09-02 isotropic structural color examples 1

[Image above] Side views of twisted and hexagonal vases manufactured by multimaterial 3D printing of colloidal inks containing 200 nm (blue), 250 nm (green), and 300 nm (pink) silica particles. Scale bar: 1 cm. Credit: Studart et al., Nature Communications (CC BY 4.0)

When I see my cat snoozing blissfully in the afternoon sun, I often envy his carefree life of sleeping, eating, and getting away with occasionally destructive hijinks.

But I had the chance to experience the life of a cat through the newly released and acclaimed video adventure game Stray.

The story follows a stray cat who falls into an underground city populated by robots. As the cat attempts to return to the surface with the help of a drone companion, there are ample opportunities to act out your feline dreams of scratching walls, kneading carpets, meowing repeatedly, and knocking things off tables and shelves.

Regarding the last point, paint cans show up frequently throughout the various levels. Knocking these over leads to colorful puddles that, when walked through, let you track kitty prints all over the floor.

Of course, if speculation is correct that this story takes place millions of years in the future, there is no way this paint would be in usable condition for spilling (unless the robots mixed fresh batches). An unopened can of acrylic paint, the longest lasting type, stays good for up to 15 years, a far cry from the video game’s timescale.

The reason paint cannot last forever is because it relies on pigment for color. These chemicals naturally react with environmental stimuli, such as light, heat, and bacteria, causing the chemical bonds to break down. Over time, this degradation results in the paint clumping or fading, and likely mold growth.

Pigment is not the only way to endow a material with color, however.

Structural color is produced by the interaction of light with micro- or nanostructures. Examples of structural color in nature abound, from the flashing feathers of hummingbirds to the colorful luster of pearls.

Because structural color does not rely on chemicals to induce color, it is not affected by chemical changes and can last for much longer—scientists have even identified examples of structural color in fossils!

Iridescence, or a change in color based on viewing angle and orientation, is often a key indicator that a material relies on structural color rather than pigment. However, not all structural colors are iridescent. When there is enough disorder present at the nanoscale, different wavelengths of incoming light will scatter evenly, producing a single color that looks the same regardless of the viewing angle. That is the case for bluebird feathers.

Researchers found they can replicate this angle-independent “isotropic” structural color using colloidal particles, or small solid particles that are suspended in a fluid phase. However, current examples are limited to the two-dimensional and simple geometries generated by casting, coating, and pressing processes.

“To partially fill this gap, additive manufacturing technologies have recently been exploited to produce three-dimensional photonic objects with intricate geometries and increased shape complexity,” researchers write in a recent open-access paper.

The researchers come from ETH Zurich in Switzerland and include physicist Henning Galinski, who we previously wrote about on CTT for helping to create shimmery chocolate via structural color.

To date, experiments using additive manufacturing have been restricted either to 2D demonstrations or to 3D crystalline structures with angle-dependent photonic properties. In the new study, the ETH Zurich researchers looked to additively manufacture complex-shaped objects with isotropic structural color.

They chose direct ink writing (DIW) as the additive manufacturing process. Designing colloidal inks that were printable via this process required close evaluation of the ink’s viscoelastic properties.

“On the one hand, top-down printing through the DIW technique demands a colloidal ink with rheological properties that allow for the extrusion of filaments into distortion-free printed structures. On the other hand, the bottom-up assembly of particles into a colloidal glass depends on the formation of a highly packed arrangement of particles without the onset of crystallization,” they explain.

To meet these opposing demands, they designed a water-based colloidal ink that consisted of silica particles, carbon black, and rheology modifiers. (The carbon black particles act as an absorption medium between the silica particles to reduce multiple scattering of the incoming light.)

The volume fraction of particles within the ink was initially low to enable direct ink writing. But it eventually reached the maximum packing limit desired for glass formation when the liquid and gel phase of the ink was removed by heat treatment.

Drying of the as-printed object at 25°C changed its color from black to gray. However, further heat treatment at the higher temperature of 200°C led to a strong and vivid green color.

“The fact that the color is angle-independent suggests that a photonic colloidal glass with length scales on the order of the wavelengths of visible light was formed upon drying at this higher temperature,” the researchers write.

They used scanning electron microscopy to confirm that the 200°C-treated sample featured a glass-like microstructure of densely packed silica particles without long-range order. Surprisingly, they achieved this structure using a single set of monodisperse particles. Previous studies relied on increased polydispersity of the colloids to prevent crystallization and induce glass formation.

“Thus, it is likely that the predominantly elastic behavior of the ink is not only important for printing distortion-free three-dimensional structures, but also plays a role in suppressing crystallization of the monodisperse colloidal particles,” they write.

The researchers then demonstrated the potential of their process by printing several samples featuring different colors, which were tuned by changing the size of the silica colloidal particles present in the initial ink.

Grid-like structures printed from inks containing a) 200 nm, b) 250 nm, and c) 300 nm silica colloids resulted in blue, green, and magenta structural colors, respectively. Credit: Studart et al., Nature Communications (CC BY 4.0)

Additional testing showed that heat does not significantly affect the structural color. Samples subjected to a heat treatment at 350°C for 1 hour in air experienced only a slight reduction in color intensity, likely due to the partial oxidation and removal of the carbon black. By using a protective argon atmosphere, the researchers successfully preserved the color even after heating a sample up to 1,000°C.

The researchers also note that, with a silica weight fraction of more than 97%, “our objects are recyclable and do not display the toxicity issues of typical colored pigments.” Recycling can be achieved through mechanical pulverization of the object and further addition of carbon black and rheological modifiers to yield a new ink.

“Our approach provides a powerful tool to design and fabricate multicolored 3D objects through the multimaterial printing of inks of programmable structural color,” they conclude. “Just like photonic materials created by living organisms in nature, our structurally colored objects can be produced using widely available, nontoxic, and sustainable chemistries.”

The open-access paper, published in Nature Communications, is “Three-dimensional printing of photonic colloidal glasses into objects with isotropic structural color” (DOI: 10.1038/s41467-022-32060-2).