• Question: Why are bubbles round?

    Asked by cyanogen_ to Francesca, Laura, Matthew, Andrew, Rebecca on 11 Nov 2014.
    • Photo: Matthew Camilleri

      Matthew Camilleri answered on 11 Nov 2014:

      Well, bubbles are full of air, and air tends to take the volume of whatever objects they are in. If air expands from the same place, as happens in bubbles then it will diffuse equally in all directions, which would produce a sphere, making the bubble shape as round.

    • Photo: Andrew McKinley

      Andrew McKinley answered on 11 Nov 2014:

      Ok, so let’s talk about energy!
      Everything tends to a minimum energy configuration. A ball on the ground has less energy than a ball on a shelf; so given the opportunity, the ball will fall to its lower energy configuration. Magnets are the same – you have to put energy into each magnet to pull two magnets apart, so it is a lower energy configuration if they are stuck together.

      Think now about elastic bands – it takes energy to stretch an elastic band, so a relaxed elastic band has less energy. Take that elastic band, and wrap it around a handful of pencils and set it on the desk. What shape is the elastic band? It is still stretched, but hopefully you will see the band is “circular”. You can deform the elastic band – roll the pencils, and try to separate them so that all the pencils are lying flat. The band will stretch, you’ll have to put energy in to stretch it, but now the band is ‘rectangular’. As soon as you release the pencils, the band will spring back into a circular shape. This configuration allows the “lowest energy per pencil”, or, to put it another way, a circular band is the most energy efficient way to store the maximum number of pencils.

      How does this apply to bubbles? Well, we’re now working in three dimensions – a soap membrane (the bubble’s surface) is stretchy like a rubber band, only this time it is being stretched by the air inside. The sphere is simply a 3-dimensional version of a circle, and the same principle applies – a sphere allows us to minimise the energy required to store a given volume of air.

      I hope that helps?

    • Photo: Laura Schofield

      Laura Schofield answered on 11 Nov 2014:

      I think the boys have covered this one! Andrew’s description of the energy levels is spot on! The air inside a bubble is always moving around, the membrane is what holds the shape. It stays as a sphere because the air inside is pushing out in all directions so the lowest energy shape (the easiest one to maintain) is a sphere.

Comments