Tag Archives: structural color

Chameleon-like artificial skin

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A March 12, 2015 news item on phys.org describes artificial skin inspired by chameleons,

Borrowing a trick from nature, engineers from the University of California at Berkeley have created an incredibly thin, chameleon-like material that can be made to change color—on demand—by simply applying a minute amount of force.

This new material-of-many-colors offers intriguing possibilities for an entirely new class of display technologies, color-shifting camouflage, and sensors that can detect otherwise imperceptible defects in buildings, bridges, and aircraft.

“This is the first time anybody has made a flexible chameleon-like skin that can change color simply by flexing it,” said Connie J. Chang-Hasnain, a member of the Berkeley team and co-author on a paper published today in Optica, The Optical Society’s (OSA) new journal.

A March 12, 2015 OSA news release (also on EurekAlert), which originated the news item, provides more information about this structural color project,

The colors we typically see in paints, fabrics, and other natural substances occur when white, broad spectrum light strikes their surfaces. The unique chemical composition of each surface then absorbs various bands, or wavelengths of light. Those that aren’t absorbed are reflected back, with shorter wavelengths giving objects a blue hue and longer wavelengths appearing redder and the entire rainbow of possible combinations in between. Changing the color of a surface, such as the leaves on the trees in autumn, requires a change in chemical make-up.

Recently, engineers and scientists have been exploring another approach, one that would create designer colors without the use of chemical dyes and pigments. Rather than controlling the chemical composition of a material, it’s possible to control the surface features on the tiniest of scales so they interact and reflect particular wavelengths of light. This type of “structural color” is much less common in nature, but is used by some butterflies and beetles to create a particularly iridescent display of color.

Controlling light with structures rather than traditional optics is not new. In astronomy, for example, evenly spaced slits known as diffraction gratings are routinely used to direct light and spread it into its component colors. Efforts to control color with this technique, however, have proved impractical because the optical losses are simply too great.

The authors of the Optica paper applied a similar principle, though with a radically different design, to achieve the color control they were looking for. In place of slits cut into a film they instead etched rows of ridges onto a single, thin layer of silicon. Rather than spreading the light into a complete rainbow, however, these ridges — or bars — reflect a very specific wavelength of light. By “tuning” the spaces between the bars, it’s possible to select the specific color to be reflected. Unlike the slits in a diffraction grating, however, the silicon bars were extremely efficient and readily reflected the frequency of light they were tuned to.

Fascinatingly, the reflected colors can be selected (from the news release),

Since the spacing, or period, of the bars is the key to controlling the color they reflect, the researchers realized it would be possible to subtly shift the period — and therefore the color — by flexing or bending the material.

“If you have a surface with very precise structures, spaced so they can interact with a specific wavelength of light, you can change its properties and how it interacts with light by changing its dimensions,” said Chang-Hasnain.

Earlier efforts to develop a flexible, color shifting surface fell short on a number of fronts. Metallic surfaces, which are easy to etch, were inefficient, reflecting only a portion of the light they received. Other surfaces were too thick, limiting their applications, or too rigid, preventing them from being flexed with sufficient control.

The Berkeley researchers were able to overcome both these hurdles by forming their grating bars using a semiconductor layer of silicon approximately 120 nanometers thick. Its flexibility was imparted by embedding the silicon bars into a flexible layer of silicone. As the silicone was bent or flexed, the period of the grating spacings responded in kind.

The semiconductor material also allowed the team to create a skin that was incredibly thin, perfectly flat, and easy to manufacture with the desired surface properties. This produces materials that reflect precise and very pure colors and that are highly efficient, reflecting up to 83 percent of the incoming light.

Their initial design, subjected to a change in period of a mere 25 nanometers, created brilliant colors that could be shifted from green to yellow, orange, and red – across a 39-nanometer range of wavelengths. Future designs, the researchers believe, could cover a wider range of colors and reflect light with even greater efficiency.

Here’s a link to and a citation for the paper,

Flexible photonic metastructures for tunable coloration by Li Zhu, Jonas Kapraun, James Ferrara, and Connie J. Chang-Hasnain. Optica, Vol. 2, Issue 3, pp. 255-258 (2015)
http://dx.doi.org/10.1364/OPTICA.2.000255

This paper is open access (for now at least).

