A bubble is a minimal-energy surface of the type that is formed by soap film. The simplest bubble is a single sphere, illustrated above
(courtesy of J. M. Sullivan). More complicated forms occur when multiple
bubbles are joined together. The simplest example is the double
bubble, and beautiful configurations can form when three or more bubbles are
conjoined (Sullivan).
An outstanding problem involving bubbles is the determination of the arrangements of bubbles with the smallest surface area which enclose
and separate
given volumes in space.