Revisiting the universal drainage of large viscous bubbles

Bubbles that have risen to an air-liquid interface will rest at this surface until their spherical cap drains sufficiently to spontaneously rupture. For large spherical film caps, the memory of initial conditions is believed to be erased due to a drainage flow with a velocity that steadily increases from the top of the bubble to its base. Consequently, the film thickness has been calculated to be relatively uniform as it thins, regardless of whether the drainage is regulated by shear or elongational stresses. Here, using a combination of interferometry and mathematical modeling, we demonstrate that for large viscous bubbles, the film thickness is highly non-uniform throughout drainage with a thickness that is orders of magnitude larger near the bubble base than near its apex. We link the spatial divergence in the film thickness profile to a universal non-monotonic drainage flow, where the location and maximum of the velocity depend on the rate the bubble thins. These results highlight an unexpected coupling between drainage velocity and bubble thickness profile and provide critical insight needed to understand the retraction and breakup dynamics of these bubbles upon rupture.