Final note: I recently wrote about research into how real chameleons are able to effect colour changes in a March 16, 2015 post.

Chameleons (male panther chameleons in particular)—colour revelation

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Caption: These are male panther chameleons (Furcifer pardalis) photographed in Madagascar. Credit: © Michel Milinkovitch

Caption: These are male panther chameleons (Furcifer pardalis) photographed in Madagascar.
Credit: © Michel Milinkovitch

Researchers at Switzerland’s University of Geneva/Université de Genève (UNIGE) have revealed the mechanisms (note the plural) by which chameleons change their colour. From a March 10, 2015 news item on phys.org,

Many chameleons have the remarkable ability to exhibit complex and rapid color changes during social interactions. A collaboration of scientists within the Sections of Biology and Physics of the Faculty of Science from the University of Geneva (UNIGE), Switzerland, unveils the mechanisms that regulate this phenomenon.

In a study published in Nature Communications [March 10, 2015], the team led by professors Michel Milinkovitch and Dirk van der Marel demonstrates that the changes take place via the active tuning of a lattice of nanocrystals present in a superficial layer of dermal cells called iridophores. The researchers also reveal the existence of a deeper population of iridophores with larger and less ordered crystals that reflect the infrared light. The organisation of iridophores into two superimposed layers constitutes an evolutionary novelty and it allows the chameleons to rapidly shift between efficient camouflage and spectacular display, while providing passive thermal protection.

Male chameleons are popular for their ability to change colorful adornments depending on their behaviour. If the mechanisms responsible for a transformation towards a darker skin are known, those that regulate the transition from a lively color to another vivid hue remained mysterious. Some species, such as the panther chameleon, are able to carry out such a change within one or two minutes to court a female or face a competing male.

A March 10, 2015 University of Geneva press release on EurekAlert (French language version is here on the university website), which originated the news item, explains the chameleon’s ability as being due to its ability to display structural colour,

Besides brown, red and yellow pigments, chameleons and other reptiles display so-called structural colors. «These colors are generated without pigments, via a physical phenomenon of optical interference. They result from interactions between certain wavelengths and nanoscopic structures, such as tiny crystals present in the skin of the reptiles», says Michel Milinkovitch, professor at the Department of Genetics and Evolution at UNIGE. These nanocrystals are arranged in layers that alternate with cytoplasm, within cells called iridophores. The structure thus formed allows a selective reflection of certain wavelengths, which contributes to the vivid colors of numerous reptiles.

To determine how the transition from one flashy color to another one is carried out in the panther chameleon, the researchers of two laboratories at UNIGE worked hand in hand, combining their expertise in both quantum physics and in evolutionary biology. «We discovered that the animal changes its colors via the active tuning of a lattice of nanocrystals. When the chameleon is calm, the latter are organised into a dense network and reflect the blue wavelengths. In contrast, when excited, it loosens its lattice of nanocrystals, which allows the reflection of other colors, such as yellows or reds», explain the physicist Jérémie Teyssier and the biologist Suzanne Saenko, co-first authors of the article. This constitutes a unique example of an auto-organised intracellular optical system controlled by the chameleon.

The press release goes on to note that the iridophores have another function,

The scientists also demonstrated the existence of a second deeper layer of iridophores. «These cells, which contain larger and less ordered crystals, reflect a substantial proportion of the infrared wavelengths», states Michel Milinkovitch. This forms an excellent protection against the thermal effects of high exposure to sun radiations in low-latitude regions.

The organisation of iridophores in two superimposed layers constitutes an evolutionary novelty: it allows the chameleons to rapidly shift between efficient camouflage and spectacular display, while providing passive thermal protection.

In their future research, the scientists will explore the mechanisms that explain the development of an ordered nanocrystals lattice within iridophores, as well as the molecular and cellular mechanisms that allow chameleons to control the geometry of this lattice.

Here’s a link to and a citation for the paper,

Photonic crystals cause active colour change in chameleons by Jérémie Teyssier, Suzanne V. Saenko, Dirk van der Marel, & Michel C. Milinkovitch. Nature Communications 6, Article number: 6368 doi:10.1038/ncomms7368 Published 10 March 2015

This article is open access.

