As reported on Wired.
- BY NADIA DRAKE
-
Normally, the colors we perceive are determined by the wavelengths of light reflected by objects in the world around us. But not all surfaces reflect light the same way. Picture an iridescent butterfly, for example. It might look drab from one direction, but explode into bright yellows or purples from another. That’s because of microscopic structures that alter the way light bounces off the butterfly’s wings.At the NanoPhotonics Centre at the University of Cambridge, scientists are tinkering with tiny structures like the ones in butterfly wings to create crazy new materials that manipulate light and change color in strange ways.
“A lot of this stuff is not completely mainstream,” said Jeremy Baumberg, who directs the center. “People think it’s a bit weird.”
During a recent visit to Cambridge, I sat down with Baumberg to talk about some of the projects he and his colleagues — engineers, physicists, chemists, materials scientists, and biologists — are working on. This gallery shows off a few highlights.
The secrets behind these multicolored materials lie in the tiny nanostructures they’re made from: spheres, helices, tangled gyroids, lattices, super-thin membranes, and stacks. “The nice thing about all these materials is they’re a very visual example of nanotechnology,” Baumberg said. “The features and the color all come from structure.”
Above:
Multicolored balloons
Super-thin, transparent nanolayers reflect different colors depending on their thickness and the interface between them. If those layers are flexible enough, they can be folded and stacked to create inflatable membranes that change color as they inflate. That’s how the lab created these psychedelic looking bubbles. “You blow air through the membrane, and you get these color-changing bubbles,” Baumberg said.
In addition to groovier party balloons, such technology could inspire practical products like tires that change to green when inflated to the right air pressure.
Image and Project: Gen Kamita, NanoPhotonics Cambridge
-
Color-changing wallpaper
Your white walls are so boring. What if you could flick a switch and watch them become something less predictable?
Baumberg and his group are working on wall coatings that can change color. These coatings would contain transparent nanostructures arranged in precise matrices. Changing the structures’ separation – by running an electrical current through the coating, for example – would shift the wavelength, and the color, they reflect. The group has already figured out how this could work, and they’ve made sheets almost big enough to cover a cellphone. The challenge now is coming up with a good way to make sheets big enough to liven up your dreary office.
“The thing that’s important to me is not making a tiny bit of material using expensive technology,” Baumberg says. Instead, he’s aiming for materials that are low-cost and easy to produce.
Image and Project: Jan Martens and Laura Brooks, NanoPhotonics Cambridge
-
Biomimetic berry-based structures
Dry a bunch of flowers for a few months and they’ll lose much of their color, fading into dusky, wilted versions of their original hues. But not Pollia condensata. These brilliant berries maintain their bluish purple iridescence for decades. They’re the most intensely colored biological thing on Earth.
Last year, Baumberg’s team, including collaborator Silvia Vignolini, worked out the secret to the berries’ transfixing, glittery color. Rather than incorporating pigments that fade – like those in the flower petals – the berry’s surface cells build color-generating structures in their membranes. The structures are made from cellulose nanofibers, laid down in periodic, twisting helices. The tiny spirals act as reflective cavities and bounce light around, ultimately amplifying and reflecting the bluish purple hue.
Now the group is working on replicating these color-producing structures using cellulose nanocrystals. “It’s not easy to do at the moment,” Baumberg said. There are some good reasons for trying, though. Remember those blue M&Ms that are no longer around? Turns out, most blue food dyes are carcinogenic. Maybe, if the group gets it right, they’ll come up with a nontoxic dye that can be used to resurrect those long-lost chocolaty treats.
Image and Project: Silvia Vignolini, NanoPhotonics Cambridge
-
Holey gold
A 3-D gyroid metamaterial made from gold sounds totally whack. And it kind of is. The various twists and turns assumed by the metal affect the way light moves through it.
