The transition from three-dimensional to two-dimensional foam structures

A two-dimensional foam consists of a monolayer of bubbles. It can be created by squeezing the more familiar three-dimensional foam between two parallel glass plates. We describe and explain the minimum plate separation H which must be reached to fully effect the transition from three- to two- dimensional foam. We find that H/V^ 1/3 is close to one, where V is the average bubble volume, and increases slightly when the side-walls of the container are taken into account.