title: A microtensiometer to probe the effect of radius of curvature on surfactant transport to a spherical interface.
abstract: Diffusion of surfactant to a spherical interface depends on the radius of curvature of the interface; the smaller the radius of curvature is, the faster the dynamics. This paper presents and validates an experimental apparatus, denoted a "microtensiometer", to study the dependence of surfactant dynamics on radius of curvature. Dynamic surface tension is monitored for a range of bubble radii from 17 to 150 microm, and the dynamics are compared with those obtained using the classic pendant drop experiment for a nonionic surfactant at the air-water interface. Experiments reveal that dynamic surface tension follows a diffusion-limited scaling, in which radius of curvature is a key parameter. Despite the clear scaling behavior of the experimental equilibration time, the full dynamic curve for an initially clean interface cannot be predicted by a diffusion-limited transport model using the molecular diffusion coefficient and a single isotherm. However, the same model is shown to correctly predict compression-expansion experiments. Aside from elucidation of surfactant transport, this device provides a tool for rapid measurements of interfacial properties using a significantly lower volume of sample than current methods.
authors.affiliation: Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA.
authors.forename: Nicolas J
authors.initials: NJ
authors.lastname: Alvarez
authors.forename: Lynn M
authors.initials: LM
authors.lastname: Walker
authors.forename: Shelley L
authors.initials: SL
authors.lastname: Anna