“Structural Paints” Could Create Brilliant Colors That Never Fade

Have you ever taken a moment to admire the brilliant blue of a bluebird’s feathers or the vibrant green of a beetle’s wings and wondered why you can’t buy that color in a paint can? Nature has long since perfected a kind of coloration that we humans still struggle with—but it may just be a matter of time before it decorates your living room.

The over-the-top colors often found in nature commonly arise from what’s known as structural color, which relies on microscopic structures rather than molecular properties to reflect colors back to your eyes.
Image Credit: Vignolini, et al.

Most of the colors around us are generated when the molecules that make up a substance absorb some colors of light and reflect others; for example, a paint might take in blue light but reflect green and red hues, which are then interpreted by your brain as yellow. In this scenario, the absorbed photons transfer their energy to the electrons in the paint, which causes damage and fading over time.

Structural colors, on the other hand, derive their brilliance from the principle of interference. When white light—that is, a mixture of many different colors—hits a ridged or grooved surface, it has a tendency to split into its many components to reveal a rainbow. The tiny reflective scales that cover a butterfly’s wing, for example, have this effect, which is why a damaged wing appears a dull brown. However, when light comes in from many angles, those miniature rainbows overlap and interfere to the point that many colors are annihilated altogether, much like two waves in the ocean can suddenly create still waters where they overlap. An object with structural color is one where the reflective surfaces are arranged so that, when all is said and done, only one color remains as a single, especially-intense hue. Crucially, none of the light’s energy is absorbed by the pigment itself; rather the light is pitted against itself to self-select certain colors and destroy the rest.

It is actually possible to recreate such structural colors on an industrial scale, taking inspiration from detailed studies of the numerous examples found in nature. There’s just one catch: this approach tends to reflect different colors in different directions, resulting in a shimmery iridescence. You can see this effect in the shimmer of a peacock’s feathers or the way a CD catches the light. Although iridescence can be beautiful and eye-catching, it’s hardly ideal when it comes to painting your walls.

Deeper investigation shows that this iridescence arises from the regular, crystalline pattern of many structural colors—which begs the question: is it possible to have structural color without such a microscopic pattern? As it turns out, yes. We’ve already found a number of examples in nature that incorporate a ridged or grooved surface—which is necessary to trigger interference—on the smallest of scales, and yet are disordered at slightly larger scales. That is to say, when looking at regions a few dozen microns wide (about the width of a hair) the structure appears irregular, while at scales much less than that small patterns are still able to catch the light. The overall effect is that the interference which characterizes structural pigment still takes place, while the iridescent effect is suppressed.

When a structural pigment has a crystalline form (left), it is the regular pattern of molecules that selectively reflects light. In this case, however, the geometry of the system—and thus the color of the reflected light—depends on the viewing angle, resulting in iridescence. On the other hand, an amorphous pigment like SiO2@Fe3O4 relies on the structure of the individual molecules to reflect certain colors. Since the overall geometrical structure is irregular (it looks more or less the same no matter how it’s rotated), the viewing angle doesn’t fundamentally change which color is reflected.
Image Credit: Dongpeng Yang

This much has been known for years. However, the challenge comes in realizing an artificial structural paint that is homogeneous enough to allow for a smooth coat yet still irregular at long scales, while remaining ordered on short scales. Furthermore, current techniques face serious challenges when it comes to coating curved surfaces.

Now a research team, led by Dongpeng Yang and Shaoming Huang of the Guangdong University of Technology, thinks they’ve found a promising lead. As they write in ACS Omega, by adhering numerous Fe3O4 molecules to a central silicon dioxide core, they found a highly regular molecule that nevertheless tended to arrange itself randomly relative to its counterparts due to electrostatic interactions. The size of such a colloid—that is, the SiO2@Fe3O4 molecule—determines the specific color enhanced by the pigment, and thanks to the long-range disorder the viewing angle doesn’t change that color. Importantly, they also found that this substance is quite good at adhering to common materials like paper, glass, plastics, resins, ceramics, and wood, and it can easily be painted onto a curved 3-dimensional surface.

Images from the ACS Omega paper show several painted surfaces at a wide variety of angles.
Image Credit: Dongpeng Yang via ACS Omega

There are a number of potential advantages to a paint like this. To begin with, the SiO2@Fe3O4 colloid is relatively cheap and easy to make on an industrial scale. Additionally, since the paint isn’t actually absorbing any light, it’s much more resistant to fading and doesn’t break down under UV exposure like many paints currently on the market, cutting back on environmental pollutants.

Although this structural paint isn’t quite as brilliant as, say, a bluebird’s feathers, Yang thinks that there’s plenty of room for improvement. Moving forward, he hopes to experiment with other colloids that have a higher refractive index, a property that will allow for richer colors. Then maybe we will have the best of both worlds: fade-resistant, environmentally friendly paint that boasts the stunning colors of nature.

Eleanor Hook

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