Researchers at MIT and Penn State University accidentally stumbled upon an entirely new way of creating iridescence. It turns out that provided the conditions are right, plain old water droplets on a transparent surface can transform into a spectrum of dazzling color. No dye or ink necessary.
According to a paper published in the journal Nature, the process hinges on a phenomenon called "structural color" – aka the idea that an object may display color just because of the way the light hits (and responds to) its geometric structure. In this case, hemispheres.
Remarkably, and just as with the "invention" of the X-ray, the discovery was made by chance. Amy Goodling and Lauren Zarzar, both at Penn State, were examining transparent droplet emulsions made from oils of different density when they noticed that they appeared blue. It was then that they approached Mathias Kolle, assistant professor of mechanical engineering at MIT.
Initially, he suspected refraction – the physical mechanism behind rainbows and why pools appear shallower than they actually are. Light is refracted when rays change direction as they move from one medium to another. Sometimes certain mediums deflect different colors by different amounts, and you end up with the rainbow.
But there was just one problem. The droplets where hemispheres on a flat surface, not spherical. This meant the light should behave differently.
What you get is something called total internal reflection (TIR). TIR is the same process that makes fiber optics possible. It describes the way light reflects and disperses when it hits an interface between a substance with a higher refractive index medium (like water) and a lower refractive index medium (say, air) at a high angle. Light rays become "trapped" in that medium and are unable to pass through.
When light enters one of these hemispheric droplets, it is reflected by TIR along the concave interface. How the light rays then combine when they leave the droplet affects the pattern of color.
This process means that the color emerges from the edges of (rather than from within) the droplets, forming halo-like structures around the edges. They also change color depending on the viewing angle. Sometimes from pink to yellow, the researchers say. Other times from green to blue, or no color at all. The size and shape of the droplet, again, affects the color of the light.
"We were really racking our brains for quite some time," Zarzar says of the discovery. "No other explanation came close to matching the effect."
Even though it is the first time the process has been formally described, Zarzar says that the phenomenon is widely recognized. "People have come up to me and said, ‘Oh, I know exactly what you’re talking about! I’ve seen it, too,'" she said.
The team hope that the process could one day be used to create color-changing powders in ink and make-up without the need for potentially harmful chemicals.
"Synthetic dyes used in consumer products to create bright colors might not be as healthy as they should be," Kolle explained.
"As some of these dyes are more strongly regulated, companies are asking, can we use structural colors to replace potentially unhealthy dyes? Thanks to the careful observations by Amy Goodling and Lauren Zarzar at Penn State and to Sara's modeling, which brought this effect and its physical explanation to light, there might be an answer."
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