Nanocellulose and an intensity of structural colour

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I love the topic of structural colour (or color, depending on your spelling preferences) and have covered it many times and in many ways. One of the best pieces I’ve encountered about structural colour (an article by Christina Luiggi for The Scientist provided an overview of structural colour as it’s found in plants and animals) was featured in my Feb. 7, 2013 posting. If you go to my posting, you’ll find a link to Luiggi’s article which I recommend reading in its entirety if you have the time.

As for this latest nanocellulose story, a June 13, 2014 news item on Nanowerk describes University of Cambridge (UK) research into films and structural colour,

Brightly-coloured, iridescent films, made from the same wood pulp that is used to make paper, could potentially substitute traditional toxic pigments in the textile and security industries. The films use the same principle as can be seen in some of the most vivid colours in nature, resulting in colours which do not fade, even after a century.

Some of the brightest and most colourful materials in nature – such as peacock feathers, butterfly wings and opals – get their colour not from pigments, but from their internal structure alone.

Researchers from the University of Cambridge have recreated a similar structure in the lab, resulting in brightly-coloured films which could be used for textile or security applications.

A June 13, 2014 University of Cambridge news release, which originated the news item, describe the phenomenon of structural colour as it applies to cellulose materials,

In plants such as Pollia condensata, striking iridescent and metallic colours are the result of cellulose fibres arranged in spiral stacks, which reflect light at specific wavelengths. [emphasis mine]

Cellulose is made up of long chains of sugar molecules, and is the most abundant biomass material in nature. It can be found in the cells of every plant and is the main compound that gives cell walls their strength.

The news release goes on to provide a brief description of the research,

The researchers used wood pulp, the same material that is used for producing paper, as their starting material. Through manipulating the structure of the cellulose contained in the wood pulp, the researchers were able to fabricate iridescent colour films without using pigments.

To make the films, the researchers extracted cellulose nanocrystals from the wood pulp. When suspended in water, the rod-like nanocrystals spontaneously assemble into nanostructured layers that selectively reflect light of a specific colour. The colour reflected depends on the dimensions of the layers. By varying humidity conditions during the film fabrication, the researchers were able to change the reflected colour and capture the different phases of the colour formation.

Cellulose nanocrystals (CNC) are also known as nanocrystalline cellulose (NCC).

Here’s a link to and a citation for  the paper,

Controlled, Bio-inspired Self-Assembly of Cellulose-Based Chiral Reflectors by Ahu Gumrah Dumanli, Gen Kamita, Jasper Landman, Hanne van der Kooij, Beverley J. Glover, Jeremy J. Baumberg, Ullrich Steiner, and Silvia Vignolini. Optical Materials Article first published online: 30 MAY 2014 DOI: 10.1002/adom.201400112

© 2014 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

While the researchers have supplied an image of the Pollia condensata, I prefer this one, which is also featured in my Feb. 7, 2013 posting,

AGELESS BRILLIANCE: Although the pigment-derived leaf color of this decades-old specimen of the African perennial Pollia condensata has faded, the fruit still maintains its intense metallic-blue iridescence.COURTESY OF P.J. RUDALL [downloaded from http://www.the-scientist.com/?articles.view/articleNo/34200/title/Color-from-Structure/]

AGELESS BRILLIANCE: Although the pigment-derived leaf color of this decades-old specimen of the African perennial Pollia condensata has faded, the fruit still maintains its intense metallic-blue iridescence.COURTESY OF P.J. RUDALL [downloaded from http://www.the-scientist.com/?articles.view/articleNo/34200/title/Color-from-Structure/]

Stunning, non?

Structural color and cephalopods at the University of California Santa Barbara

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I last wrote about structural color in a Feb.7, 2013 posting featuring a marvelous article on the topic by Cristina Luiggi in the The Scientist. As for cephalopods, one of my favourite postings on the topic is a Feb. 1, 2013 posting which features the giant squid, a newly discovered animal of mythical proportions that appears golden in its native habitat in the deep, deep ocean. Happily, there’s a July 25, 2013 news item on Nanowerk which combines structural color and squid,

Color in living organisms can be formed two ways: pigmentation or anatomical structure. Structural colors arise from the physical interaction of light with biological nanostructures. A wide range of organisms possess this ability, but the biological mechanisms underlying the process have been poorly understood.

