Butterflies Prevent Forgery?

The iridescence seen in butterflies and other animals is, without a doubt, one of the most beautiful optical natural phenomena. Being able to effectively mimic it has a whole host of applications, particularly in the printing industry where many believe that these complex optical effects could be incorporated into banknotes and credit cards, making them more difficult to forge.

The iridescence of butterfly wings works on the same principle as the effect that is seen on soap bubbles, oil spills and various gemstones. Rather than being caused by pigments, the way in which the colour on a surface changes depending on the angle it is viewed from relies on multiple reflections of light due to multiple structures on the micro-scale and nano-scale, hence the term structural colour. The effect is seen in many different butterflies and it is believed to have evolved independently in each species, suggesting it has a clear evolutionary advantage. In the example of the Indonesian butterfly species, Papilio blumei, iridescence is caused by microscopic dimples, or "concavities" on the surface of the wing. Here, the juxtaposition of colour is caused by light reflecting from different parts of the microstructure; light reflected at the edges of the concavities is blue and light from the centre is yellow-green.

This allows the butterfly to pull off a clever evolutionary trick. Because different species perceive colour in different ways, the butterfly can be highly visible to a potential mate while being effectively camouflaged to predators. Similarly, to the naked eye, the blue and yellow-green colours are both present but viewing the surface using the right optical equipment extinguishes the yellow-green part of the colour, providing a simple but effective method for checking whether a certain micro-structure is present.

A number of different groups have been working on replicating this effect in the lab. Most notably, scientists at the University of Cambridge last year succeeded in mimicking the Papilio blumei micro-structure using a combination of 5 techniques, including self-assembly of micro-particles and using a technique called atomic layer deposition to coat the sample with several thin alternating layers of different materials.
Being able to achieve this kind biomimicry in the lab is an important first step is rolling out such a technique for printing applications. While the relative complexity of the microstructure which causes the iridescence has been a barrier to mimicking it, this also has a clear advantage, as the harder an effect is to achieve, the harder it will be for criminal forgers to replicate it. And while it's inevitable that the complexities of forgeries will advance with the complexities of the genuine article, knowing that a banknote relies on mastering the art of atomic layer deposition will give no doubt lend some reassurance to banknote issuers in the meantime.

References: Mimicking the colourful wing scale structure of the Papilio blumei butterfly, M. Kolle et al, Nature Nanotechnology 5, 511-515 (2010)