Microdrops create controllable color

Thin films, prisms, and nanoscale periodic lattices can all use their geometries to turn white light into a range of colors. Even simple water droplets can disperse sunlight into a rainbow. Building on those well-understood phenomena, Amy Goodling, Sara Nagelberg, and their collaborators at the Pennsylvania State University, MIT, and Sandia National Laboratories have uncovered a new mechanism for generating color using curved micron-scale interfaces.

The unexpected colors first appeared when the researchers were observing biphasic oil droplets dispersed in water. When the drops were illuminated from above, those with an upward-facing concave interface between the two oil phases were encircled by colored rings. The color depended on the size of the drop and the observation angle. Suspecting that the curved interface was responsible, the researchers adapted the experiment to a simpler system: sessile water drops condensed on the underside of a clear plastic surface. The illuminated droplets developed the same colored rings as the oil droplets under white light, as shown in the first figure.

The researchers turned to the basic optical principles of total internal reflection and interference to explain their observations. Light rays can enter through the upper surfaces and undergo repeated internal reflections along the inner curved surface, as illustrated in the second figure. Exiting rays with different numbers of reflections have taken different path lengths, so they interfere constructively or destructively depending on their wavelengths. By considering the possible light paths through a drop, the researchers identified the wavelengths that gained intensity due to constructive interference at a given viewing angle. That approach enabled accurate predictions of the drops’ size- and angle-dependent colors.

Goodling, Nagelberg, and colleagues then made large-scale patterns by varying the wetting angles of the drops in different regions. In one example, a light-responsive surfactant enabled the ability to dynamically change the patterns. Solid particles can display the same coloration as their liquid counterparts and could be used in inks or paints. However, their orientations would somehow have to be controlled to get the right color. (A. E. Goodling et al., Nature 566, 523, 2019 .)