The bath is regarded by many as an area for quiet contemplation, and it is often the humble soap bubble that comes under the consideration of the idle mind. Indeed, they are actually quite interesting things.

What are They and Why Are They Round?

Soap tends to float on the surface of water like a strong flexible layer, and when bubbles rise from below (for instance when air has been pushed underneath by the flow from a tap), they are unable to break through the soap barrier. Instead, as they rise they become coated in a thin layer of soap, forming a little balloon which floats like a sealed bottle on the surface.

The bubble is spherical because all the air molecules within it are pushing outward against the soap film with the same force in all directions. The bubble may start to deform because of external pressures, for instance if you blow on it, but it will always attempt to return to its most stable shape. The molecules both in the soap film itself and in the air inside the bubble are in continual motion, buffeting and poking, so any weak spots such as corners and seams are quickly broken - the bubble's shape always returns to a sphere.

Why Do Bubbles Group?

A bubble floats on top of the water (rather than being embedded within the surface film), and it drags up with it the nearest parts of the water surface, lifting the surrounding film into a small peak with the bubble floating on top. Two bubbles floating close to one another will drift according to local currents or air breezes, but when they come too close each tries to float up the slope of the other's peak, causing them to suddenly rush together. Once collision occurs at the top of the beak, a double bubble is formed.

Why Do Clumps of Bubbles Reform into One Large One?

A double bubble has seams and a flat plane¹. As mentioned above, the continual buffeting of the bubble's molecules will soon find these weak spots and break them down to leave a larger, more stable sphere.