Iridescent clusters and marginal regeneration in vertical soap films*

Liquid foams consist of bubbles separated by thin films. Individual films consist of two surfactant-laden surfaces that are ∼ 5 nm - 10 micron apart. Sandwiched between these interfacial layers is a fluid that drains primarily under the influence of gravitational, viscous and interfacial forces, including disjoining pressure. Understanding and controlling the drainage kinetics of thin films is an important problem that underlies the stability, lifetime and rheology of foams and emulsions. We experimentally follow the drainage kinetics of foam films using imaging, color science and cluster imaging velocitometry. Interference between light reflected from two surfactant-laden surfaces that are ∼ 100 nm - 10 micron apart leads to thickness-dependent iridescent colors in the visible region. Below 50 nm the thin films appear as black. In this study we characterize, track and analyze the motion of thick and thin regions that travel under combined influence of gravity, buoyancy and due to advection by capillary-driven flows. In particular, we study the origin of marginal regeneration, i. e. the complex flow patterns that originate near the borders of foam films.