Tag Archives: peacock feathers

Structural colo(u)r from transparent 3D printed nanostructures

Caption: Light hits the 3-D printed nanostructures from below. After it is transmitted through, the viewer sees only green light — the remaining colors are redirected. Credit: Thomas Auzinger [downloaded from http://visualcomputing.ist.ac.at/publications/2018/StructCol/]

An August 17, 2018 news item on ScienceDaily announces the work illustrated by the image above,

Most of the objects we see are colored by pigments, but using pigments has disadvantages: such colors can fade, industrial pigments are often toxic, and certain color effects are impossible to achieve. The natural world, however, also exhibits structural coloration, where the microstructure of an object causes various colors to appear. Peacock feathers, for instance, are pigmented brown, but — because of long hollows within the feathers — reflect the gorgeous, iridescent blues and greens we see and admire. Recent advances in technology have made it practical to fabricate the kind of nanostructures that result in structural coloration, and computer scientists from the Institute of Science and Technology Austria (IST Austria) and the King Abdullah University of Science and Technology (KAUST) have now created a computational tool that automatically creates 3D-print templates for nanostructures that correspond to user-defined colors. Their work demonstrates the great potential for structural coloring in industry, and opens up possibilities for non-experts to create their own designs. This project will be presented at this year’s top computer graphics conference, SIGGRAPH 2018, by first author and IST Austria postdoc Thomas Auzinger. This is one of five IST Austria presentations at the conference this year.

SIGGRAPH 2018, now ended, was mentioned in my Aug. 9, 2018 posting.but since this presentation is accompanied by a paper, it rates its own posting. For one more excuse, there’s my fascination with structural colour.

An August 17, 2018 Institute of Science and Technology Austria press release (also on EurekAlert), which originated the news item, delves into the work,

The changing colors of a chameleon and the iridescent blues and greens of the morpho butterfly, among many others in nature, are the result of structural coloration, where nanostructures cause interference effects in light, resulting in a variety of colors when viewed macroscopically. Structural coloration has certain advantages over coloring with pigments (where particular wavelengths are absorbed), but until recently, the limits of technology meant fabricating such nanostructures required highly specialized methods. New “direct laser writing” set-ups, however, cost about as much as a high-quality industrial 3D printer, and allow for printing at the scale of hundreds of nanometers (hundred to thousand time thinner than a human hair), opening up possibilities for scientists to experiment with structural coloration.

So far, scientists have primarily experimented with nanostructures that they had observed in nature, or with simple, regular nanostructural designs (e.g. row after row of pillars). Thomas Auzinger and Bernd Bickel of IST Austria, together with Wolfgang Heidrich of KAUST, however, took an innovative new approach that differs in several key ways. First, they solve the inverse design task: the user enters the color they want to replicate, and then the computer creates a nanostructure pattern that gives that color, rather than attempting to reproduce structures found in nature. Moreover, “our design tool is completely automatic,” says Thomas Auzinger. “No extra effort is required on the part of the user.”

Second, the nanostructures in the template do not follow a particular pattern or have a regular structure; they appear to be randomly composed—a radical break from previous methods, but one with many advantages. “When looking at the template produced by the computer I cannot tell by the structure alone, if I see a pattern for blue or red or green,” explains Auzinger. “But that means the computer is finding solutions that we, as humans, could not. This free-form structure is extremely powerful: it allows for greater flexibility and opens up possibilities for additional coloring effects.” For instance, their design tool can be used to print a square that appears red from one angle, and blue from another (known as directional coloring).

Finally, previous efforts have also stumbled when it came to actual fabrication: the designs were often impossible to print. The new design tool, however, guarantees that the user will end up with a printable template, which makes it extremely useful for the future development of structural coloration in industry. “The design tool can be used to prototype new colors and other tools, as well as to find interesting structures that could be produced industrially,” adds Auzinger. Initial tests of the design tool have already yielded successful results. “It’s amazing to see something composed entirely of clear materials appear colored, simply because of structures invisible to the human eye,” says Bernd Bickel, professor at IST Austria, “we’re eager to experiment with additional materials, to expand the range of effects we can achieve.”

