Modelling and investigation of magnetic soap films

Adding magnetic particles to a soap film allows for a magnetic field to control the drainage in the film, which has key implications on the film stability since thinner films are less stable and more prone to rupture. Interference colours exhibited by soap films reveal the time evolution of the film thickness. An accurate model for the film thickness, which can be directly compared with experimental results, will allow for predictions of the film lifetime with different magnetic field arrangements. Existing models for the film thickness of magnetic soap films disregard essential physics by assuming a uniform film magnetisation and immobile interfaces. In this study, the lubrication approximation is applied to the Navier-Stokes equations to derive a model for the film thickness that incorporates additional physics compared to previous models. This new model consists of a coupled system of equations for the film thickness, magnetic nanoparticle concentration, surface velocity, and interfacial surfactant concentration. Simulations are performed by solving the system of equations with the finite element method, and results are compared with experiments. Our findings suggest that the migration of nanoparticles and the surface velocity have a significant impact on film thinning. With further research, it is envisaged that magnetic soap bubbles could be used as responsive capsules: magnetic fields could be used to control the capsule location and trigger the release of substances.