For centuries, humans have been searching for better ways to make colors and often look to nature for inspiration. The oldest colors used in art and clothing were natural pigments and dyes, which selectively absorb certain wavelengths of visible light. In contrast, the complex colors of butterfly wings and mother-of-pearl are produced not only by pigmentation, but also by the scattering of light by microscopic structures whose size corresponds approximately to the wavelengths of visible light. This effect is called structural staining. In a paper in Nature, Goodling et al.1 describe another method for obtaining brilliant colors, based on the scattering of light from small droplets. This phenomenon is parallel to some of the most beautiful color sets found in the sky.
Goodling and his colleagues observed that asymmetric liquid droplets at the micrometer scale showed pronounced coloration when a beam of white light was reflected by them. It was surprising because the droplets were naturally colorless. The coloring must therefore result from interactions of light with the structure of the droplets.
When the authors examined the droplets under the microscope, they found that the colored light emerged specifically from the droplet edges and thus formed circular halos around the edges (Fig. 1). In addition, the droplets were iridescent: they changed color depending on the viewing angle, sometimes changing from pink to yellow, from green to blue, or even the absence of color. For a fixed viewing angle, the color of the light reflected by the droplets strongly depended on the size and morphology of the droplets. For example, suspensions of droplets of different sizes were of a shimmering white, while suspensions of droplets of similar size had a uniform color.
Goodling et al. conducted a series of experiments and modeling studies to study the physical mechanism underlying the staining effect. Unlike the rainbow of colors obtained when white light refracts through glass, the dispersion of the material (variation of the refractive index of a material as a function of the wavelength) does not does not depend on the viewing angle or the observed color range.
Rather, the authors propose that light rays entering a droplet along one edge be redirected along the curved surface of the droplet by a process called total internal reflection. The light rays pass along the inner surface of the droplet and exit from the opposite edge thereof, acquiring a distinct color due to interference between emerging light rays – the interference enhances or cuts the different wavelengths of the light. spectrum of visible light. The acquired color also depends on the specific path taken by the light rays through the droplet, which is why the coloring is very sensitive to the size, morphology and viewing angle of the droplets. Further refinement of modeling methods, perhaps involving 3D simulations of the electromagnetic fields of white light in the droplet, will undoubtedly provide a better understanding of the physics underlying this colored effect.
Goodling and his collaborators are not the first to observe colors because of the scattering of light by tiny droplets. Atmospheric optical effects, such as rainbows, crowns and glories, owe their brilliant color palette to the intricate interplay between sunlight and submillimeter water droplets.2,3. The phenomenon of glories, in particular, has some similarities with the color effects observed by the authors.
Glories usually appear when clouds are seen from above (for example, from a plane) and appear as concentric color rings around the shadow of the plane. observer (or, if the observer is in the plane, around the shadow of the plane). They are caused by interference from the sun's rays that have been scattered by droplets in the clouds.4,5and can be explained by a well-established set of solutions to Maxwell's equations known as Mie's theory.3. However, Mie's theory describes scattering only from spherical particles and so can not be directly used to explain the observations of Goodling and his colleagues, which involve non-spherical particles. Further research is needed to determine whether the droplet coloring of the authors has the same physical origin as the atmospheric glories.
Goodling et al. report that their droplets can be used in 2D arrays to create pixelated images. They manipulate the color of each pixel by adapting the shape and size of the droplets, or the liquid composition. In addition, the coloring effect can be obtained using a wide range of materials and geometric shapes – in addition to droplets made of different liquids, Goodling et al. demonstrate that solid particles and polymeric microstructures can also exhibit this effect.
The integration of this technology into screens and sensors is a promising prospect, but will be difficult to achieve. Unlike pigments, the colors obtained using this method are visible only in reflected light according to certain viewing angles and require lighting in a fixed direction, which may limit the range of light. possible applications. The extent to which the staining effect can be used to manipulate and adapt the spectral signatures of the reflected light remains unknown. However, this question can easily be explored, for example by incorporating pigments into the droplets to absorb specific wavelengths of light.
Another question is whether the entire range of visible colors can be produced by a systematic adjustment of the shape and composition of the droplets. This remains to be seen, but the range of colors obtained is already impressive and the reported spectra are quite complex. It seems possible, therefore, that we may soon be able to fabricate surface structures producing patterns of light that are designed and iridescent and very sensitive to the environment and the position of the observer.
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