Metallica's nano color

Many living things contain pigment molecules that bring color to their world, but many also exploit structural colors instead. The dazzling display of a blue jay's feathers, for instance, produce their remarkable patterns and colors through the nanoscopic characteristic of the feathers. Similarly, the blue tarantula, Poecilotheria metallica, needs no pigment to reflect light waves and creates its wonderful patina. While many such structural colors are fixed, many whether found in or on animal, vegetable or mineral are iridescent and so shift in color depending on the angle of observation; this has been a problem for designers hoping to utilize the non-toxic, vibrant and durable nature of structural colors.

Now, scientists at The University of Akron, Ohio, the University of Nebraska-Lincoln, Karlsruhe Institute of Technology (KIT), Germany, and Ghent University in Belgium, have succeeded in engineering nanostructures to display the same color regardless of viewing angle as is the case with the blue jay's feathers and the tarantula's metallic hue. [TA Blackledge et al, Adv Opt Mater (2016); DOI: 10.1002/adom.201600599]. The discovery could have implications for a wide range of industries including, textiles, packaging, and cosmetic industries, as well as in the world of art and beyond.

The key characteristic of many natural structural colors that is different from industrially produced materials is that nature uses amorphous or irregular structures, which always give the same color whereas synthetic structures are commonly regular or patterned and so iridescent.

The researchers have found that the blue tarantula has periodic structures on its hairs but despite this does not display iridescence. On closer examination, they could see that the hairs are multi-layered, and have a flower-like structure. Computer modeling of these structures allowed the team to analyze the reflection behavior but then using a nano-3D printer they could make real-life models to corroborate their simulations. They were thus able to print a flower-like nanostructure that exhibits the same color over a viewing angle of 160 degrees. This is the largest viewing angle of any synthetic structural color so far achieved.

It is the hierarchical nature of the structure from micro- to nano-scales that gives it homogeneous reflection intensity and precludes the color shift when viewed at different angles. Moreover, by printing a different sized "flower", the team was able to adjust the color displayed, which would make it particularly interesting for industrial applications. "This could be a key first step towards a future where structural colorants replace the toxic pigments currently used in textile, packaging, and cosmetic industries," explains Akron's Bor-Kai Hsiung. He suggests that one of the first applications might be in colorful textiles manufactured without toxic dyes and their waste products.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".