We measure the liquid content, the bubble speeds, and the distribution of
bubble sizes, in a vertical column of aqueous foam maintained in steady-state
by continuous bubbling of gas into a surfactant solution. Nearly round bubbles
accumulate at the solution/foam interface, and subsequently rise with constant
speed. Upon moving up the column, they become larger due to gas diffusion and
more polyhedral due to drainage. The size distribution is monodisperse near the
bottom and polydisperse near the top, but there is an unexpected range of
intermediate heights where it is bidisperse with small bubbles decorating the
junctions between larger bubbles. We explain the evolution in both bidisperse
and polydisperse regimes, using Laplace pressure differences and taking the
liquid fraction profile as a given.Comment: 7 pages, 3 figure