Colors in the natural world mainly come from pigments, or through light passing through an object that breaks down visible light into a spectrum. Colors are also generated from nanostructures. The most common color generated from a nanostructure is blue. Examples in nature include the blue tarantula or peacock feathers.
Colors are created with tarantulas and peacocks from nano-sized structures that allow reflected light waves to overlap (these are termed photonic structures). This light bending effect leads to dynamic color effects.
A research group have been examining color patterns and nanostructures in an attempt to replicate the effects in the laboratory. Reporting on a breakthrough, researchers working at the Karlsruhe Institute of Technology have managed to replicate nanostructures that generate the same color irrespective of the viewing angle.
The research, Controlled Environments reports, has a potential practical application. With pigments, although a variety of colors can be created, the pigments are often toxic. In contrast, colors created through nanostructures are often non-toxic (depending on the materials selected). In addition, the colors created are often more intense, vibrant and longer-lasting.
The downside with artificially nanostructured colors is that the perception of the color and its degree of intensity alters depending upon the viewing angle. This can be seen by looking at the underside of CD, holding it up to the light and changing the viewing angle (that is the color is iridescent). This effect doesn’t happen in nature. If you look at a kingfisher, the bird’s feathers appear blue irrespective of the viewing angle.
The difference comes down to the structure and the lack of regularity with nanostructures in nature. The problem in replicating this is that most industrial processes are designed to produce an object in a regular way (to obtain consistency in production, for example).
The Karlsruhe Institute of Technology research team, led by Dr. Radwanul Hasan Siddique, has been studying the blue tarantula. One thing the researchers quickly realized was that the blue tarantula always appears blue, as perceived from any angle, despite it having hairs on its legs. This puzzled the researchers, since the hairs should lead to iridescence, rather than an even blue hue.
The cobalt blue tarantula, scientific name Haplopelma lividum, is noted for its blue legs. It is a fast spider and very defensive, able to produce potent venom. The reason for the uniform blue-color, Dr. Siddique found, was due to the pattern of the hairs. These resemble a multi-layered, flower-like structure.
Studying this closer, the scientists looked at the reflection behavior of the hairs and ran these through a computer program. Using this information, the researchers constructed replicas using special nano-3D printers.
The results of this led to the generation of a flower-like structure that could produce the color over a viewing angle of 160 degrees. Although not providing a complete uniformity of color from any direction, this represents the largest viewing angle of any synthetic structural color achieved to date. Key to this success was getting the appropriate multi-layered structure and rotational symmetry, together with a hierarchical structure that ensured homogeneous reflection intensity. These factors combined prevented any color changes over the viewing angle.
The researchers also found that where the size of the flower structure, the color generated could be adjusted. Developing this further, the researchers hope that structural colorants can replace the toxic pigments currently used in clothes, as well as cosmetics. The main obstacle, and the one the researchers are working on, is how to scale-up the process so it will work on an industrial scale.
The research has been published in the journal Advanced Optical Materials, and the paper is headed “Tarantula-Inspired Noniridescent Photonics with Long-Range Order.”
This article is part of Digital Journal’s regular Essential Science columns. Each week Tim Sandle explores a topical and important scientific issue. Last week we profiled new technology where surgeons are using ultrasound to monitor for cancer. The previous week examined whether fungi became more pathogenic in space, and if this phenomenon posed a risk to astronauts.
