Instability of the Interface Due to Surfactant Diffusion in the System of Immiscible Liquids

The loss of stability of the horizontal interface in a two-layer system of immiscible liquids with a diffusing component was investigated experimentally. To this end, the evolution of the system, which was based on a mixture of chlorobenzene and benzene (lower layer) and water or an aqueous solution of sodium chloride (upper layer) was examined. At the beginning of the experiment, the diffusing component (acetic acid), which was a surface-active substance (surfactant), was located in the lower layer of the system. Since the initial densities of the base fluids were close to or equal to the acid density, all further density changes in the system were only the result of the contraction effect. The prepared liquids filled the vertical Hele-Shaw cell and were separated by a movable partition. The removal of the partition led to the development of the diffusion process of acid, which turned to be the reason for the loss of stability of the system provoking the deformation of the inter-phase boundary and the detachment of droplets from it. The dependences of changes in density and interfacial tension at the point of maximum deflection of the boundary on time and initial acid concentration in the lower layer were determined. For case of discontinuity of the interfacial surface a neutral curve was obtained. The analysis of the system behavior revealed a stabilizing role of the solutal Marangoni convection.