Two years ago, an interdisciplinary team from UC Santa Barbara [University of California Santa Barbara a.k.a. UCSB] discovered the mechanism by which a neurotransmitter dramatically changes color in the common market squid, Doryteuthis opalescens. That neurotransmitter, acetylcholine, sets in motion a cascade of events that culminate in the addition of phosphate groups to a family of unique proteins called reflectins. This process allows the proteins to condense, driving the animal’s color-changing process.

The July 25, 2013 UC Santa Barbara news release (also on EurekAlert), which originated the news item, provides a good overview of the team’s work to date and the new work occasioning the news release,

Now the researchers have delved deeper to uncover the mechanism responsible for the dramatic changes in color used by such creatures as squids and octopuses. The findings –– published in the Proceedings of the National Academy of Science, in a paper by molecular biology graduate student and lead author Daniel DeMartini and co-authors Daniel V. Krogstad and Daniel E. Morse –– are featured in the current issue of The Scientist.

Structural colors rely exclusively on the density and shape of the material rather than its chemical properties. The latest research from the UCSB team shows that specialized cells in the squid skin called iridocytes contain deep pleats or invaginations of the cell membrane extending deep into the body of the cell. This creates layers or lamellae that operate as a tunable Bragg reflector. Bragg reflectors are named after the British father and son team who more than a century ago discovered how periodic structures reflect light in a very regular and predicable manner.

“We know cephalopods use their tunable iridescence for camouflage so that they can control their transparency or in some cases match the background,” said co-author Daniel E. Morse, Wilcox Professor of Biotechnology in the Department of Molecular, Cellular and Developmental Biology and director of the Marine Biotechnology Center/Marine Science Institute at UCSB.

“They also use it to create confusing patterns that disrupt visual recognition by a predator and to coordinate interactions, especially mating, where they change from one appearance to another,” he added. “Some of the cuttlefish, for example, can go from bright red, which means stay away, to zebra-striped, which is an invitation for mating.”

The researchers created antibodies to bind specifically to the reflectin proteins, which revealed that the reflectins are located exclusively inside the lamellae formed by the folds in the cell membrane. They showed that the cascade of events culminating in the condensation of the reflectins causes the osmotic pressure inside the lamellae to change drastically due to the expulsion of water, which shrinks and dehydrates the lamellae and reduces their thickness and spacing. The movement of water was demonstrated directly using deuterium-labeled heavy water.

When the acetylcholine neurotransmitter is washed away and the cell can recover, the lamellae imbibe water, rehydrating and allowing them to swell to their original thickness. This reversible dehydration and rehydration, shrinking and swelling, changes the thickness and spacing, which, in turn, changes the wavelength of the light that’s reflected, thus “tuning” the color change over the entire visible spectrum.

“This effect of the condensation on the reflectins simultaneously increases the refractive index inside the lamellae,” explained Morse. “Initially, before the proteins are consolidated, the refractive index –– you can think of it as the density –– inside the lamellae and outside, which is really the outside water environment, is the same. There’s no optical difference so there’s no reflection. But when the proteins consolidate, this increases the refractive index so the contrast between the inside and outside suddenly increases, causing the stack of lamellae to become reflective, while at the same time they dehydrate and shrink, which causes color changes. The animal can control the extent to which this happens –– it can pick the color –– and it’s also reversible. The precision of this tuning by regulating the nanoscale dimensions of the lamellae is amazing.”

Another paper by the same team of researchers, published in Journal of the Royal Society Interface, with optical physicist Amitabh Ghoshal as the lead author, conducted a mathematical analysis of the color change and confirmed that the changes in refractive index perfectly correspond to the measurements made with live cells.

A third paper, in press at Journal of Experimental Biology, reports the team’s discovery that female market squid show a set of stripes that can be brightly activated and may function during mating to allow the female to mimic the appearance of the male, thereby reducing the number of mating encounters and aggressive contacts from males. The most significant finding in this study is the discovery of a pair of stripes that switch from being completely transparent to bright white.

