Why are bubbles round?

Episode #5 of the course “Science questions everyone should know how to answer”

Everyone likes blowing bubbles. Who knew a simple combination of water and soap would provide fun for centuries, from young to old. For as fun as bubbles are, they really are rather ordinary. Only two ingredients, and always round. Why never square or star-shaped? If you’ve ever wondered why bubbles are round, there are two simple reasons:

1. Surface tension. Part of the reason bubbles maintain their round shape is the same reason mosquitoes can walk on water. All liquids have what’s called surface tension, which is a certain level of resistance it has to a force penetrating it. This surface tension allows the soap molecules to bind to the water molecules, both inside and out. When you blow on the hoop that produces bubbles, you’re stretching the capacity of these molecules to bind to each other until one pinches off and floats away.

2. A sphere is the smallest possible shape. Nature does not exert more energy than it needs to. The natural world around us has figured out the most economical way to be. The reason bubbles maintain a spherical or round shape is because that is the smallest shape to hold the volume of air and soap inside the ring of water. In other words, water does not want to create more work for itself than it needs to, and a round shape allows it to do just that.

What this also means is that since there’s no pressure in outer space (meaning nothing to push back against the surface tension of the bubble), a bubble can grow to a much greater size. It will still pop eventually, however, because the soap will become too thinly stretched, as opposed to the water on Earth.

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