Sunday, 20 May 2012

Lecture : Butterflies and Battleships

Nottingham Trent University held a research conference recently which featured a talk by Prof Roy Sambles that had the rather cryptic title “Electromagnetic surfaces, from butterflies to battleships”.

Prof Sambles is a lecturer and researcher at the University of Exeter and has had a long and productive career investigating the interaction of light with materials, notching up some 400 publications and getting elected as a fellow of the Royal Society along the way.

Prof Sambles, who incidentally is of such a height that you wonder whether he might have been just as successful in the NBA, began his talk with an explanation of why butterflies have such colourful wings. The structural coloration of butterfly wings is created by minute scales and whilst blacks and browns are the result of pigmentation, the blues, greens, reds and iridescence are usually caused by a complex microstructure in the scale which causes the light to be scattered and reflected in a particular way. Examples of these exquisite structures can be found at the webpage of Prof Sambles research group and include the bright blue iridescence of the Morpho Retenor butterfly, which can be seen. You can find some further incredible imagery in this paper from the University of Southampton. Although not mentioned in the talk, a flavour of the kind of structures described can be seen in the following, increasingly magnified, images from the wing of a European Peacock butterfly (Aglais io)


Part of wing (x50)

Single scale ( x1000)

Detail of scale (x5000)

Prof Sambles also talked about the “flicker” iridescence of the Ancyluris Meliboeus butterfly, again showing an incredible level of detail in the wing scale microsctructure. The male of this species has an iridescence that is only visible over a short range of angles, so a slowly beating wing will appear to suddenly flash on and off again. The iridescent pattern is underneath the wing, probably because the males are in the foliage of the forest and wish to signal to the females below them in the leafmould. You can read about this effect here.

Another example given was that of Papilio Palinurus, which has a beautiful velvety green colour to its wings. This turns out to be the result of dimples in the scales - the centres of the dimples reflect only yellow light, whilst the edges bounce light around to reflect only blue light. And it is this combnination of yellow and blew that gives the butterfly its characteristics green colour.

You can see images of the incredibly detailed scales of these insects in this paper and, especially, in this Nature review article

After giving some more examples of clever lighting trickery in butterflies, Prof Sambles moved on to talking about the synthetic structures his group had been working on to investigate how to manipulate light and other electromagnetic waves using some of the techniques from nature.

Some everyday examples already in production are anti-counterfeiting images and marks on credit cards, banknotes etc.

Whilst future developments cover a whole range of applications ranging from clever wallpaper that allows mobile signals to come in whilst stopping wi-fi getting out; to stealth technology for military applications.

Regarding this latter application, Prof Sambles pointed out that the clever structures on the surfaces of butterfly wings operated at the scale of several wavelengths. For similar techniques to work with microwaves, say, structures with a thickness of a few centimetres would be required - which is a bit impractical for a coating or a wallpaper.

Or so everyone thought until the Prof's research group found out that they could get the same effect by changing the alignment of the structures, as described in this paper. Critically, this kind of technology allows millimetre sized waves to be absorbed by micrometre thick structurs

The Prof discussed this area in some detail. Unfortunately, pretty much all of went over NSB’s head, so if you have the maths and are interested, you can find out more at Prof Sambles list of publications. Sorry. Oh, you'll be wondering about the Battleships - that was covered in passing by mentioning the Sea Shadow, which, of course has a need to absorb incoming microwave radar and is one of the possible applications for the innovative metallic structures that Prof Sambles is researching.

If you liked this post, why not check out some of the other interveiws and event reports by clicking on the relevant tab at the top of the page.
Sea Shadow

1 comment:

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