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.
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.