Humans have been searching for better ways of making colours for centuries, frequently turning to nature for inspiration. The earliest colours used in art and clothing were naturally occurring pigments and dyes, which selectively absorb certain wavelengths of visible light. By contrast, the complex colours found in butterfly wings and mother-of-pearl are produced not only from pigmentation, but also by the scattering of light from microscopic structures whose sizes are roughly the same as the wavelengths of visible light — an effect known as structural coloration. In a paper in Nature, Goodling et al.1 describe another method for achieving brilliant colours that is based on the scattering of light from small droplets. This phenomenon parallels some of the most beautiful displays of colour found in the sky.
Goodling and colleagues observed that asymmetrical, micrometre-scale liquid droplets showed pronounced coloration when a beam of white light was reflected from them. This was surprising because the droplets were inherently colourless. The coloration must therefore arise from interactions of the light with the structure of the droplets.
When the authors examined the droplets under a microscope, they observed that the coloured light emerged specifically from the edges of the droplets, and therefore forms circular haloes around the edges (Fig. 1). Moreover, the droplets were iridescent: they changed colour depending on the viewing angle, in some cases from pink to yellow, to green to blue, to no colour at all. For a fixed viewing angle, the colour of the light reflected from the droplets depended strongly on the droplet size and morphology. For example, suspensions of droplets of different sizes were a shimmering white, whereas suspensions of droplets of a similar size were a uniform colour.
Goodling et al. carried out a series of experiments and modelling studies to investigate the physical mechanism behind the coloration effect. Unlike the rainbow of colours obtained when white light refracts through glass, the dependence on viewing angle and the range of colours observed from the droplets cannot be explained by material dispersion (the variation of a material’s refractive index as a function of wavelength).
Instead, the authors propose that light rays entering a droplet along an edge are redirected along the curved surface of the droplet by a process known as total internal reflection. The light rays pass along the droplet’s interior surface and exit from the opposite edge of the droplet, acquiring a distinct colour that is due to interference between emerging light rays — the interference accentuates or mutes different wavelengths in the visible light spectrum. The acquired colour also depends on the specific path taken by light rays through the droplet, which explains why the coloration is highly sensitive to droplet size, morphology and viewing angle. Further refinement of the modelling methods, perhaps involving 3D simulations of the electromagnetic fields of the white light in the droplet, will undoubtedly uncover more details of the physics underlying this colourful effect.
Goodling and co-workers are not the first to observe colours due to light scattering from tiny droplets. Atmospheric optical effects, such as rainbows, coronas and glories, owe their brilliant displays of colour to the intricate interplay between sunlight and submillimetre-scale water droplets2,3. The phenomenon of glories, in particular, bears some similarity to the coloration effects observed by the authors.
Glories are most commonly seen when clouds are viewed from above (for example, from an aeroplane), and occur as concentric rings of colour around the shadow of the observer (or, if the observer is in the plane, around the plane’s shadow). They are caused by the interference of rays of sunlight that have been scattered by droplets in clouds4,5, and can be explained by a well-established set of solutions to Maxwell’s equations known as Mie theory3. However, Mie theory describes scattering only from spherical particles, and therefore cannot be directly used to explain Goodling and colleagues’ observations, which involve non-spherical particles. Further exploration is needed to determine whether the coloration of the authors’ droplets shares the same physical origin as atmospheric glories.
Goodling et al. report that their droplets can be used in 2D arrays to create pixelated images. They manipulate the colour of each pixel by tailoring the droplet shape and size, or liquid composition. Furthermore, the coloration effect can be achieved using a wide range of materials and geometric shapes — besides droplets composed of different liquids, Goodling et al. demonstrate that solid particles and polymeric microstructures can also exhibit this effect.
The incorporation of this technology into displays and sensors is an exciting prospect, but will be challenging to achieve. Unlike pigments, colours produced using this method are seen only in reflected light at certain viewing angles, and require lighting from a fixed direction, which might limit the range of possible applications. The extent to which the coloration effect can be used to manipulate and tailor the spectral signatures of 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 full range of visible colours can be produced through systematic tuning of droplet shape and composition. This remains to be seen, but the range of colours achieved is already impressive, and the reported spectra are quite complex. It therefore seems possible that we could soon be able to fabricate surface structures that produce designed, iridescent patterns of light that are highly responsive to the environment and to the observer’s location.
Nature 566, 458-459 (2019)