Flowing Menisci: Coupled Dynamics and Liquid Exchange with Soap Films

Liquid foams exhibit menisci whose lengths range from hundreds of microns in microfoams to several centimeters in macroscopic bubble arrangements. These menisci are known to thin under gravity until reaching a steady thickness profile, where hydrostatic and capillary pressures are balanced. However, current drainage models neglect the liquid exchanges between the menisci and the adjacent soap films, assuming the latter provide a negligible liquid reservoir. Using controlled experiments, we systematically measure the shape of an isolated meniscus placed in a vertical soap film. By increasing the film thickness, we identify a flowing regime in which the flux from the film significantly enlarges the meniscus. We present an analytical model that extends the drainage equation to incorporate this dynamical effect and introduce a gravito-exchange length, which sets the minimum meniscus thickness. The model is in quantitative agreement with experiments, capturing the transition between hydrostatic and flowing menisci. This Letter has implications for flowing or rearranging foams, where thick films are commonly observed.