Making a Rainbow with Water Drops!

Featured Image: Reprinted by permission from Goodling, A. E.; Nagelberg, S.; et al. Nature 2019, 556, 525-527. Copyright 2019.

Title: Coluration by total internal reflection and interference at microscale concave interfaces

Authors: A. E. Goodling, S. Nagelberg, B. Kaehr, C. H. Meredith, S. I. Cheon, A. P. Saunders, M. Kolle, and L. D. Zarzar

Journal: Nature

Year: 2019

https://dx.doi.org/10.1038/s41586-019-0946-4

Have you ever looked at a rainbow and been amazed how all those colors can come from millions and millions of tiny droplets of water in the air? That’s why rainbows form after it rains, because all the drops of water refract (or split up) the sunlight into the different colors of the spectrum. But refraction is not the only way that this can happen. In this paper, researchers have found a method to use droplets of water to see all the colors in a completely new way.

The physical effect that causes these beautiful colors is known as total internal reflection. This is when a beam of light passing through a material is completely reflected off of the edge where it meets the air, bouncing back into the material (Figure 1A). Fiber optic cables and other tools use this to “trap” light inside of them and direct where it goes.

Figure 1: A) Example of total internal reflection of a laser beam within a material. B) Total internal reflection inside of hanging droplet leading to interfering light beams. Image in A by Zátonyi Sándor via CC BY-SA 3.0. Image in B adapted by permission from Goodling, A. E.; Nagelberg, S.; et al. Nature 2019, 556, 525-527. Copyright 2019.

In this paper, the researchers found that they could see total internal reflection inside of drops of liquid hanging off a surface. When light was shined from the top, it traveled through the droplet until it hit the curved bottom of the drop. Then it bounced across the droplet before shooting back out the top again (Figure 1B). However, because different rays of light are reflected off of different parts of the curved surface of the droplet, they each travel different distances, and come out of the top with their wavelengths shifted. A complicated process called wave interference happens because of this, where some wavelengths fade away and others are brighter. What this means for the rainbow is that only certain colors of light come out of the top of the droplets!

While the researchers first observed this with complicated emulsion droplets (which are one type of liquid mixed with another type) that had two different types of oil inside of them, they then found that this could be done with just water as well. Because this effect is determined by the size of the droplets, all the droplets of a certain size are the same color. So, a collection of droplets of all different sizes shows a rainbow of different colors (Figure 2A). A dish of these droplets even shows colors to the naked eye when illuminated! And because different colors come off of the droplets at different angles, if the dish is rotated, all the droplets appear to change color (Figure 2B). In the Supplementary Information for the article (which is linked above), there is great video of this happening!

Figure 2: A) Different size water droplets showing different colors of light. Scale bar in inset 1 cm. Scale bar of droplets 200 µm. B) Rotating the dish of droplets changes the color that is seen. Scale bar 2 mm. Adapted by permission from Goodling, A. E.; Nagelberg, S.; et al. Nature 2019, 556, 525-527. Copyright 2019.

This interesting effect could even be applied to more complicated systems. Like mentioned above, the scientists first explored this using emulsion droplets with different oils inside of them. But by changing the way these oils interact with each other, they could change the way the light that came off of them behaved, including both the colors and position of the light. They used special emulsion droplets that were sensitive to ultraviolet light and could change their curvature when exposed. A photomask that blocked light in the shape of a cartoon penguin was placed over the dish and ultraviolet light was shined on the droplets (Figure 3A). The droplets that were exposed then had a different shape than the droplets that weren’t, meaning that they appeared as different colors, revealing the cute penguin (Figure 3B)!

Figure 3: A) Cartoon of how the droplets are exposed to UV light. B) Droplets that were exposed to UV light have different structures and different colors. Scale bar 50 µm. Adapted by permission from Goodling, A. E.; Nagelberg, S.; et al. Nature 2019, 556, 525-527. Copyright 2019.

The exciting thing about these droplets and their colors is that, although the way they seem to work is completely new, the experiments are very simple to set up. Any clear surface that has water droplets underneath it can cause these colors to show up. So, the next time you are boiling water in your kitchen, just stretch some clear plastic wrap over top of it and see the rainbow for yourself!

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