When exhaling underwater, why do multiple bubbles come out and not a big one?
In: Physics
When the air that you breathe out starts to form a bubble, the pressure of the surrounding water is immediately trying to crush that bubble. It is ultimately unable to reduce the size of the displaced air to zero, which means that the bubble remains a bubble, with its size determined by the surface tension of water and the prevailing pressure conditions. The deeper you are, the more pressure, which overcomes the surface tension of the water to a higher degree and creates a smaller bubble. When people are SCUBA diving at deeper depths, hundreds of tiny bubbles are breathed out instead of one or two or three bigger bubbles.
Because the air that you exhale starts rising up to the surface right away, and doesn’t wait for you to “inflate” the bubble to make it bigger. If you could swim upwards as fast as the air bubbles, you could theoretically inflate a big bubble like a balloon, while it rises up to the surface.
The surrounding water pressure, pushing on the air, wants to compress it into a shape where the forces all balance out: a sphere. The air, being so much lighter than water, wants to move upward. Those processes combine to break up the air stream into little approximately-spherical bits. As you blow more air out, the previous air has already been surrounded by water and has risen too far away for more air to enter it.
If you scuba dive, if you are capable of ascending approximately as fast as the bubble you can keep enlarging it. The same forces are doing the same work, but if your movement never lets the bubble get far enough away for the water to pinch it closed, it just keep expanding.
Dolphins do this, as well as blowing air rings, which they then chase upward. They even play around to see who can swim through the rings or ‘pop’ the bubbles (breaking them into a cloud of smaller bubbles).
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