Deformational instability of a plane interface with transfer of matter. Part 3.—Effects of thickness of diffusion zone and of low surface tension

Abstract

Some correction are made to previous calculations on spontaneous interfacial convection driven by differences in surfactant adsorption on liquid–liquid interfaces. When those corrections are applied in the computer calculations, the previously reported differences between the dispersion curves with the Sternling–Scriven model and that of the present authors at low surface tension disappear in the case of a linear concentration profile. With exponential profiles, simulating profiles of the erf-type, there is still found a destabilisation for low interfacial tensions which might be a mechanism active in spontaneous emulsification phenomena. The influence of decreasing the thickness of the concentration zone while keeping the interfacial transfer of substance constant is a stabilising one, for oscillatory as well as non-oscillatory instabilities due to the decreasing concentration difference between surfactant in the two liquid phases. The Sternling–Scriven formalism is modified to cover the case of exponential profiles. In cases with not too low surface tensions (above 1 dyne cm^–1) this model gives the same results as the Lyngby approach, i.e., effects due to interfacial deformation, gravity and absolute value of the surface tension are often unimportant.