Stability of two-dimensional liquid foams under externally applied electric fields

Liquid foams are highly complex systems consisting of gas bubbles trapped within a solution of surfactant. Electroosmotic effects may be employed to induce fluid flows within the foam structure and impact its stability. The impact of external electric fields on the stability of horizontally oriented monolayer of foam (2D foam) composed of anionic, cationic, non-ionic and zwitterionic surfactants was investigated, probing the effects of changing the gas-liquid and solid-liquid interfaces. Time-lapse recordings were analysed to investigate the evolution of foam over time subject to varying electric field strengths. Numerical simulations of electroosmotic flow of the same system were performed using the Finite Element Method. Foam stability was affected by the presence of an external electric field in all cases and depended on the surfactant type, strength of the electric field and the solid material used to construct the foam cell. For myristyltrimethylammonium bromide (MTAB) foam in a glass cell, time to collapse 50% foam was increased from ~ 25 minutes under no electric field to ~85 minutes under an electric field strength of 2000 V/m. In comparison, all other surfactants trialled exhibited faster foam collapse under external electric fields. Numerical simulations provided insight as to how different zeta potentials at the gas-liquid and solid-liquid interfaces affect fluid flow in different elements of the foam structure under external electric fields, leading to a more stable or unstable foam.