Letting air out of a hole underwater

Let's take a look!

What type of experiment is this?

Experimental procedure and explanation:

• Create a hole of about 2 mm at the bottom of a plastic cup (in this experiment, a pin vise was used to make a hole about 2 mm in diameter).
• When you submerge the cup in water, the air inside the cup is forced out through the hole until the outflow stops. However, a small amount of air remains inside the cup because the surface tension of the water creates an interface that maintains a balance between the slightly higher air pressure inside the hole and the water pressure above the hole.
• Next, experiment with making the hole smaller, with a diameter of about 0.5 mm (made with the tip of a thumbtack).
• A smaller hole results in a stronger surface tension, and the outflow of water stops when the air pressure is higher than the one is described above, which results in more air remaining inside.
• Generally, a smaller hole gives rise to a higher surface tension.
• In the video with the 0.5-mm hole, it may seem like air is being sucked in, but this is an optical illusion caused by the video's 30 frames per second playback. In reality, air bubbles are coming out of the hole and rising to the surface.

[Note]	Please be careful not to hurt yourself when creating a hole in the plastic cup. Please do this with the help of an adult.
[Keywords]	Surface tension
[Related items]	Water that won't fall (on a board with a hole)
[Reference]	“The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs, p. 62–67. “Illustrated Fluid Dynamics Trivia,” by Ryozo Ishiwata, Natsume Publishing, p. 48–49.

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