Why Are Soap Bubbles Round?

What Is Surface Tension?

Water molecules strongly attract each other. Deep inside a liquid, each molecule is pulled equally in all directions by its neighbours — so the forces cancel out. But a molecule at the surface has no neighbours above it. It gets pulled sideways and downward, creating a net inward force.

This inward pull makes the surface behave like a stretched elastic membrane. We call this surface tension. It is why water droplets are round, why small insects can walk on water, and why soap bubbles are spheres.

Why a Sphere?

Surface tension makes the water surface want to be as small as possible. It costs energy to have surface area — the more surface, the more molecules are at the "unhappy" boundary.

Given a fixed volume of air, what shape has the smallest surface area? The answer, proven by mathematics, is a sphere. A sphere encloses more volume per unit of surface area than any other shape.

So when a bubble forms, surface tension pulls the film until it achieves the minimum possible surface area for the air trapped inside — a perfect sphere.

How Does Soap Help?

Pure water forms droplets, not lasting bubbles. This is because pure water has too high a surface tension — the film tears apart immediately.

Soap molecules (surfactants) are special: one end of the molecule loves water (hydrophilic) and the other end hates it (hydrophobic). When soap is added to water, the soap molecules line up at the surface with their water-hating ends pointing outward. This dramatically lowers surface tension from about 72 mN/m to around 25–40 mN/m.

A soap bubble is actually a very thin film with two soap layers: one on the outside face of the film, one on the inside. The water layer sandwiched between them is only a few hundred nanometres thick — which is why bubbles produce colours!

The Laplace Pressure

Because the bubble surface is curved and under tension, the air pressure inside a bubble is higher than outside. This extra pressure keeps the bubble inflated. The relationship was worked out by Pierre-Simon Laplace in 1805.

ΔP = 4γ / R

Here ΔP is the pressure difference, γ is the surface tension of the soap film, and R is the radius of the bubble. The factor 4 (instead of 2) comes from the fact that a bubble has two film surfaces.

This also explains why small bubbles have higher internal pressure than large ones. When two bubbles merge, air flows from the smaller (higher pressure) into the larger — that's why the smaller one shrinks as the larger one grows.

Why Are Bubbles Colourful?

The thin soap film causes thin-film interference. Light reflects off both the outer and inner surface of the film. When the path difference between these two reflections equals a whole number of wavelengths of a particular colour, that colour is amplified (constructive interference). Other colours cancel out (destructive interference).

As the film drains and thins over time, different colours appear in sequence. Just before a bubble pops, the film is so thin (less than 100 nm) that all colours cancel — the surface looks black or transparent, a sign the bubble is about to burst.

Try It Yourself

• Mix 6 parts water, 2 parts dish soap, and 1 part glycerine for very long-lasting bubbles. The glycerine slows evaporation.
• Try blowing a bubble inside a bucket of misty air — you can sometimes see the thin-film colour bands clearly.
• Hold two bubbles together. Notice how the connecting wall is always a flat disc (when they're the same size) — this is Plateau's law.