INFLUENCE OF SURFACTANT TYPE AND CONCENTRATION ON THE DRAINAGE OF LIQUID-FILMS

Measurements of drainage are reported of films drawn from surfactant solutions in a vertical frame. The drainage was measured either by interference or by the downward velocity of polystyrene particles. The latter could be observed by means of light extinction. The drainage time of films of 1.5 mum thickness (measured with polystyrene particles) was found to be independent of the film height within the investigated range (13-17 mm) and was proportional to the bulk viscosity for solutions containing water/glycerol and CTAB. The drainage rate is independent of concentration above the critical micelle concentration (cmc), and films drain faster below the cmc. Liquid films drawn from CTAB solutions are mobile below the cmc. Thicker and thinner regions at the film/Plateau border transition alternate; the corresponding wavelength does not vary strongly with the bulk viscosity. From the drainage time of films drawn from solutions with a complicated rheological behavior, the effective shear stress and shear rate in the dominant process of film thinning can be estimated.