“This is the first time that switchable white cells based on the reflectin proteins have been discovered,” Morse noted. “The facts that these cells are switchable by the neurotransmitter acetylcholine, that they contain some of the same reflectin proteins, and that the reflectins are induced to condense to increase the refractive index and trigger the change in reflectance all suggest that they operate by a molecular mechanism fundamentally related to that controlling the tunable color.”

Could these findings one day have practical applications? “In telecommunications we’re moving to more rapid communication carried by light,” said Morse. “We already use optical cables and photonic switches in some of our telecommunications devices. The question is –– and it’s a question at this point –– can we learn from these novel biophotonic mechanisms that have evolved over millions of years of natural selection new approaches to making tunable and switchable photonic materials to more efficiently encode, transmit, and decode information via light?”

In fact, the UCSB researchers are collaborating with Raytheon Vision Systems in Goleta to investigate applications of their discoveries in the development of tunable filters and switchable shutters for infrared cameras. Down the road, there may also be possible applications for synthetic camouflage. [emphasis mine]

There is at least one other research team (the UK’s University of Bristol) considering the camouflage strategies employed cephalopods and, in their case,  zebra fish as noted in my May 4, 2012 posting, Camouflage for everyone.

Getting back to cephalopod in hand, here’s an image from the UC Santa Barbara team,

This shows the diffusion of the neurotransmitter applied to squid skin at upper right, which induces a wave of iridescence traveling to the lower left and progressing from red to blue. Each object in the image is a living cell, 10 microns long; the dark object in the center of each cell is the cell nucleus. [downloaded from http://www.ia.ucsb.edu/pa/display.aspx?pkey=3076]

This shows the diffusion of the neurotransmitter applied to squid skin at upper right, which induces a wave of iridescence traveling to the lower left and progressing from red to blue. Each object in the image is a living cell, 10 microns long; the dark object in the center of each cell is the cell nucleus. [downloaded from http://www.ia.ucsb.edu/pa/display.aspx?pkey=3076]

Fro papers currently available online, here are links and citations,

Optical parameters of the tunable Bragg reflectors in squid by Amitabh Ghoshal, Daniel G. DeMartini, Elizabeth Eck, and Daniel E. Morse. doi: 10.1098/​rsif.2013.0386 J. R. Soc. Interface 6 August 2013 vol. 10 no. 85 20130386

The Royal Society paper is behind a paywall until August 2014.

Membrane invaginations facilitate reversible water flux driving tunable iridescence in a dynamic biophotonic system by Daniel G. DeMartini, Daniel V. Krogstadb, and Daniel E. Morse. Published online before print January 28, 2013, doi: 10.1073/pnas.1217260110
PNAS February 12, 2013 vol. 110 no. 7 2552-2556

The Proceedings of the National Academy of Sciences (PNAS) paper (or the ‘Daniel’ paper as I prefer to think of it)  is behind a paywall.

“Egyptian blue” the first synthetic pigment in history inspires nanomaterials

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Some chemists at the University of Georgia (US) have analyzed the blue pigment found in Egyptian monuments and elsewhere to discover that it has some unique properties at the nanoscale which ancient Egyptians and others capitalized on in their artworks. From the Feb. 20, 2013 news item on Nanowerk,

Tina T. Salguero [University of Georgia] and colleagues point out that Egyptian blue, regarded as humanity’s first artificial pigment, was used in paintings on tombs, statues and other objects throughout the ancient Mediterranean world. Remnants have been found, for instance, on the statue of the messenger goddess Iris on the Parthenon and in the famous Pond in a Garden fresco in the tomb of Egyptian “scribe and counter of grain” Nebamun in Thebes.

They describe surprise in discovering that the calcium copper silicate in Egyptian blue breaks apart into nanosheets so thin that thousands would fit across the width of a human hair. The sheets produce invisible infrared (IR) radiation similar to the beams that communicate between remote controls and TVs, car door locks and other telecommunications devices.

