A team of researchers from the University of Cambridge has shown why intense, pure red colors in nature are mainly produced by pigments, instead of the structural color that produces bright blue and green hues.
The team used a numerical experiment to determine the limits of matt structural color and found that it extends only as far as blue and green in the visible spectrum.
The results of the study appeared recently in the PNAS journal.
What is Structural Color?
Structural color is something which is not caused by pigments or dyes, but internal structure alone. The appearance of the color, whether matt or iridescent depends on how the structures are arranged at the nanoscale. Like
1. If it is ordered, or crystalline, the structure would result in iridescent colors i.e shows luminous colors that seem to change when seen from different angles.
2. If it is disordered, or correlated, the structures result in angle-independent matt colors i.e look the same from any viewing angle.
A point to note is that these structural colors do not fade. These angle-independent matt colors would be highly useful for applications such as paints or coatings, where metallic effects are not wanted.
“In addition to their intensity and resistance to fading, a matt paint which uses structural color would also be far more environmentally-friendly, as toxic dyes and pigments would not be needed,” said first author Gianni Jacucci from Cambridge’s Department of Chemistry. “However, we first need to understand what the limitations are for recreating these types of colors before any commercial applications are possible.”
“Most of the examples of structural color in nature are iridescent – so far, examples of naturally-occurring matt structural color only exist in blue or green hues,” said co-author Lukas Schertel. “When we’ve tried to artificially recreate matt structural color for reds or oranges, we end up with a poor-quality result, both in terms of saturation and color purity.”
The team based in the lab of Dr. Silvia Vignolini used numerical modeling to determine the limitations of creating saturated, pure, and matt red structural color. They modeled the optical response and color appearance of nanostructures, as found in the natural world.
The results showed that saturated, matt structural colors cannot be recreated in the red region of the visible spectrum, which might explain the absence of these hues in natural systems.
“Because of the complex interplay between single scattering and multiple scattering, and contributions from correlated scattering, we found that in addition to red, yellow and orange can also hardly be reached,” said Dr. Silvia Vignolini.
Gianni Jacucci et al, The limitations of extending nature’s color palette in correlated, disordered systems, Proceedings of the National Academy of Sciences (2020). DOI: 10.1073/pnas.2010486117
Press Release: Cambridge University