Dynamics of Thin Surfactant Films
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Gravitational drainage from plane vertical films of various ordinary surfactants, cationic, anionic, non-ionic and superspreaders are studied theoretically and experimentally using a microinterferometric technique. The ordinary surfactants showed ordered interferometric color bands and the film thickness decreased linearly in time and surface elasticity was calculated from the measured time dependent film thickness. The superspreaders showed complicated dynamic turbulent-like interferometric patterns and had an order of magnitude longer life time before bursting compared to their “cousin” non-superspreaders. The stabilization of the superspreader films and the nonlinear decrease of the film thickness with time is attributed to significant disjoining pressure measured using gravitational drainage of vertical films and is associated with the van der Waals repulsion of the fluffy surfaces of the film formed by long superspreader bilayers hanging from the free surfaces. Two surfactant mixture solutions at different mixing ratios were used to find a relation between the lifetime of planar vertical film and foamability of surfactant solution. The results show that solutions with longer lifetimes in planar film drainage reveal a higher foamability. Also, the foamability of the mixed surfactant systems was found to be greater than the foamability of each of the individual components. The higher foamable surfactant solution was added to gypsum slurry during the manufacturing of dry wallboard leading to increased fluidity of the gypsum-foam slurry, thereby reducing the total water required for the process. Gravitational drainage from thin planar surfactant films in the presence of inorganic salts was experimentally studied. Strong ion-specific effects of the counter ions were found to affect the stability and the rate of drainage of the planar foam films as a function of concentration of the inorganic salts. The counter-ions can either stabilize (below the critical concentration) or destabilize the foam films. Ionic surfactant solutions were also used to study gravitational drainage from thin vertical planar films supported on a frame with the upper and lower parts being electrodes. The imposed electric field resulted in various additional physical phenomena. The interplay of these phenomena stabilized the film drainage irrespectively of polarity. Similar effects were observed with foams.