WHY ARE BUBBLES ROUND?

How surface tension plays a big role in keeping soap bubbles spherical

- WORDS SCOTT DUTFIELD

With a bubble wand in hand and lungs full of air, your breath can create a perfectly spherical soap bubble. But why are bubbles this shape, rather than a cube or prism? A bubble is essentially a ball of trapped air from your lungs that has been encased in a thin film.

This film is created when soap molecules, also known as surfactant molecules, sandwich water molecules with a type of force applied to liquids called surface tension. This force is responsible for keeping the water molecules strongly attracted to one another. Surface tension also reduces the surface area of a liquid by pulling the molecules together. When this soapy film is filled with your breath, surface tension makes quick work of pulling all the water molecules together, shrinking into a shape with the least surface area: a sphere.

A bubble's obsession with being spherical comes from its desire to minimise its surface area. The fluid dynamics of a bubble, like other principles in physics, is all about trying to find equilibrium between the different forces acting on the bubble. The force of pressure pushing on the inside of the bubble, the air pressure applied to the outside of the bubble and the surface tension are balanced when a bubble becomes completely spherical.