Baumberg and his team make this weird material by starting with a polymer that self-assembles into the gyroid lattice. “It will do that if you cool it slowly enough, and at the right concentration,” Baumberg said. The lattice, he notes, “is really beautiful.” Then, they add a second polymer that fills in the empty space. Removing the original, self-assembling polymer leaves a veiny mold that can be filled in with gold. Once the gold is in, they dissolve away the mold, leaving behind a 3-D metal lattice. “I call it holey gold,” Baumberg said. “It has completely different properties from gold. Light worms its way through differently.”
These types of structures could be used to enhance the conversion of light into electricity in solar cells, or to create metals with completely different properties than the original.
Image: Stefano Salvatore, NanoPhotonics Cambridge
-
Polymer opals
Named for their resemblance to an opal’s flickering colors, these materials change color when stretched. What initially looks like a shiny piece of yellow elastic shifts through green to blue. Patterns printed onto the color-changing stretchable also morph as the material’s shape shifts.
These materials are made by laying a matrix of tiny spheres onto a stretchable fabric. Spheres of a certain size, separated by a certain distance, will reflect light of a certain color. But as you yank on the fabric and increase the distance between the spheres, the wavelength of light they reflect shifts. Suddenly, instead of orange, you have something green.
It’s tempting to imagine where these color-changing materials might find a home. Perhaps not surprisingly, Baumberg says the most interest the team has gotten has been from the fashion industry. Already, runways in Paris have showcased garments incorporating these stretchables. Maybe we’ll be able to wear a trippy, color-changing jumpsuit to Burning Man next year?
Designer, and Image: Jasna Rokegem/Qibin Zhao, NanoPhotonics Cambridge; Video: YouTube
-
Woodpile nanostructures
In photonics, woodpile structures are layered materials stacked in much the same way as an actual woodpile – except of course that they’re not made of wood. Also, they’re very tiny. And they control light in weird ways.
“They’re very technically difficult structures to make, on a 100-nanometer scale,” Baumberg said. Working in this size range allows the material to trap and reroute light because it creates regions within the structure where electromagnetic radiation, including visible light, cannot pass. Baumberg and his team are working on creating metal-containing woodpile structures, using gold wires and other flexible materials. Because these structures can direct how light (and information) moves through a system, they’re useful in communications systems and sensors.
Image and Project: Lindsey Ibbotson, NanoPhotonics Cambridge
-
Stretchable metals
Single-layered, stretchy mats made of gold nanoparticles and elastic materials could function very effectively in cell phone sensors. Someday.
When these mats are relaxed and the gold particles are near one another, the material looks like gold and acts like a metal. That is, electrons can hop from particle to particle. But if you stretch the mat and increase the distance between the gold nanoparticles, the material morphs. First, its color changes to red. And second, it starts behaving like an insulator since electrons have a tougher time moving from particle to particle.
Baumberg envisions these kinds of nanoparticle mats finding a place in sensors – perhaps in the form of disposable films that could be coupled to a mobile phone. The mats could be made sensitive to different things by intercalating different molecules between the gold particles. “For instance, it could measure the contaminants in the environment, or in something you ate, or something in our sweat,” Baumberg said. When the right molecules interact with the particle mat, it will swell and turn red.
Image and Project: Matthew Millyard, NanoPhotonics Cambridge
-
Plasmonic gold lattice
This tube is made from a rolled up, superthin lattice of gold cups. Each of the cups traps and concentrates light, allowing certain colors to spiral around the tube. Those colors change as the tube changes shape, and varying the intensity of light shined on the tubes causes them to roll and unroll. The tube, and its cups, are able to control and manipulate light on a nanoscale. Related technologies are used in molecular diagnostics.
Image: Fumin Huang, NanoPhotonics Cambridge
-
Color-changing wrapping paper
Thin, clingy films that reflect a myriad of colors can not only be styled into balloons (see the first slide in this gallery), they can be used to wrap stuff up. Baumberg and his team do this by floating the filmy layers on water, to keep them flat and prevent them from clinging to everything, then rolling them around objects, like the human hairs in the image above. We know it’s highly improbable that this kind of technology will ever find its way into hair salons, but if does – yes, please.
Image: Mathis Kolle and Nick Gibbons, NanoPhotonics Cambridge