(this article is first published in the fruit shell)

when the moist and warm air encounters cool glasses, mirrors, or other similar surfaces, condensed water droplets always put a layer of "mist" on it.

what color does this fog look like? You might say, of course it's white! After all, it is made up of colorless, transparent water droplets.

but it's not that simple. Under certain circumstances, the original colorless water droplets can actually form a layer of colorful fog!

(this is really water. Original video source: ref. 1)

and this is not the best. What is really most powerful is that this colorful layer of water fog was published in Nature and appeared on the cover of the February 28 issue:

(cover picture: Sara Nagelberg)

what is the color fog all about? Let me explain it in detail.

how to have the same color water fog?

to observe the same color fog as the Nature cover study, you first need to prepare a transparent plastic petri dish made of polystyrene. As long as you have such props, the rest is simple: just pour some hot water into the petri dish, cover it, and illuminate it with a beam of white light. strong>

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(the author without PS has a hand-shaking selfie. After acceleration, the actual time is about 10 seconds)

from the actual observation, the color distribution of water fog has been changing since the lid is closed, and the visible time of these colors to the naked eye will not last very long.

and if you look at these colored water fog under a microscope, you will find that there is a bright "color aperture" on the edge of these micron droplets. The "aperture" colors of droplets of different sizes are different.

(under a microscope, there is a colored aperture on the edge of each droplet. I know this picture is very similar, but it is not _ (: pictures "∠) _ picture from the original paper, the scale at the bottom of the big picture is 200 μ m, and the scale of the small picture is 1cm. )

where does color come from?

first of all, I need to note that this is not a mini version of a rainbow, nor is it an ultra-small soap bubble. The cause of color formation and the law of color distribution on water fog are different from these two common phenomena.

researchers have found that color water fog is actually the result of two optical phenomena: one is total reflection, the other is interference.

Total reflection occurs when light is incident from a large refractive index medium and the angle exceeds a critical value.

(dynamic image source: QuantumBoffin)

and when the light is incident from above, it is easy to have a total reflection along the edge of the droplet on the lower wall of the petri dish (see below). Light may be fully reflected from different paths, which have different lengths, so there is a phase difference when the light is finally emitted. Light with phase difference interferes with each other, turning white light into a different color.

(the combined action of total reflection and interference makes micron droplets produce color. Screenshot source: MIT)

this effect depends not only on the refractive index of the droplets, but also on the size and shape of the droplets. Be sure to choose a plastic petri dish instead of glass in order to make the water droplets on the wall form the right shape.

Why are you studying this?

the iridescent phenomenon of small droplets was first discovered by Amy Goodling and Lauren Zarzar. At that time, they were studying transparent droplets on some petri dishes (but not water), and suddenly found that the droplets looked particularly blue, which aroused their research interest. strong>

in addition to satisfying curiosity, this study also provides a new idea for people to make structural colors. Although the small droplets on the petri dish lid cannot remain stable, replacing the droplets with small polymer particles can also achieve the same effect. And the researchers have also established a model for this phenomenon, which can predict the color rules under different conditions.

however, the effect of this iridescent phenomenon depends very much on the light source and viewing angle, which will limit its application prospects.

(turning the angle, the iridescent of the droplet looks completely different. What is shown here is not water droplets, but the principle is similar. The original video is from Ref. 1)

but I am very happy to have a Nature cover with the same style at home.

Resources:

https://www.nature.com/articles/s41586-019-0946-4

http://news.mit.edu/2019/water-droplets-structural-color-0227