What makes a soap bubble round? — MZ, Massachusetts
The molecules in a liquid are touching one another and this touching reduces the molecules’ potential energies. Separating the molecules and reducing the extent of their touching requires energy and is something that the liquid won’t normally do on its own. The molecules at the surface of a liquid have fewer neighbors than they would have if they were in the body of the liquid. Those molecules thus have higher potential energies than they would have in the body of the liquid. To minimize the overall potential energy of a liquid, it naturally tends to minimize its surface area.
When a soap solution has trapped some air to form a bubble, that solution can no longer shrink into a tiny droplet. The air keeps bubble large and it can’t avoid having lots of molecules on its surfaces, where they have higher than normal energies. But what the soap solution can do to minimize its total potential energy is to minimize the number of its molecules that are on the surface. The soap solution experiences what is called “surface tension”—an elastic tightening of its surface. This surface tension tends to minimize the surface area of the soap solution to minimize its potential energy. The soap solution minimizes its surface area around the trapped air by forming a spherical shape. A spherical shell makes the most efficient use of its surface area in enclosing a volume.