On the shape of giant soap bubbles

We study the effect of gravity on giant soap bubbles and show that it becomes dominant above the critical size ℓ = a²/e₀, where e₀ is the mean thickness of the soap film and a = γ b / ρ g is the capillary length (γ b stands for vapor–liquid surface tension, and ρ stands for the liquid density). We first show experimentally that large soap bubbles do not retain a spherical shape but flatten when increasing their size. A theoretical model is then developed to account for this effect, predicting the shape based on mechanical equilibrium. In stark contrast to liquid drops, we show that there is no mechanical limit of the height of giant bubble shapes. In practice, the physicochemical constraints imposed by surfactant molecules limit the access to this large asymptotic domain. However, by an exact analogy, it is shown how the giant bubble shapes can be realized by large inflatable structures.