Why does the Water Dome Grow Larger?

Let's take a look!

What type of experiment is this?

Experimental procedure and explanation:

• In June 2017, an experiment called the Water Dome was published. When water is poured onto the cap of a plastic bottle, it forms a small dome of water.
• A related phenomenon gained popularity on social media when a child noticed that touching the dome with a finger causes it to grow larger. Despite many discussions, the exact cause of this effect remains unclear.
• Several theories have been proposed, including 1) the pressure inside the dome differs from atmospheric pressure, and touching it equalizes the pressure, causing the dome to expand; 2) the Coandă effect might explain the dome’s growth; 3) surface tension could play a key role.
• In this experiment, I set out to examine the various hypotheses and identify the correct underlying principle.
• It’s remarkable that a child was the first to notice this phenomenon. This highlights the importance of intellectual curiosity in everyday life—observing the world, recognizing the unusual, and asking “why?” is the essence of discovery.
• We’ll explore and evaluate each proposed explanation below. Once we understand the true cause, we can apply this knowledge to other phenomena and even design new experiments. Why not try your own version of the experiment during summer vacation?
• This video was produced with the support of the JSPS Grant-in-Aid for Scientific Research (18K03956).

[Attention]	Water is used throughout these experiments, but it’s important to remember that water is a valuable resource. To minimize waste, consider reusing the water after each experiment—for bathing, cleaning, watering plants, and other practical purposes.
[Keywords]	Surface tension, inertia
[Related items]	Water dome
[References]	Ryozo Ishiwata and Mitsumasa Nemoto, “The Wonder of Flow,” Kodansha Bluebacks, pp. 62–67. Ryozo Ishiwata, “Illustrated Fluid Dynamics Trivia,” Natsume Publishing, pp. 48–49.

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