The article can be found here,

Nanoscience of an Ancient Pigment by Darrah Johnson-McDaniel, Christopher A. Barrett, Asma Sharafi, and Tina T. Salguero. J. Am. Chem. Soc., 2013, 135 (5), pp 1677–1679 DOI: 10.1021/ja310587c Publication Date (Web): December 10, 2012

Copyright © 2012 American Chemical Society

The article is behind a paywall but the abstract is open to everyone and there is this image,

Credit: Researchers at the University of Georgia [downloaded from http://pubs.acs.org.proxy.lib.sfu.ca/doi/full/10.1021/ja310587c#]

Credit: Researchers at the University of Georgia [downloaded from http://pubs.acs.org.proxy.lib.sfu.ca/doi/full/10.1021/ja310587c#]

If I understand this rightly, Egyptian blue can be categorized as both a traditional pigment and a structural color due to nanoscale structures. (I recently wrote about structure, color, and the nanoscale in a Feb. 7, 2013 posting.)

As these things do from time to time, it reminded me of a song,

Enjoy!

Structure of color

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AGELESS BRILLIANCE: Although the pigment-derived leaf color of this decades-old specimen of the African perennial Pollia condensata has faded, the fruit still maintains its intense metallic-blue iridescence.COURTESY OF P.J. RUDALL [downloaded from http://www.the-scientist.com/?articles.view/articleNo/34200/title/Color-from-Structure/]

AGELESS BRILLIANCE: Although the pigment-derived leaf color of this decades-old specimen of the African perennial Pollia condensata has faded, the fruit still maintains its intense metallic-blue iridescence.COURTESY OF P.J. RUDALL [downloaded from http://www.the-scientist.com/?articles.view/articleNo/34200/title/Color-from-Structure/]

Hard to believe those berries were collected more than four decades ago, according to Cristina Luiggi in her Feb. 1, 2013 article, Color from Structure, for The Scientist magazine. Her focus is on biological nanostructures and it is a fascinating article which I urge you to read in its entirety if you have the time and this kind of thing interests you. As you can see, the pictures are great.

Here are a few excerpts from the piece,

Colors may be evolution’s most beautiful accident. Spontaneous mutations that perturbed the arrangement of structural components, such as cellulose, collagen, chitin, and keratin, inadvertently created nanoscale landscapes that catch light in the most vibrantly diverse ways—producing iridescent greens, fiery reds, brilliant blues, opalescent whites, glossy silvers, and ebony blacks.

Structural colors, in contrast to those produced by pigments or dyes, arise from the physical interaction of light with biological nanostructures. These color-creating structures likely developed as an important phenotype during the Cambrian explosion more than 500 million years ago, when organisms developed the first eyes and the ability to detect light, color, shade, and contrast. “As soon as you had visual predators, there were organisms that were either trying to distract, avoid, or communicate with those predators using structural coloration,” says Yale University evolutionary ornithologist Richard Prum.

Ever since, structural coloration has evolved multiple times across the tree of life, as a wide range of organisms developed ways to fine-tune the geometry of some of the most abundant (and often colorless) biomaterials on Earth, engineering grooves, pockets, and films that scatter light waves and cause them to interfere with each other in ways we humans happen to find aesthetically pleasing.

Here’s why color derived from structure doesn’t fade, from Luiggi’s article,

Pigments and dyes are molecules that produce colors by the selective absorption and reflection of specific wavelengths of electromagnetic radiation. Structural colors, on the other hand, rely exclusively on the shape of the material and not its chemical properties. While pigments and dyes degrade and their colors fade over time, some types of structural coloration, which rely on the same materials that make up tree bark, insect exoskeletons, and claws or nails, can persist hundreds, thousands, and even millions of years after the death of the organism.

Structural color can be found in a lot of plant life,

Although there are only a handful of known examples of structural colors in fruits, there are plenty to be found in the leaves and petals of plants. Within every family of flowering plants, there is at least one species that displays structural colors.

