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Study Finds Clear Droplets Produce Iridescent Colors

Thursday, April 18, 2019

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According to findings from a combined research team from Penn State University (University Park, Pennsylvania) and the Massachusetts Institute of Technology (Cambridge, Massachusetts), under proper conditions, ordinary clear water droplets can produce colors, without the addition of dyes or inks.

The iridescent effect comes from the droplet’s “structural color,” or how an object generates color based on its geometric assembly interactions with light.

The Research

A team of researchers from Penn State and MIT published a paper in the journal Nature in February 2019 describing how a surface covered in same-size droplets, like a fine mist, and lit with a white light lamp, ultimately produce bright, iridescent colors.

Images: Penn State

According to findings from a combined research team from Penn State University (University Park, Pennsylvania) and  the Massachusetts Institute of Technology (Cambridge, Massachusetts), under proper conditions, ordinary clear water droplets can produce colors, without the addition of dyes or inks.

“The typical way you get color is with dyes or pigments, which have molecules that selectively absorb and scatter specific wavelengths of light,” said Lauren Zarzar, assistant professor of chemistry at Penn State and a leader of the research team.

“Structural color is different. It’s a product of light interacting with a material in a way that causes light interference. Structural color is often iridescent and the color we see depends on the angle we are looking from and the angle of the light. We see it in things like opals, butterfly wings, beetles and bird feathers.”

While studying different mixtures of oils and water-based surfactants—compounds that reduce surface tension in liquids and are commonly used in soaps and detergents—through transparent droplet emulsions, the team noticed the light and color interactions.

Looking a little deeper, the team then produced the same outcome by allowing water droplets to condense on the lids of Petri dishes filled with warm water.

Amy Goodling, a graduate student in materials science and engineering at Penn State and co-first author of the paper added, “We have been working with these droplets for a while because the combination of oils and surfactants create droplets with a particular internal structure. It wasn’t until we started making droplets that were uniform in size that the color became noticeable.”

Having first thought that the colors were produced in a similar way to rainbows, the team continued to test how light entered the droplets.

Since the droplets are not complete spheres, like in the rainbow theory, and more consistent with a hemispheric or dome-shape, when the light entered the droplet, it reacted much differently. Instead of bending or refracting the light and reflecting it back, researchers found that the light would take multiple paths, sometimes bouncing two or more times before exiting at a new angle.

Because of the curve in the hemisphere and the change in refractive index (the measurement used for how fast light passes through an object), this creates total internal reflection, which isn’t possible in perfect spheres.

“It’s like kids making waves in a pool,” said Mathias Kolle, assistant professor of mechanical engineering at MIT and an author of the paper. Kolle also adding that if the kids just splash around, there’s no constructive effort, but by pushing and pulling together, creating those waves in phase so that they come out together, the color is going to be more intense.

The research team has since devised a model that predicts what color a droplet will produce, given the droplet's specifics, such as the size and curvature, in addition to optical conditions and the droplet’s total internal reflection.

Sara Nagelberg, a graduate student at MIT who headed up the modeling effort to try to explain the effect, and Kolle then incorporated all parameters into a mathematical model. Zarzar and Goodling then tested the model’s predictions against droplets they had produced in the lab.

What Happens Next

Expectations point to that idea that the model could someday be used for designing droplets and particles for an array of color-changing applications such as droplet-based litmus tests, color-changing powders, inks in makeup products, paints or displays.

“Synthetic dyes used in consumer products to create bright colors might not be as healthy as they should be,” said Kolle.

“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 and Lauren at Penn State and Sara’s modeling, which brought this effect and its physical explanation to light, there might be an answer.”

Expectations point to that idea that the model could someday be used for designing droplets and particles for an array of color-changing applications such as droplet-based litmus tests, color-changing powders, inks in makeup products, paints or displays.

Expanding beyond just water droplets, the researchers have also 3D-printed solid caps and domes from various transparent polymer-based materials to observe a similar color effect that are able to be predicted by the same model.

“You can tailor a droplet’s size, morphology and observation conditions to create the color you want,” Kolle concluded.

In addition to the researchers quoted in this article, the team also included: Caleb H. Meredith, Seong Ik Cheon and Ashley P. Saunders at Penn State; and Bryan Kaehr at Sandia National Laboratories in Albuquerque, New Mexico. The research was supported by the Department of Materials Science and Engineering, the Department of Chemistry, the Materials Research Institute, and the Office of Science Engagement at Penn State; the U.S. Army Research Office through the Institute for Soldier Nanotechnologies at MIT; the U.S. National Science Foundation, and the Thomas and June Beaver Fellowship.

   

Tagged categories: Coating Materials - Commercial; Colleges and Universities; Color; Colorants; Good Technical Practice; NA; North America; Polymers; Research; Research and development

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