Jedidiah Becker for redOrbit.com — Your Universe Online
Read my exclusive interview with Professor Shu Yang about her research.
Have you ever looked at a peacock´s feathers, a butterfly´s wing or an oily puddle on the road and wondered why they have those shimmering, vibrant colors?
Unlike the colors you see in spring grass, an animals´ fur or fading autumn leaves, these iridescent hues are not the result of pigmentation but rather of a naturally occurring phenomenon known as “structural color.” And while the royal azure of the male peacock´s feather may resemble the deep indigo of a ripe blueberry, the mechanisms that produce these colors are fundamentally different at the most basic level.
According to a study published in the journal Advanced Functional Materials, a team of researchers from the University of Pennsylvania has found a new way to artificially recreate structural colors in a laboratory while also combining them with another highly useful physical property: the ability to strongly repel water known as “superhydrophobicity.”
“A lot of research over the last 10 years has gone into trying to create structural colors like those found in nature, in things like butterfly wings and opals,” says Shu Yang, the team´s lead researcher. Yang is an associate professor at the university´s Department of Materials Science and Engineering and a leading expert in the fields of biomaterials, polymers and nanostructured materials.
Yang also says that a lot of research has been devoted in recent years to creating materials that exhibit superhydrophobicity, a characteristic which could have innumerable applications in both the industrial and domestic spheres. However, Yang´s research team has used a bit of outside-the-box thinking to become one of the few laboratories to successfully create a material that combines the properties of structural colors as well as those of superhydrophobicity.
THE SCIENCE OF STRUCTURAL COLORS & SUPERHYDROPHOBICITY
The mechanisms that produce both structural colors and superhydrophobicity rely on the basic physical structure or geometry of a material rather than on its chemical properties.
To understand how structural colors work and what makes them so unique, it helps to first remember how pigments — a more familiar form of coloration — work.
At the molecular level, when light strikes a pigment, certain wavelengths of light are absorbed by the pigment´s electrons while other waves are simply reflected. Thus the colors that the human eye perceives are actually those wavelengths of light that weren´t absorbed by the pigment. When you look at a bowl of ripe blueberries or the petals of a violet, you´re essentially seeing the spectrum of light that was ℠rejected´ by a natural pigment called anthocyanin.
In contrast to pigments, structural colors are the result of light interacting with tiny repeating patterns and structures on the surface of a material. As with pigments, these “microstructures” or “nanostructures” correspond with different wavelengths of light.
Unlike pigments, however, these microstructures don´t create color by absorbing light of certain wavelengths. Instead, they interact and interfere with the path of the light rays, subjecting them to a variety of optical phenomena such as thin film interference, diffraction grating effects, multilayer interference, photonic crystal effects, and light scattering.
In turn, these different types of optical phenomena cause light of particular wavelengths to be reflected through constructive and destructive interference which can intensify the color or give it that shimmery, iridescent quality.
And since structural colors depend entirely on the arrangement of molecules on the nanoscale rather than on electronic absorption at the chemical level, a peacock feather or butterfly wing that is ground into a fine powder will not retain its color since the grinding process destroys these nanostructures.
Like structural colors, a superhydrophobic surface — one that is extremely hard to make wet — also depends on the basic microstructure of the material. However, it relies on the roughness of a material at the nanoscale. Since water adheres best to flat surfaces where it can maximize contact area, “rough” surfaces make it difficult for water get “grip” and are thus water repellant.
A LITTLE LAB MAGIC
In attempts to create structurally colored surfaces that are also ultra water repellant, researchers have experimented with a variety of methods that involve different combinations of complex, intricate steps. These attempts have typically involved first creating the colored surface using 3D polymers. Once this basic foundation is laid which creates the structural color, they then use different techniques to attempt to “roughen” the surface and make it water repellant without damaging the delicate nanostructures that gives the surface its optical properties.
Yang´s team, however, decided to take a creative new approach for creating both of the desired properties.
First, they began with a technique known as holographic lithography which uses a laser to create a three-dimensional network of lattices on a synthetic material called photoresist. Those parts of the photoresist not exposed to the laser beam were then removed by washing the material in a solvent. This left “holes” in the unlasered areas which, in turn, gave the material the surface properties needed to produce its structural color.
Then it was time to make the material super water repellant. Whereas most previous processes have used techniques known as nanoparticle assembling or plasma etching to create the desired roughness, Yang´s team was able was able to make the material´s surface rough by simply using a different solvent after the photoresist was removed.
Yang explained that the secret was to use a poor solvent after the photoresist had been washed off. While good solvents try to maximize their contact with the material´s surface, poor solvents produce the exact opposite effect — a property that the researchers were able harness.
“The good solvent causes the structure to swell,” explained Yang.
“Once it has swollen, we put in the poor solvent. Because the polymer hates the poor solvent, it crunches in and shrivels, forming nanospheres within the 3D lattice.”
The tiny nanospheres created by the poor solvent give the surface the roughness it needs to become superhydrophobic without disturbing the network of lattices that produce it´s structural color.
“We found that the worse the solvent we used, the more rough we could make the structures,” Yang said.
APPLICATIONS
One of the most urgent forces propelling the research and development of superhydrophobic materials is their potential to reduce energy consumption. Because all kinds of optical devices — from LCD´s to solar panels — rely on the efficient transmission of light through transparent surfaces, the ability to apply a water-repellent, self-cleaning coating to these surfaces could have a tremendous impact on the energy efficiency of countless electronic devices
Yet while both researchers and industry leaders see a wide variety of practical applications for materials that exhibit both superhydrophobicity and structural color, Yang´s team has also embraced vision for this combination of properties that is at once aesthetic and pragmatic.
“Specifically, we´re interested in putting this kind of material on the outside of buildings. The structural color we can produce is bright and highly decorative, and it won´t fade away like conventional pigmentation color dies. The introduction of nano-roughness will offer additional benefits, such as energy efficiency and environmental friendliness.”
“It could be a high-end facade for the aesthetics alone, in addition to the appeal of its self-cleaning properties. We are also developing energy efficient building skins that will integrate such materials in optical sensors.”
Yang´s research was supported by the Office of Naval Research and the National Science Foundation.
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