Have you ever seen a condensation of fine water on a transparent surface that shines like a rainbow? Scientists have now discovered exactly how this happens – and used this new knowledge to make water droplets produce a dazzling array of colors.
If the droplets are on a transparent surface and illuminated by a single lamp, they will glow. In addition, droplets of the same size produce the same color.
The trick lies in how the shape of a water droplet interacts with light, producing the different wavelengths we perceive as color.
When light enters the droplet, it jumps around the inner surface before it resurfaces. As the size of the drop has an effect on the heel, it influences the resulting color we see.
This phenomenon, according to which the structure of an object produces colors, is called a structural color. It is common in nature – the wings of a butterfly or the skin of a chameleon, for example, appear colored to us when light jumps from the microstructures.
By studying how light interacts with water droplets, engineers have developed a model to predict the color a drop of water will produce under specific structural and optical conditions.
The project began when Amy Goodling and Lauren Zarzar of Penn State studied droplet emulsions. They noticed that the drops looked blue and asked the MIT mechanical engineer, Mathias Kolle, for an explanation.
At first, the researchers thought it might be similar to the refraction mechanism that produces rainbows from spherical droplets of water.
"Initially, we followed this rainbow effect," said MIT engineer Sara Nagelberg. "But it turned out to be something quite different."
The droplets that rest on a surface are hemispherical, rather than spherical like the rainbow-generating mist, and that is a critical difference. This hemispherical shape allows for a phenomenon that is not possible with spheres.
It is called total internal reflection – when light enters a dense medium from a less dense medium at a high angle, this results in 100% of the light being reflected in that denser medium (in this case, it was the glass surface of the plate Petri). ), without any refraction.
When a ray of light enters a droplet, it will jump there before exiting at another angle. As the light rays combine in the output is a factor that contributes to the color production.
Other factors that affect the resulting color include the size, bending and refractive index of the drop, as well as the light used. The predictive model took all this into account.
Then the team tested by spraying an even layer of droplets on a petri dish or transparent film, lighting it and moving the camera around. This last part changed the angle at which the reflected light would enter the eye, affecting the color.
The size of the drop also made a difference. Larger droplets produced more red colors, which are produced by longer wavelengths, while smaller droplets tended to blue, produced by shorter wavelengths.
In addition, the team experimented with a coating to produce droplets of different sizes in a single film, to determine if several colors could be obtained (could), as well as to make solid transparent microstructures in the form of droplets to determine if the model could also be applied to these (could).
With these results, the team believes we can produce pigments that do not depend on chemical dyes.
"As some of these dyes are more heavily regulated, companies are asking, can we use structural colors to replace potentially unhealthy dyes?" "Dyes used in consumer products to create bright colors may not be as healthy as they should be." Kolle said.
With this model, maybe we can.
The research was published in Nature.