Capillary Drainage in Horizontal Soap Films: Theoretical Review and Experimental Illustrations

Soap films and bubbles are inherently unstable systems that evolve over time. Their thickness is primarily governed by the competition between capillary and viscous forces. The presence of surfactants introduces Marangoni stresses, which limit interfacial extension and significantly increase film lifetime. While the bulk flow is typically well-described by a simple Poiseuille profile between the two interfaces, the interfacial dynamics can induce complex behaviors, even in the simple case of horizontal film drainage, which is used as a paradigmatic example in this review. The interfacial velocity is dictated by the thickness gradients and by the interfacial rheology, which, in many practical cases, reduces to the condition of an incompressible interface. This simplified framework allows for analytical predictions and scaling laws in axisymmetric flows. It is also consistent with the spontaneous symmetry breaking that may be observed in horizontal films—a phenomenon associated with the marginal regeneration process, which remains only partially understood. This review presents the most elementary theoretical frameworks capable of capturing the essential features of these flows and provides quantitative comparisons with available experimental data.