“The presence of structural colors, especially in flowers, is likely used by pollinators to spot the position of the flower and to recognize it better,” Vignolini [Silvia Vignolini, a physics postdoc at the University of Cambridge] explains. But in some plants, the evolutionary purpose of structural coloration is harder to pin down. The leaves of the low-lying tropical spikemoss Selaginella willdenowii, for example, produce blue-green iridescence when young and growing in the shade, and tend to lose the structural coloration with age and when exposed to high levels of light. The iridescence is achieved by cells in the leaves’ upper epidermis, which contain a few layers of cellulose microfibrils packed with different amounts of water. This ultrastructure is often absent in the leaves of the same species growing in direct sunlight. Researchers hypothesize that the spikemoss turns off its iridescence by changing the water content of the leaves’ cell walls, says Heather Whitney, a research fellow at the University of Bristol who studies iridescence in plants.

This capability is not limited to plants. Insects (jewel beetles and the morpho butterfly are often cited) and fish also have evolved to include structural color as protective or attractive devices, from Luiggi’s article,

The brightest living tissues on the planet are found in fish. Under ideal conditions, for example, the silvery scales of the European sardine and the Atlantic herring can act like near-perfect mirrors—reflecting up to 90 percent of incoming light. It is an irony of nature that these shiniest of structures are not meant to be flaunted, but are intended as camouflage.

“When you’re out in the open water, if you drop down below 10 to 30 meters, in any direction you look, the intensity of light is the same,” explains Nicholas Roberts, a physicist at the University of Bristol who specializes in bio-optics. At that depth, a perfect reflector, or mirror, would seem invisible, because light is equally reflected from all sides and angles.

It will be interesting to see if there’s any future discussion of the giant squid in the context of structural color since, according to very recent research (as per my Feb. 1, 2013 posting), it appears to be covered in gold leaf when observed in its habitat.

Luiggi’s article starts with an ornithologist and circles back in a discussion about the difficulty of creating nanostructures, soft matter condensed physics, and birds,

To create structural colors, organisms must master architecture at the nanoscale—the size of visible-light wavelengths. “But it turns out that biology doesn’t do a good job of creating nanostructures,” Prum says.

Instead, organisms create the initial conditions that allow those nanostructures to grow using self-organizing physical processes. Thus, organisms exploit what’s known as soft condensed matter physics, or “the physics of squishy stuff,” as Prum likes to call it. This relatively new field of physics deals with materials that are right at the boundaries of hard solids, liquids, and gases.

“There’ve been huge advances in this field in the last 30 years which have created rich theories of how structure can arise at the nanoscale,” Prum says. “It has been very applicable to the understanding of how structural colors grow.”

Soft condensed matter physics has been particularly useful in understanding the production of the amorphous nanostructures that imbue the feathers of certain bird species with intensely vibrant hues. The blue color of the male Eastern bluebird (Sialia sialis), for example, is produced by the selective scattering of blue light from a complex nanostructure of b-keratin channels and air pockets in the hairlike branches called feather barbs that give the quill its lift. The size of the air pockets determines the wavelengths that are selectively amplified.

While there’s better understanding of the mechanisms involved in structural color, scientists are a long way from replicating the processes, from the article,

“The three-dimensional nature of the structures themselves is just so complex,” says Vukusic. [physicist Peter Vukusic, a professor of natural photonics at the University of Exeter, UK] “Were it to be a simple periodic system with a regular geometry, you could easily put that into a computer model and run simulations all day. But the problem is that they are never perfectly periodic.”

This article is open access so, as I noted earlier, all you need is the time. As of my Feb. 6, 2013 posting, there was some new research announced about scientists making observations about the structural color in peacock feathers and applying some of those ideas to develop better resolution in e-readers.

Peacocks and their structural colour inspire better resolution in e-readers

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Thank goodness birds, insects, and other denizens of the natural world have not taken to filing patents otherwise we’d be having some serious problems in the courts as I have hinted in previous postings including this March 29, 2012 posting titled, Butterflies give and give … .

This time, it’s the peacock which is sharing its intellectual property as per this Feb. 5, 2013 news item on ScienceDaily,

Now, researchers at the University of Michigan have found a way to lock in so-called structural color, which is made with texture rather than chemicals. A paper on the work is published online in the current edition of the Nature journal Scientific Reports.

