Electroosmotic flow in free liquid films: Understanding flow in foam plateau borders

Liquid flow in foams mostly proceeds through Plateau borders where liquid content is the highest. A sufficiently thick (~180 µm) free liquid film is a reasonable model for understanding of electrokinetic phenomena in foam Plateau borders. For this purpose, a flow cell with a suspended free liquid film has been designed for measurement of electrokinetic flow under an imposed electric potential difference. The free liquid film was stabilised by either anionic (sodium lauryl sulfate (NaDS)) or cationic (trimethyl(tetradecyl) ammonium bromide (TTAB)) surfactants. Fluid flow profiles in a stabilised free liquid film were measured by micron-resolution particle image velocimetry (µ-PIV) combined with a confocal laser scanning microscopy (CLSM) setup. Numerical simulations of electroosmotic flow in the same system were performed using the Finite Element Method. The computational geometry was generated by CLSM. A reasonably good agreement was found between the computed and experimentally measured velocity profiles. The features of the flow profiles and the velocity magnitude were mainly determined by the type of surfactant used. Irrespective of the surfactants used, electroosmotic flow dominated in the midfilm region, where the film is thinnest, while backflow due to pressure build-up developed near the glass rods, where the film is thickest.