The superspreading effect of trisiloxane surfactant solutions

The problem of superspreading has recently attracted much attention both from the theoretical point of view and because of the practical use of the phenomenon. However, up to now there is no general agreement about the mechanism of the effect and on the necessary conditions for its realization, in particular regarding different types of surfactants. We report here the results of our investigation of the spreading of vesicular solutions of the trisiloxane surfactant D-8 at a concentration of 0.16 wt %. At this concentration a maximum rate of droplet spreading was observed by Zhu et al. for this surfactant. By including the disjoining pressure isotherm of wetting films, Pi (h) into the hydrodynamic equations, quantitative agreement with experimental data was attained. The same approach was used for describing the rate of film climbing over a hydrophobed inclined plate. Rapid spreading was explained by formation of extremely thick wetting films, stabilized by mutual repulsion of vesicles, and, partly, by long-range electrostatic forces. The role of vesicles may also consist of damping capillary waves on the film surface and protecting the thick film against rupture, that is, working as enhanced dilatational (second) viscosity. The proposed form of the Pi (h) isotherm agrees with the shapes of the profile of the thick film edge, calculated on the basis of video camera images. Parameters of the Pi (h) isotherm are estimated from rates of stationary flow of wetting films over hydrophobed walls between two menisci in glass capillaries of various radii. The critical disjoining pressure at which the film lost its stability is small and corresponds just to the saturated vapor pressure. The proposed approach explains the link between turbidity and superspreading and the requirement for a saturated water vapor atmosphere. The phenomenon of flow and stability of micron thick wetting films containing small interacting particles is a subject of general interest. The problem demands further investigation in more detail with application of different structure-sensitive methods and computer simulation.