It’s a question that has baffled scientists for decades. How does one create true colours that are eco-friendly, do not fade over time and are not subject to viewing angle distortions? The humble snout weevil, native to the Philippines, could decode the puzzle — with help from a scientist of Indian origin and other researchers.
The weevil, whose scientific name is Pachyrrhynchus congestus pavonius , has achieved celebrity status among biophysicists and optical engineers across the world, after a recent discovery by Yale-NUS College (Singapore) Assistant Professor of Science Vinod Kumar Saranathan and Dr Bodo D Wilts from the Adolphe Merkle Institute at the University of Fribourg, Switzerland.
Saranathan, originally from Trichy in Tamil Nadu, became interested in the mechanisms of colour creation in the natural world because of a lifelong fascination with ornithology, the study of birds, and his undergraduate training in physics. As he delved further into this esoteric and highly specialised sub-field of biophysics, he discovered that insects — the most diverse group of organisms on our planet — may hold the key to understanding how vivid colours can appear in nature even in the absence of pigments.
He painstakingly went through hundreds of drawers of insect specimens in several natural history museums, including those at the University of Oxford and Yale University, and then shortlisted 120 species from a wide variety of insect families for his research. Saranathan, who did his bachelors in physics from Ohio Wesleyan University, US, and his MS in 2007 and a PhD in 2011 from Yale University, was a Royal Society Newton Fellow at the Edward Grey Institute of Field Ornithology, Department of Zoology, Oxford.
Examining these 120 species, Saranathan and his colleagues discovered that some insects possess crystal structures at a size and scale that allow light to interact with them in a precise and predictable fashion. These crystal nanostructures, which are 500 times smaller than the width of a human hair, are essentially the building blocks of such insects’ scales. “They are like Lego blocks,” Saranathan says, and explains that they repeat within an insect’s scale cells at periodicities close to the wavelengths of the colours that constitute light. The result is that when white light strikes these scale cells, they reflect specific colours.
As the reflected light varies in colour according to the size and composition of the crystals, insects bearing such nanostructures are able to display a stunning array of vivid colours. Scale cells with larger crystal structures reflect red light and its hues. The most striking demonstration of this phenomenon is offered in the snout weevil, which presents nearly all seven colours of the rainbow in dazzling detail, which is why it is also called the rainbow weevil.
Unlike colours produced in nature by pigments — in flowers, for instance — the hues obtained in the presence of crystalline nanostructures are well-defined.
“Generally in insects, the animal displays different facets of the same crystal nanostructure side-by-side, to produce an overall mixed colour,” says Saranathan. It is like a pointillist painting, he explains, referring to the form of art in which images are composed of small and distinct dots of colour.
“However, in the rainbow weevil, every scale is tuned to make one colour by varying both the sizes of the Lego block and the volume of [constituent] materials inside every such building block,” he says. This colour-generation mechanism also allows the weevil and similar insects to display complex hues of blue and green — shades which are rarely achieved in nature through pigments.
Currently, Saranathan and his group are studying the mechanism of growth of these nanocrystals. Their eventual hope is that it will be possible to mimic these nanostructures and their allied colour generation abilities in the lab. Devising a way to do so would pave the way for myriad real-world applications in a spectrum of industries ranging from cosmetics to fibre optics. A phone or television screen with such technology would display true colour from any viewing angle.
These nanostructures can be used to improve the efficiency of fibre-optic transmissions and of solar cells. As the light reflected by these crystals change colour in the presence of foreign particles and chemicals, they can also be used as sensors to determine the presence of toxins or pollutants in the environment. The discovery could help the cosmetics and fashion industries create vivid colours that do not fade.
“Our team is working to figure out with renewed vigour the exact mechanism by which insects grow these crystal nanostructures within their cells,” says Saranathan, whose team was recently awarded a grant from the National Research Foundation in the Prime Minister’s Office of Singapore. It will, however, be at least a decade before a prototype is developed that mimics the exact method that insects use, he says.
Unlike colours that come from pigments, the hues displayed by the interference of light in crystalline nanostructures do not fade with time. This reduces waste. Moreover, these nanostructures are also biodegradable, which means colour generation can be eco-friendly. Rather than a pot of gold, a cleaner, greener planet is what Saranathan and his team hope to find, through their efforts, at the end of their rainbow.
Amrita V Nair is a freelance writer and public policy specialist