{"id":2754,"date":"2019-03-01T19:46:39","date_gmt":"2019-03-01T10:46:39","guid":{"rendered":"http:\/\/163.180.4.222\/lab\/?p=2754"},"modified":"2019-03-04T17:16:29","modified_gmt":"2019-03-04T08:16:29","slug":"colour-from-colourless-droplets","status":"publish","type":"post","link":"https:\/\/biochemistry.khu.ac.kr\/lab\/?p=2754","title":{"rendered":"Colour from colourless droplets & \uc0c9\uc18c \uc5c6\uc774 \ubb3c\ubc29\uc6b8\ub85c\ub9cc \uadf8\ub9b0 \uadf8\ub9bc"},"content":{"rendered":"

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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.<\/h5>\n

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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 \u2014 an effect known as structural coloration. In\u00a0a paper in\u00a0Nature<\/i><\/a>, Goodling\u00a0et al.<\/i>1<\/a><\/sup>\u00a0describe 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.<\/p>\n

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.<\/p>\n

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.<\/p>\n

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Figure 1 | Iridescent reflected light.<\/b>\u00a0a<\/b>, Goodling\u00a0et al<\/i>.1<\/a><\/sup>\u00a0report 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.\u00a0b<\/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.<\/span><\/p>\n<\/figcaption><\/figure>\n

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Goodling\u00a0et al<\/i>. 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\u2019s refractive index as a function of wavelength).<\/p>\n

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\u2019s interior surface and exit from the opposite edge of the droplet, acquiring a distinct colour that is due to interference between emerging light rays \u2014 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.<\/p>\n

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