The effect of surfactants on the stability of fluid interfaces during phase transformation

The influence of surface active solutes on the stability of fluids during phase transformation is examined both experimentally and theoretically. Experiments on the interfacial stability of triethanolamine as it steadily evaporates into a partial vacuum verifies that for high surface tension fluids, spontaneous convection induced by differential vapor recoil is stabilized by the addition of surfactants and that the degree of stability increases with the surface activity of the surfactant. The hydrodynamic stability of surfactant solutions undergoing phase transformation is then examined using linear stability analysis. Results reveal that surfactants have a marked stabilizing effect on the differential vapor recoil mechanism but have essentially no effect on the fluid inertia and moving boundary destabilizing mechanisms. Consequently, the maximum stabilizing effect of surface active solutes on liquids evaporating under vacuum is determined by the criterion for fluid inertia instability.