In a peacock’s mother-of-pearl tail, precisely arranged hairline grooves reflect light of certain wavelengths. That’s why the resulting colors appear different depending on the movement of the animal or the observer. Imitating this system—minus the rainbow effect—has been a leading approach to developing next-generation reflective displays.

The University of Michigan Feb. 5, 2013 news release, which originated the news item, provides information about potential applications and more details about the science,

The new U-M research could lead to advanced color e-books and electronic paper, as well as other color reflective screens that don’t need their own light to be readable. Reflective displays consume much less power than their backlit cousins in laptops, tablet computers, smartphones and TVs. The technology could also enable leaps in data storage and cryptography. Documents could be marked invisibly to prevent counterfeiting.

Led by Jay Guo, professor of electrical engineering and computer science, the researchers harnessed the ability of light to funnel into nanoscale metallic grooves and get trapped inside. With this approach, they found the reflected hues stay true regardless of the viewer’s angle.

“That’s the magic part of the work,” Guo said. “Light is funneled into the nanocavity, whose width is much, much smaller than the wavelength of the light. And that’s how we can achieve color with resolution beyond the diffraction limit. Also counterintuitive is that longer wavelength light gets trapped in narrower grooves.”

The diffraction limit was long thought to be the smallest point you could focus a beam of light to. Others have broken the limit as well, but the U-M team did so with a simpler technique that also produces stable and relatively easy-to-make color, Guo said.

“Each individual groove—much smaller than the light wavelength—is sufficient to do this function. In a sense, only the green light can fit into the nanogroove of a certain size,” Guo said.

The U-M team determined what size slit would catch what color light. Within the framework of the print industry standard cyan, magenta and yellow color model, the team found that at groove depths of 170 nanometers and spacing of 180 nanometers, a slit 40 nanometers wide can trap red light and reflect a cyan color. A slit 60 nanometers wide can trap green and make magenta. And one 90 nanometers wide traps blue and produces yellow. The visible spectrum spans from about 400 nanometers for violet to 700 nanometers for red.

“With this reflective color, you could view the display in sunlight. It’s very similar to color print,” Guo said.

Particularly interesting (for someone who worked in the graphic arts/printing industry as I did) are the base colours being used to create all the other colours,

To make color on white paper, (which is also a reflective surface), printers arrange pixels of cyan, magenta and yellow in such a way that they appear to our eyes as the colors of the spectrum. [emphasis mine] A display that utilized Guo’s approach would work in a similar way.

To demonstrate their device, the researchers etched nanoscale grooves in a plate of glass with the technique commonly used to make integrated circuits, or computer chips. Then they coated the grooved glass plate with a thin layer of silver. When light—which is a combination of electric and magnetic field components—hits the grooved surface, its electric component creates what’s called a polarization charge at the metal slit surface, boosting the local electric field near the slit. That electric field pulls a particular wavelength of light in.

The base colours in printing are CMYK (cyan, magenta, yellow, black). At least, that was the case when I worked in the graphic arts industry and quick search on the web suggests that standard still holds.(Have I missed a refinement?) In any event, here’s an image that demonstrates how this new colour scale can be used,

University of Michigan researchers created the color in these tiny Olympic rings using precisely-sized nanoscale slits in a glass plate coated with silver. Each ring is about 20 microns, smaller than the width of a human hair. They can produce different colors with different widths of the slits. Yellow is produced with slits that are each 90 nanometers wide. The technique takes advantage of a phenomenon called light funneling that can catch and trap particular wavelengths of light, and it could lead to reflective display screens with colors that stay true regardless of the viewer's angle. Image credit: Jay Guo, College of Engineering

University of Michigan researchers created the color in these tiny Olympic rings using precisely-sized nanoscale slits in a glass plate coated with silver. Each ring is about 20 microns, smaller than the width of a human hair. They can produce different colors with different widths of the slits. Yellow is produced with slits that are each 90 nanometers wide. The technique takes advantage of a phenomenon called light funneling that can catch and trap particular wavelengths of light, and it could lead to reflective display screens with colors that stay true regardless of the viewer’s angle. Image credit: Jay Guo, College of Engineering

You can find more about this work in the ScienceDaily news item, which includes a link to the abstract, or in the University of Michigan news release, which includes more images from the scientists.