Why Are Bubbles in a Foam No Longer Round?

An isolated soap bubble is nearly spherical because, for a fixed volume, a sphere has the smallest surface area. Surface tension therefore pulls the film towards a lower-area shape. When many bubbles are crowded together, however, they cannot all remain complete spheres and also fill the available space. Neighbouring bubbles share films and press one another into polyhedral forms with curved faces and edges.

Those shared films do not fold at random. If two bubbles are similar in size and pressure, the film between them is nearly flat. A smaller bubble usually has higher internal pressure, so the shared film bows towards the larger bubble. In a stable foam, three films commonly meet along an edge at angles of about 120 degrees. This local balance of surface tension is part of the pattern described by Plateau's laws.

Bubbles in foam therefore do not deform because surface tension has disappeared. They deform because surface tension is still reducing total surface area under crowded conditions. Real foams also drain, rupture and merge, so their geometry keeps changing; nor is every bubble simply a hexagon. The important feature is how shared boundaries find a local balance among volume, pressure and area.

The structure of singularities in soap-bubble-like and soap-film-like minimal surfaces