Imagine stretching a piece of film to reveal a hidden message. Or check the color of a bracelet to measure muscle mass. Or wear a swimsuit that changes color as you do laps. Such chameleon-like, color-changing materials could be on the horizon thanks to a photographic technique revived and repurposed by MIT engineers.
By applying a 19th-century color photography technique to modern holographic materials, an MIT team has printed large-scale images onto elastic materials that can change color when stretched and reflect different wavelengths as the material is stressed.
The researchers produced stretchable films printed with detailed bouquets of flowers that change from warm to cooler hues as the films are stretched. They also printed films showing the imprint of objects such as a strawberry, a coin, and a fingerprint.
The team’s findings provide the first scalable manufacturing technique for producing detailed, large-scale materials with “structural color” – color that arises as a result of a material’s microscopic structure and not through chemical additives or dyes.
“The scaling of these materials is not trivial because you have to control these structures at the nanoscale,” says Benjamin Miller, a graduate student in MIT’s Department of Mechanical Engineering. “Now that we’ve cleared that scaling hurdle, we can explore questions like: Can we use this material to make robotic skin that has a human-like sense of touch? And can we design touch-enabled devices for things like virtual augmented reality or medical education? It’s a big area that we’re looking at now.
The team’s results appear today in natural materials. Miller’s co-authors are MIT graduate student Helen Liu and Mathias Kolle, MIT associate professor of mechanical engineering.
Kolle’s group develops optical materials inspired by nature. Researchers have studied the light-reflecting properties of seashells, butterfly wings and other iridescent organisms that appear to shimmer and change color due to microscopic surface textures. These structures are angled and layered to reflect light like miniature colored mirrors or what engineers call Bragg reflectors.
Groups like Kolle’s have attempted to reproduce this natural, textural color in materials using a variety of techniques. Some efforts have produced small samples with precise nanostructures, while others have produced larger samples but with less optical precision.
As the team writes, “an approach that offers both [microscale control and scalability] remains elusive, despite multiple potential high-impact uses.”
While pondering how to solve this problem, Miller happened to visit the MIT Museum, where a curator took him through an exhibit on holography, a technique that creates three-dimensional images by superimposing two beams of light on a physical material.
“I realized what they’re doing in holography is pretty much the same as what nature is doing with structural color,” says Miller.
This visit spurred him to learn about holography and its history, which took him back to the late 19th century and Lippmann photography – an early color photography technique invented by Franco-Luxembourgish physicist Gabriel Lippmann, who later Technology received the Nobel Prize in Physics.
Lippmann created color photos by first placing a mirror behind a very thin, transparent emulsion—a material 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 grains of the emulsion to reconfigure their position like many small mirrors, reflecting the pattern and wavelength of the exposure light.
Using this technique, Lippmann structurally projected colored images of flowers and other scenes onto his emulsions, although the process was laborious. It involved making the emulsions by hand and waiting days for the material to get enough exposure to light. Because of these limitations, the technique largely disappeared into history.
A modern twist
Miller wondered if Lippmann photography coupled with modern holographic materials could be accelerated to produce large format, structurally colored materials. Like Lippmann’s emulsions, current holographic materials are composed of photosensitive molecules that can crosslink when exposed to incident photons to form colored mirrors.
“The chemistry of these modern holographic materials is now so responsive that it is possible to perform this technique in a short time simply with a projector,” Kolle notes.
In their new study, the team stuck an elastic, transparent holographic film onto a reflective, mirror-like surface (in this case, aluminum foil). The researchers then set up a commercially available projector several feet from the film and projected images onto each sample, including Lippman-esque bouquets of flowers.
As they suspected, in minutes instead of days, the films produced large, detailed images that vividly reproduced the colors of the original images.
They then peeled the foil off the mirror and taped it to a black elastic silicone pad for support. They stretched the foil and observed how the colors changed – a consequence of the material’s structural color: as the material stretches and thins, its nanoscale structures reconfigure to reflect slightly different wavelengths, for example from red to blue.
The team found that the color of the film is very sensitive to stress. After making a completely red foil, they glued it to a silicon backing of different thicknesses. Where the support was thinnest, the film stayed red, while thicker sections stretched the film, turning it blue.
Similarly, they found that pressing various objects into samples of red film left detailed green imprints caused, for example, by the seeds of a strawberry and the creases of a fingerprint.
Interestingly, they were also able to project hidden images by tilting the film at an angle to the incoming light when creating the colored mirrors. Essentially, this tilt caused the material’s nanostructures to reflect a red-shifted light spectrum. For example, green light used during material exposure and development would cause red light to be reflected, and red light exposure would result in structures that reflect infrared — a wavelength that is invisible to humans. When the material is stretched, this otherwise invisible image changes color and shows up as red.
“This is how you could encrypt messages,” says Kolle.
Overall, the team’s technique is the first to enable large-scale projection of detailed, structurally colored materials.
In fact, Kolle notes that the new color-changing materials are easy to incorporate into textiles.
“With Lippmann’s materials, he couldn’t even have produced a Speedo,” he says. “Now we could do a whole leotard.”
Beyond fashion and textiles, the team is researching applications such as color-changing bandages to monitor bandage pressure levels in the treatment of conditions such as venous ulcers and certain lymphatic diseases.
What color is a mirror? Explain mirrors and how they work.
Benjamin Harvey Miller et al, Scalable optical fabrication of dynamic structural colors in stretchable materials, natural materials (2022). DOI: 10.1038/s41563-022-01318-x
Provided by the Massachusetts Institute of Technology
Citation: Engineers repurpose 19th-century photography technique to make stretchy, color-changing films (2022, August 1), retrieved August 2, 2022 from https://phys.org/news/2022-08-repurpose-19th-century -photography-technique -stretchy.html
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