Color that never fades or Structural Color Effect

by admin on September 13, 2012

University of Cambridge researchers Silvia Vignolini and Beverley Glover have determined that the powerful, non-fading “structural color” of a berry called Pollia condensata could have widespread applications in a variety of industries. Food dyes, makeup, car paint, colored security features in currency, and many other products could be enhanced by this discovery.

Read More about the fruit that retains its color after 100 years here

Though it is an aggregate berry like a blackberry, the Pollia fruit is smaller than a blueberry. Its iridescent blue skin reflects roughly 30 percent of the light that hits it, which makes it the most intensely colored living thing known to science. (The morpho butterfly and scarab beetle are the Pollia’s closest rivals.) The Pollia does not contain colored cells; instead, its cells are coiled in a twist and form sheets. When sunlight filters down through these layers of cellulose, the vast majority of the cells reflect only the blue wavelengths. A few cells reflect other colors, which gives the fruit its characteristic shimmer.

This effect, known as structural color and also found in peacock feathers and butterfly wings, could replace pigments in many consumer products. Samples of the fruit from the 19th century remain as brightly and intensely colored as those picked in 2012. The biological foundation of most green plants, cellulose is very common in nature and thus inexpensive to obtain and process. Its astonishing brightness, low cost, and non-toxicity could make it an effective replacement for conventional pigments.

The scientists  discovered  each individual cell generates colour independently, producing a pixelated or pointillist effect (like those in the paintings of Seurat). This colour is produced by the reflection of light of particular wavelengths from layers of cellulose in the cell wall. The thickness of the layers determines which wavelength of light is reflected. As a result, some cells have thinner layers and reflect blue; others have thicker layers and reflect green or red.

Pollia berries have no nutritional value; birds carry them to their nests as decorations, which may help them attract mates. This process allows Pollia seeds to spread and the fruit to perpetuate itself. Unlike traditional pigments, which begin to fade after absorbing a certain amount of light, the Pollia fruit’s structural color lasts indefinitely.

The paper and cosmetics industries have already employed structural color in some of their products. L’Oreal’s pigment-free makeup is based on butterfly research in which specimens from the 18th century remained as vibrantly colored as newly hatched butterflies. In addition, BMW and other car companies have created iridescent paint based on the principles of structural color.

This From MSNBC:
LONDON — Scientists have found nature’s way of creating color that never fades, a technique they say could replace pigments used in industry with natural plant extracts in products from food coloring to security features in banknotes.
Layers of cellulose that reflect specific wavelengths of light — “structural color” found in peacock feathers, scarab beetles and butterflies — make a particularly intense blue in the Pollia condensata plant, scientists say.
Samples of the fruit in plant collections dating back to the 19th century had not lost any shine or intensity, they found.
“By taking inspiration from nature, it is possible to obtain smart multifunctional materials using sustainable routes with abundant and cheap materials like cellulose,” said University of Cambridge physicist Silvia Vignolini.
“It is 10 times more intense and bright than any color achieved with a pigment,” said Vignolini, who led the study with plant scientist Beverley Glover.
Although the fruit has no nutritional value, birds were attracted by its bright color, possibly as a decoration for their nests or to impress mates, helping in seed dispersal.
“This obscure little plant has hit on a fantastic way of making an irresistible, shiny, sparkly, multi-colored, iridescent signal to every bird in the vicinity, without wasting any of its precious photosynthetic reserves on bird food,” said Glover.
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And, unlike pigments, structural color does not fade over time as it is not broken down by absorbing light.
“Edible, cellulose-based nanostructures with structural color can be used as substitutes for toxic dyes and colorants in food,” said Vignolini. The paper industry is already set up to extract and use cellulose and its processes could also be adapted for security labeling or cosmetics, she said.
“Cellulose-based structures have a really strong optical response and are completely inert in the human body,” she said.
Another advantage of the technique is that the desired color can be achieved by adding layers in the structure to reflect different wavelengths, rather than buying new pigment.
Similar research by Peter Vukusic at Exeter University into the structure that creates color in butterfly wings has spawned a pigment-free photonic make-up from French cosmetics company L’Oreal.
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“I saw how brilliant optically some of these structural colors are,” Vukusic told Reuters. “Some species collected as far back as the 18th century are as bright today as freshly hatched butterflies.”
Although some car companies, including BMW, have also exploited the phenomenon to produce iridescent paint that changes color when viewed from different angles, “this is nothing compared to what you see in nature,” Vukusic said.
He said that if production challenges can be overcome, the abundance of cellulose — the basis of most green plant material — makes it a material with great potential.
The research was published Monday in the Proceedings of the National Academy of Sciences.

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