The researchers found that under certain conditions, ordinary drops of pure water on a transparent surface can create bright colors without adding ink or dyes. They have developed a model that allows you to accurately determine the color that will have a drop, considering the specific structural and optical conditions.
Scientists studied samples of transparent emulsions from various water-based oils in a Petri dish. In the process of observation, they noticed that some elements had bright blue color. At first they suggested that this could be associated with the same effect of light scattering, which causes the formation of a rainbow. However, the observed drops were not spheres, but had the form of the dome on the surface. Therefore, the group began to study this phenomenon in detail.
It turned out that drops in the form of a hemisphere caused a complete internal reflection of the rays, resulting from its passage between media with highly characterized refractive indices. In fact, when the light falls into a drop, he can go different ways, repeatedly reflected before going out. At the same time, one beam can be challenged twice, and the other three, retaining, having passed a longer route. Such a phase delay can cause an interference effect that enhances certain waves, which ultimately gives drops color.
Researchers from the Massachusetts Institute of Technology and the University of Pennsylvania found that the color of the drops depends not only on the refractive index of the drops, but also on the structural conditions, such as size and curvature. They included all these parameters in a mathematical model that allows us to determine the colors that will appear depending on the conditions and checked it in the laboratory conditions.
At first, the group decided to strengthen the effect, creating drip oil-based water-based emulsions, the dimensions of which they could accurately control using a microfluidic device. They have formed a «carpet» from the drops of the same size and directed the source of white light on it. When the observer moves, the created structure was overflowing with various colors. It was found that with an increase in the size of the drop, the color shifts to the red spectrum, and in smaller — to blue. Scientists also created tiny solid transparent caps on a 3D printer and observed a similar effect.
According to researchers, this optical feature can be used in light displays, lactium tests and cosmetics without synthetic dyes.
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