techniques including electron microscopy and optical modelling to figure out how the spider scales display such beautiful colours. To understand iridescence we need to think about light as waves that can interact and interfere with one another. If two waves meet so their peaks line up, the peaks will add together and amplify the wave, making it twice as big. This is called constructive interference. On the other hand, if a peak meets a trough it will cancel out to nothing, which is called destructive interference. Iridescence can be caused by reflecting light multiple times over several semi-transparent layers, or by using a diffraction grating which splits incoming light into several beams travelling in different directions. In both of these cases it is constructive interference that creates iridescence. The researchers found the spiders have a microscopic 3D surface that is curved a bit like an aerofoil. Over this surface they have a nanoscale diffraction grating. The interaction between the grating and the curved surface means that light hitting the scale is separated out into its individual colours resulting in a beautiful rainbow.
The researchers then used nanoscale 3D printing to try and mimic the spider’s intricate structures in order to confirm their hypothesis. Our current technologies are not capable of resolving and separating white light into individual colours at short distances, but inspiration from these spiders’ scales could help improve optical technologies. In particular it could reduce the size of spectrometers used in space missions, or even lead to wearable chemical detectors. Who knows, at some point in the future we could all be using spider-inspired technology!
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March 2018
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