EFFECTIVE VISCOSITIES IN THIN IONIC MICELLAR LIQUID-FILMS

Thin liquid films stabilized by surfactants above the critical micelle concentration exhibit stratification or stepwise dynamic thinning. A continuum hydrodynamic model is outlined for stepwise film thinning th at incorporates equilibrium micellar structuring through self-consiste nt oscillatory disjoining pressures and effective viscosities. Effecti ve viscosities as functions of thickness ave evaluated with an extensi on of the local average density model, considering dilute colloidal su spension shear viscosities and solvent effects. To establish local she ar viscosities, structured DFT micellar profiles, coarse-grained densi ties, and disjoining pressure are used. Ionic micelles and other collo idal systems with repulsive interactions show structured effective vis cosities that are generally less than the corresponding homogeneous so lution shear viscosity bounded by the pure solvent viscosity and that of the bulk micellar solution. For: 0.1 and 0.2-M sodium dodecylsulfat e micellar solutions, the effective viscosities are less than 5 and 10 %, respectively below the homogeneous fluid viscosity, except at small thicknesses, indicating that the micellar film thins faster than a pu re water film of the same thickness. Calculated thinning curves closel y resemble experimental observations in the stepwise thinning behavior , displaying decreasing slopes and increased step nations at later rim es. Despite the micellar structuring within the film, the ionic micell es do not contribute appreciably to the viscous resistance of the thin ning film. Rather; Reynolds' film thinning is obeyed, with the equilib rium oscillatory disjoining pressures driving the stepwise dynamics. T he shear viscosity of the ionic micellar film is well approximated by that of the bulk solution.