How droplets of oil or water can glow vibrant colors | Science News

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How droplets of oil or water can glow vibrant colors

The effect is a type of iridescence, with different colors seen at different angles

2:00pm, March 8, 2019
microscopic water droplets showing iridescent rings of colored light

DYNAMIC DROPLETS  When illuminated with white light, microscopic water droplets emanate iridescent rings of light whose color depends on the drops’ shape and size, as well as a viewer’s perspective.

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Oil and water may not mix, but the two have now revealed a new example of structural color, in which an object’s hue arises from its shape.

Studying droplets made of two layers of clear oil, researchers discovered that, depending on a viewer’s perspective, the tiny blobs glowed a variety of vibrant colors under white light. In a petri dish, same-sized droplets changed color as the dish was rotated (see video below). The same phenomenon, described in the Feb. 28 Nature, occurred with tiny water droplets that collected on the underside of a petri dish’s lid. 

Materials chemist Lauren Zarzar of Penn State and colleagues found that the iridescent hues appear when light strikes a bowl-shaped boundary between two substances — in this case, the water-air barrier on the underside of the water droplets hanging off a flat surface, or a basin-shaped divide between the two layers of oil. Light that enters near a droplet’s edge bounces along this this concave surface multiple times before being reflected and exiting near the opposite edge.

ROUNDABOUT RAINBOW Under illumination, a fine mist of clear oil particles inside a petri dish are iridescent, reflecting different colors from different angles.

Under a microscope, that reflected light creates an iridescent ring whose apparent color depends on the viewer’s perspective. That’s because light waves can take many different ricocheting routes through the droplet on their way from the light source to an observer. When waves of a specific wavelength — for instance, yellow light — line up, they reinforce each other and produce a bright color. But light rays of other wavelengths taking these same routes may get misaligned and wash each other out. Changing the viewing angle changes which pathways a viewer sees, and thus the color.

The researchers also found that changing a droplet’s diameter or curvature altered its apparent color even when viewed from the same angle (see image above). A petri dish filled with two differently curved kinds of oil droplets produced a two-color picture of a penguin (see image below).

The effect also worked with tiny, transparent polymer bumps on flat surfaces. Such research could help make color-changing materials that could be used in cosmetics, camouflage or other products. “You could also design surfaces where you have effectively two different images from different directions, or you see an image only from a very specific direction,” Zarzar says.

Particulate penguin

Droplets made of two oils with differently curved boundaries in between their layers (gray microscope images) produce two different iridescent colors when viewed from the same angle. That allowed researchers to create this blue-and-green picture of a penguin. Seen from a different angle, the film of oil droplets would appear as two other colors, showing someone, say, a purple-and-orange penguin instead. 

a photo of a penguin made from two types of oil droplets

A.E. Goodling et al. Colouration by total internal reflection and interference at microscale concave interfaces. Nature. Vol. 566, February 28, 2019, p. 523. doi:10.1038/s41586-019-0946-4.

Further Reading

S. Milius. Nanoscale glitches let flowers make a blue blur that bees can see. Science News. Vol. 192, November 25, 2017, p. 11.

L. Hamers. Tiny ‘supraballs’ put a new spin on creating long-lasting color. Science News. Vol. 192, October 14, 2017, p. 32.

A. Yeager. New printer creates color by shaping nanostructures. Science News Online, May 5, 2017.

E. Conover. Butterfly-inspired nanostructures can sort light. Science News. Vol. 189, June 11, 2016, p. 32.

D. Powell. Fossil moth reveals colorful hue. Science News. Vol. 180, November 5, 2011, p. 12.

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