Mechanisms of symmetric and asymmetric drainage of foam films

[摘要] The drainage of horizontal thin liquid films produced from aqueous solutions of ionic surfactants was studied experimentally, using videomicroscopy and interference techniques, for several surfactants in a wide range of concentrations. Two types of drainage were observed: asymmetric and symmetric. The film drainage was found to be much faster in the asymmetric case.First, axisymmetric drainage was investigated. In this case, a numerical model was developed to simulate the entire drainage process, including the film formation.The condition for the transition from a nearly ;;plane-parallel;; film to a dimpled film in the absence of disjoining pressure was determined. The ratio of the minimum to maximum thickness in the film and a dimensionless rate of drainage was correlated with the ratio of the maximum possible curvature in the dimple to the curvature in the meniscus.The presence of disjoining pressure makes a qualitative difference in film drainage. Low electrolyte concentrations in a film containing ionic surfactants produce a repulsive disjoining pressure that inhibits formation of the thin barrier ring and thus of the dimple itself. The film drains rapidly to its equilibrium thickness. For high electrolyte concentrations, disjoining pressure is dominated by van der Waals attraction. As a result a thin annular film forms that forces the dimple into a lens with a finite contact angle. These types of behaviors were observed experimentally.Then, the mechanisms of asymmetric thin film drainage were investigated. A simple linear stability analysis and a two dimensional numerical model were developed and showed that asymmetric drainage is caused by a hydrodynamic instability that is produced by a surface-tension-driven flow and stabilized by surface viscosity, surface diffusivity and system length scale. A criterion for the onset of instability causing asymmetric drainage was determined. Experiments performed on aqueous solutions of SDS and SDS:1-dodecanol showed the strong dependence of the drainage type on the surface shear viscosity. Experimental results were found to be in good agreement with the stability predictions.

[发布机构] Rice University

[效力级别] engineering