Soap films and soap bubbles are examples of "minimal surfaces," so-called because nature selects the shape that requires the least amount of total energy to maintain, and thus enclose a given area/volume with as little perimeter/surface area as possible. (A circle takes the least perimeter to surround a given amount of area; and a sphere is the shape of least surface area that encloses a given amount of volume.)
The general problem of determining the shape of the minimal surface constrained by a given boundary is known as Plateau's Problem, named after Joseph Antoine Ferdinand Plateau, a nineteenth-century, blind, Belgian physicist, who "observed" a handful of simple patterns that seemed to completely describe the geometry of how soap bubbles fit together. Plateau claimed that soap bubble surfaces always make contact in one of two ways: either three surfaces meet at 120-degree angles along a curve; or six surfaces meet at a vertex, forming angles of about 109 degrees.