Iridescent colours have been observed to be reflected from specially designed droplets of colourless liquids, with the reflected colour depending on the viewing angle. The finding reveals a curious mechanism for creating coloration.



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.


Figure 1 | Iridescent reflected light. a, Goodling et al.1 report that asymmetrical, micrometre-scale liquid droplets dispersed in a transparent fluid medium reflect coloured light from their edges when illuminated by a beam of white light. The coloration depends on the size and shape of the droplets. b, The colour of the reflected light also depends on the angle at which the droplets are viewed. Here, light reflected from droplets in a Petri dish is projected onto a translucent dome placed over the dish, revealing the colours produced at different viewing angles. Scale bar, 1 centimetre.



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)


doi: 10.1038/d41586-019-00638-4



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네이처 제공


이번 주 국제학술지 네이처는 프리즘처럼 백색의 빛을 색색으로 흩트리는 물방울의 모습을 표지에 담았다. 백색 빛은 서로 다른 색 파장의 합이다. 파장의 길이에 따라 굴절하는 정도가 달라 짧은 파장을 가진 보라색이 가장 많이 굴절하고 붉은색 긴 파장이 가장 적게 굴절하면서 무지개처럼 빛의 분리가 일어난다. 플라스틱 접시나 물병에서도 무지개가 가끔 보이는데 빛이 물체의 구조에 따라 굴절을 달리하며 일어나는 현상이다.

로렌 자자 미국 펜실베니아주립대 화학부 교수 연구팀은 실험용 접시에 붙은 기름이 섞인 투명한 물방울을 관찰하던 도중 물방울이 푸른색으로 빛나는 것을 발견했다. 처음에 연구팀은 무지개를 만드는 현상과 같은 원리라고 이해했다. 하지만 무지개를 만드는 원리인 미 산란은 물방울이 구형일 때만 일어나는 현상이다. 접시에 붙은 물방울은 반구 형태다.

연구팀은 물방울에서 색상이 나타난 원인을 전반사에서 찾았다. 전반사는 빛이 한 물체에서 다른 물체로 향할 때 굴절률의 차이에 따라 굴절이 커져 다른 물체로 진행할 수 없을 때 빛이 모두 반사되는 현상이다. 물방울의 바닥 면에서 들어온 빛이 물방울의 구면에서 2번 이상 전반사되어 다시 바닥 면으로 나오게 된다.

내부로 들어가 반사돼 나오는 빛은 프리즘에서 이동 경로가 다르듯, 파장에 따라서 물방울 속 다른 경로를 지나가게 된다. 두 번 전반사되는 파장의 빛이 있는가 하면 네 번 이상 전반사되는 빛도 생긴다. 밖으로 나오는 빛은 경로에 따라 파장의 마루와 골이 달라진다. 경로에 따라 파장이 강해지는 보강간섭과 약해지는 상쇄간섭이 일어나며 강해진 일부 파장만 눈에 보이게 된다. 파란색 파장이 보강간섭으로 남았다면 물방울이 파란색으로 보이는 것이다.

연구팀은 이러한 변수를 계산해서 물방울의 크기와 모양을 조절해 다양한 색상을 얻을 수 있음을 보였다. 물방울이 올라가는 표면의 소수성을 바꾸면 물방울의 반구 모양을 구의 4분의 1 모양, 4분의 3 모양처럼 다양한 모양으로 바꿀 수 있다. 이렇게 만든 물방울을 코끼리 모양으로 배치해 염료가 없이도 단지 빛과 물방울만으로 다양한 색깔로 색칠한 코끼리를 그려내는 데 성공했다. 물방울로 그려낸 최초의 그림이다.


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