Engineers at MIT and Penn State University have devised of a way to make ordinary clear water droplets produce beautiful vibrant color without the use of inks or dyes. The effect is created through the use of a fine mist of transparent droplets lit with a single lamp.
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They owe these iridescent shades to something called “structural color." The term refers to the creation of color through microscopically structured surfaces fine enough to interfere with visible light.
Now, they believe this effect could be used instead of often harmful synthetic dyes. They even created a model to predict the resulting colors.
“Synthetic dyes used in consumer products to create bright colors might not be as healthy as they should be,” says Mathias Kolle, assistant professor of mechanical engineering at MIT.
“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.”
The novel model takes everything into account including the droplets' structural conditions and their refractive indices. The researchers have already tested the model’s predictions in several experiments.
Testing the model
First, they created droplet emulsions with controlled sizes. This test resulted in what the researchers call a “carpet” of droplets of the exact same size in a Petri dish. They then illuminated it with a single white light and recorded the droplets with a camera.
As the camera turned the researchers were able to witness how the angle at which light enters the droplet affects their color. The team then tested droplets of various sizes. Impressively enough, the model's predictions proved correct in all these scenarios.
They further sought to test the importance of curvature in a droplet’s. To achieve this they used water condensation on a transparent film treated with a water-repelling solution. They found an equally beautiful pattern of color, predicted by the model, that almost looked elephant shaped.
Finally, the researchers tested their model on 3-D-printed solid caps and domes from various transparent polymer-based materials. The model was once again accurate in predicting the resulting color patterns.
“There’s a complex parameter space you can play with,” Kolle says. “You can tailor a droplet’s size, morphology, and observation conditions to create the color you want.”
The researchers believe the model may be used to design droplets for many color-changing applications ranging from litmus tests to color-changing ink in makeup. The model would serve as an efficient design guide for all these options.