Victoria Hwang

During a Massachusetts winter, the colors of the cardinal and the blue jay stand out prominently against the gray skies. The cardinal gets its color from red carotenoid pigments in its feathers that absorb all the non-red colors in the visible light spectrum, reflecting back only red. According to Victoria Hwang, a PhD candidate in applied physics, blue jays use a very different strategy to achieve their coloring.

“Their blue feathers come from an internal structure of keratin, which preferentially reflects blue while other colors are transmitted through the material,” Hwang explains. This special type of color is called structural color and because it doesn’t absorb other wavelengths of light, it doesn’t degrade like pigment-based colors. “Absorption degrades over time, meaning pigment-based colors fade easily,” Hwang says. “Structural colors can virtually last forever as long as the structure remains intact.”

This internal structure can take many forms, the simplest being packed nanospheres. Hwang explains that as the properties of these nanospheres change—their size, the material they’re made of, how tightly packed they are—so does the color they reflect. “That’s why one of the possible applications for structural colors is adding color to Kindles,” she says. “You can’t add all available pigments to a screen, but you can add a set nanospheres and theoretically display a spectrum of colors by changing the spacing between them.”

The nature of structural colors—their malleability coupled with their resistance to fading—have made them a promising tool in many industries, from automobiles to makeup. But, while the physics of structural colors is known, predicting what color arises from a particular structure has been difficult. Hwang is fixing this problem by creating a model that brings together all previous knowledge about the physics of structural color, which provides accurate predictions. “Previous models could tell you that a color is blue-ish, but they couldn’t say what kind of blue,” Hwang explains. “With our model, we can tell you the exact tone of blue as well as its saturation.”