Scientists Create Chameleon-Like, Color-Shifting Materials

Scientists Create Chameleon-Like, Color-Shifting Materials

Materials scientists at MIT have unlocked a rich and easily controlled material design space, previously unattainable using any competing technique, by harnessing the pioneering color photography work of Nobel laureate Gabriel Lippmann.

Optical manufacture of stretchable color-changing materials at the macroscale: (a) structural color patterns are recorded as periodic refractive index variations in the photo-elastomer via standing waves that result from exposure to a projected light pattern that reflects off the mirror backing; (b) the resulting structural color matches the spectral distribution of the exposure pattern; when the material is stretched or compressed, the periodicity of the recorded photonic structure changes, causing a predictable and reversible color change. Image credit: Miller et al., doi: 10.1038/s41563-022-01318-x / MIT Communication Initiatives.

Optical manufacture of stretchable color-changing materials at the macroscale: (a) structural color patterns are recorded as periodic refractive index variations in the photo-elastomer via standing waves that result from exposure to a projected light pattern that reflects off the mirror backing; (b) the resulting structural color matches the spectral distribution of the exposure pattern; when the material is stretched or compressed, the periodicity of the recorded photonic structure changes, causing a predictable and reversible color change. Image credit: Miller et al., doi: 10.1038/s41563-022-01318-x / MIT Communication Initiatives.

Structurally colored materials that change their color in response to mechanical stimuli are uniquely suited for optical sensing and visual communication.

The main barrier to their widespread adoption is a lack of manufacturing techniques that offer spatial control of the materials’ nanoscale structures across macroscale areas.

“An approach that offers both microscale control and scalability is elusive, despite several potential high-impact applications,” said Benjamin Miller, a graduate student in the Department of Mechanical Engineering at MIT.

While puzzling over how to resolve the challenge, Miller happened to visit the MIT Museum, where a curator talked him through an exhibit on holography, a technique that produces 3D images by superimposing two light beams onto a physical material.

“I realized what they do in holography is kind of the same thing that nature does with structural color,” Miller said.

That visit spurred him to read up on holography and its history, which led him back to the 1800s, and Lippmann photography — an early color photography technique invented by the Franco-Luxembourgish physicist Gabriel Lippmann, who later won the Nobel Prize in Physics for the technique.

Lippmann generated color photos by first setting a mirror behind a very thin, transparent emulsion — a material that he concocted from tiny light-sensitive grains. He exposed the setup to a beam of light, which the mirror reflected back through the emulsion.

The interference of the incoming and outgoing light waves stimulated the emulsion’s grains to reconfigure their position, like many tiny mirrors, and reflect the pattern and wavelength of the exposing light.

Using this technique, Lippmann projected structurally colored images of flowers and other scenes onto his emulsions, though the process was laborious.

It involved hand-crafting the emulsions and waiting for days for the material to be sufficiently exposed to light. Because of these limitations, the technique largely faded into history.

Miller wondered if, paired with modern, holographic materials, Lippmann photography could be sped up to produce large-scale, structurally colored materials.

Like Lippmann’s emulsions, current holographic materials consist of light-sensitive molecules that, when exposed to incoming photons, can cross-link to form colored mirrors.

“The chemistries of these modern holographic materials are now so responsive that it’s possible to do this technique on a short timescale simply with a projector,” said Dr. Mathias Kolle, also from the Department of Mechanical Engineering at MIT.

In their study, the authors adhered elastic, transparent holographic film onto a reflective, mirror-like surface (in this case, a sheet of aluminum).

They then placed an off-the-shelf projector several feet from the film and projected images onto each sample, including Lippman-esque bouquets.

As they suspected, the films produced large, detailed images within several minutes, rather than days, vividly reproducing the colors in the original images.

They then peeled the film away from the mirror and stuck it to a black elastic silicone backing for support.

They stretched the film and observed the colors change — a consequence of the material’s structural color: when the material stretches and thins out, its nanoscale structures reconfigure to reflect slightly different wavelengths, for instance, changing from red to blue.

The team found the film’s color is highly sensitive to strain. After producing an entirely red film, they adhered it to a silicone backing that varied in thickness. Where the backing was thinnest, the film remained red, whereas thicker sections strained the film, causing it to turn blue.

Similarly, the scientists found that pressing various objects into samples of red film left detailed green imprints, caused by, say, the seeds of a strawberry and the wrinkles of a fingerprint.

Interestingly, they could also project hidden images, by tilting the film at an angle with respect to the incoming light when creating the colored mirrors.

This tilt essentially caused the material’s nanostructures to reflect a red-shifted spectrum of light.

For instance, green light used during material exposure and development would lead to red light being reflected, and red light exposure would give structures that reflect infrared — a wavelength that is not visible to humans.

When the material is stretched, this otherwise invisible image changes color to reveal itself in red.

“You could encode messages in this way,” Dr. Kolle said.

The team’s paper appears this week in the journal Nature Materials.

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B.H. Miller et al. Scalable optical manufacture of dynamic structural colour in stretchable materials. Nat. Mater, published online August 1, 2022; doi: 10.1038/s41563-022-01318-x

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