The impact of diffusion and mixing in expanding fluid drops on the rate of surface tension change

The dynamics of surface tension change in expanding drops of surfactant solution was studied using drop shape analysis. Experiments explored the dynamics of surface tension change resulting from the flow of surfactant-free water into a surfactant-containing drop. Experiments were conducted for two surfactants, one cationic (cetylpyridinium chloride) and one anionic (sodium octylbenzenesulfonate), both selected because they exhibit rapid adsorption/desorption, allowing for the study of mixing behind the expanding interface. Results were interpreted through the use of a spherical diffusion model designed to simulate drop expansion resulting from influx of surfactant-free water at the core of the drop. Best-fit diffusion constants were found to be consistent with reported molecular diffusion constants for the two surfactants, suggesting that hydraulic mixing within the drop does not play a significant role in the dynamics of surface tension change for the conditions studied. The results of the work have practical implications for applications where interface motion is induced by solute delivery, in that diffusion near the interface is likely to control the dynamics of interfacial tension reduction under many conditions. (C) 2016 Elsevier B.V. All rights reserved.