A new device for assessing film stability in foams: Experiment and theory

A new device is proposed for registering thin film drainage around a single foam bubble based on the increase of the electrical resistance of the draining film. Initially, a small bubble is expanded inside an electrically conductive liquid bridge that is formed between two vertically aligned metallic electrodes. Then the liquid of the bridge is withdrawn at a pre-selected flow rate until rupture of the bridge/bubble system while monitoring its electrical resistance across the electrodes. The present work reports results for liquid withdrawal rates between 20 and 200 μl/h which correspond to total drainage time between ∼3 and ∼25 min. Experiments are performed with aqueous solutions of SDS between 50 and 2000 ppm. A simple theoretical model is used to describe the evolution of the resistance of the global liquid bridge/bubble system. The model assumes a perfectly spherical bubble which affects the shape of the liquid bridge only as a volume constraint. A simple approach is employed for the computation of the electrical resistance of the system during the last stages of drainage where the thin film around the bubble gets gradually thinner and stretches until rupture. A detailed parametric study of the model is presented. The comparison between experimental and model results demonstrates the potential of the new device for the assessment of foam stability with respect to coalescence.