“It’s particularly exciting to witness the growing role of computational tools in fabrication,” concludes Auzinger, “and even more exciting to see the expansion of ‘computer graphics’ to encompass physical as well as virtual images.”

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

Computational Design of Nanostructural Color for Additive Manufacturing by Thomas Auzinger, Wolfgang Heidrich, and Bernd Bickel. ACM Trans. Graph. 37, 4, Article 159 (August 2018). 16 pages. doi.org/10.1145/3197517.3201376

This appears to be open access.

There is also a project page bearing the same title as the paper, Computational Design of Nanostructural Color for Additive Manufacturing.

More on the blue tarantula noniridescent photonics

Covered in an Oct. 19, 2016 posting here, some new details have been released about noniridescent photonics and blue tarantulas, this time from the Karlsruhe Institute of Technology (KIT) in a Nov. 17, 2016 (?) press release (also on EurekAlert; h/t Nanowerk Nov. 17, 2016 news item) ,

Colors are produced in a variety of ways. The best known colors are pigments. However, the very bright colors of the blue tarantula or peacock feathers do not result from pigments, but from nanostructures that cause the reflected light waves to overlap. This produces extraordinarily dynamic color effects. Scientists from Karlsruhe Institute of Technology (KIT), in cooperation with international colleagues, have now succeeded in replicating nanostructures that generate the same color irrespective of the viewing angle. DOI: 10.1002/adom.201600599

In contrast to pigments, structural colors are non-toxic, more vibrant and durable. In industrial production, however, they have the drawback of being strongly iridescent, which means that the color perceived depends on the viewing angle. An example is the rear side of a CD. Hence, such colors cannot be used for all applications. Bright colors of animals, by contrast, are often independent of the angle of view. Feathers of the kingfisher always appear blue, no matter from which angle we look. The reason lies in the nanostructures: While regular structures are iridescent, amorphous or irregular structures always produce the same color. Yet, industry can only produce regular nanostructures in an economically efficient way.

Radwanul Hasan Siddique, researcher at KIT in collaboration with scientists from USA and Belgium has now discovered that the blue tarantula does not exhibit iridescence in spite of periodic structures on its hairs. First, their study revealed that the hairs are multi-layered, flower-like structure. Then, the researchers analyzed its reflection behavior with the help of computer simulations. In parallel, they built models of these structures using nano-3D printers and optimized the models with the help of the simulations. In the end, they produced a flower-like structure that generates the same color over a viewing angle of 160 degrees. This is the largest viewing angle of any synthetic structural color reached so far.

Flower-shaped nanostructures generate the color of the blue tarantula. (Graphics: Bill Hsiung, University of Akron)


The 3D print of the optimized flower structure is only 15 µm in dimension. A human hair is about three times as thick. (Photo: Bill Hsiung, Universtiy of Akron)

Apart from the multi-layered structure and rotational symmetry, it is the hierarchical structure from micro to nano that ensures homogeneous reflection intensity and prevents color changes.

Via the size of the “flower,” the resulting color can be adjusted, which makes this coloring method interesting for industry. “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,” says Radwanul Hasan Siddique of KIT’s Institute of Microstructure Technology, who now works at the California Institute of Technology. He considers short-term application in textile industry feasible.

The synthetically generated flower structure inspired by the blue tarantula reflects light in the same color over a viewing angle of 160 degrees. (Graphics: Derek Miller)  

Dr. Hendrik Hölscher thinks that the scalability of nano-3D printing is the biggest challenge on the way towards industrial use. Only few companies in the world are able to produce such prints. In his opinion, however, rapid development in this field will certainly solve this problem in the near future.

Once again, here’s a link to and a citation for the paper,

Tarantula-Inspired Noniridescent Photonics with Long-Range Order by Bor-Kai Hsiung, Radwanul Hasan Siddique, Lijia Jiang, Ying Liu, Yongfeng Lu, Matthew D. Shawkey, and Todd A. Blackledge. Advanced Materials DOI: 10.1002/adom.201600599 Version of Record online: 11 OCT 2016

© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The paper is behind a paywall. You can see the original Oct. 19, 2016 posting for my comments and some excerpts from the paper.

Structure of color

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.