Brightening the Wintertime Blues

[MexicoDoug at aol.com]

Nothing like a nice article in Nature to brighten up the wintertime blues!  
Saludos, Doug (México)

http://www.nature.com/nsu/040126/040126-4.html#b1

Butterflies boast ultrablack wings

Insects use optical trick to get the blackest black out of dark pigments. 
28 January 2004 MICHAEL HOPKIN 

'Ultrablack' is the new black - if you're a butterfly. The insects use 
optical trickery to make dark pigment in their wings look even darker, a new study 
has revealed1. Other researchers have already used a similar trick to make 
blacker-than-black surfaces.

The male mountain blue don (Papilio ulysses), a common Australian butterfly, 
has striking blue patches on its wings framed by black regions. The blacker 
the black, the more the blue stands out as a signal to rival males.

Experts thought this was simply down to a dark pigment that absorbs most of 
the incoming light. Not so, say Pete Vukusic of the University of Exeter, UK, 
and his colleagues. The pigment-containing scales also have a physical 
structure that 'traps' light, making it more likely to be absorbed.

Many animals use physics to achieve colour effects that cannot be obtained 
with pigment alone. The peacock, for example, owes its greenish glint to 
'constructive interference' - the arrangement of tiny proteins in its feathers cause 
light to be reflected in such a way that unwanted wavelengths are cancelled 
out.

Vukusic's team set out to see whether structural features can suppress colour 
as well as create it.

They found that the black parts of the butterfly's wing are constructed of 
scales covered in pits less than a micrometre across, which make a honey-comb 
like pattern. These scales take advantage of refraction to trap light, much like 
a fibre-optic cable. 

Super Black

When light passes between two substances with different 'refractive indices' 
- from water into air, say - some of it becomes trapped in the denser medium. 
Water has a refractive index of about 1.3 relative to air; the butterfly's 
wing tissue has an reflective index of about 1.6 relative to air. This maximizes 
the chance that light will be absorbed by the pigment, Vukusic explains.

The researchers tested this by immersing the wings in a chemical bath of 
bromoform, which has roughly the same reflective index as the wing tissue. "We 
effectively removed the effect of the scales' structure," says Vukusic.

As expected, the scales looked less black when their structure was cancelled 
out. In air they absorbed more than 90% of light; in bromoform the figure 
dropped to just over 50%.

So is 'ultrablack' destined to be next season's hottest colour? Perhaps so, 
says Richard Brown, a chemical engineer at Britain's National Physical 
Laboratory in Teddington, near London. In 2002 he unveiled 'Super Black', a pitted 
nickel-phosphorus alloy coating that achieves a similar effect to the scales of 
P. ulysses.

Super Black will be most useful in the construction of optical instruments, 
Brown predicts. "But we've also had interest from artists who are not satisfied 
with the blackness of their paints," he adds.

The coating absorbs 99.7% of incident light, making it more efficient than P. 
ulysses. But the fact that butterflies got there first shows the value of 
animals as a source of engineering advice, says Vukusic. "Biomimetics is growing 
in popularity," he says. "Wherever we can we should take cues from nature."

    
    
References

Vukusic, P., Sambles, J. R. & Lawrence, C. R. Structurally assisted blackness 
in butterfly scales. Proceedings of the Royal Society of London B, published 
online, doi:10.1098/rsbl.2003.0150 (2004). |Article|     

© Nature News Service / Macmillan Magazines Ltd 2004
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