(592f) The Effect of Curvature and Finite Bulk Solution Volume On Surfactant Dynamics

The growing interest in multiphase microfluidic systems highlights the need to understand the transport and adsorption of surfactants at these lengthscales.  We have developed a microtensiometer to accurately measure surfactant dynamics at spherical microscale gas-liquid and liquid-liquid interfaces. Using the principle that the rate of diffusion depends on curvature, we show that contributions of diffusion and kinetic exchange can be separated to allow for the measurement of fundamental transport coefficients. We have also revisited the role of a confined volume on surfactant transport dynamics, since depletion can play a major role when the volume of solution is small, the concentration of surfactant is low, or the surface-area-to-volume ratio is large.  All of these conditions may hold true in microfluidic devices.  A planar interface, due to a fixed surface-area-to-volume ratio, has a diffusion timescale dependent only on equilibrium surface coverage and bulk concentration of adsorbing species.  When the interface is curved, the observed adsorption dynamics can be much different than predicted by a planar analysis.  The ratio of surface area to volume for a spherical interface is a function of curvature, leading to a diffusion timescale that depends on the radius of curvature.  The characteristic time is reduced when surfactant is transported to the interface from the outside of the sphere; it is increased when surfactant is transported from within the sphere. Depletion also causes the bulk concentration to decrease and the surface concentration to be smaller than predicted by the governing isotherm, further altering the timescale. In this study, we develop a timescale analysis for diffusion-limited transport to a spherical interface both from outside and from within the sphere, accounting for depletion.  The timescales are compared using dynamic interfacial tension data for nonionic surfactants adsorbing at microscale drops and bubbles using the microtensiometer that we have developed.  A criterion describing the conditions when depletion and curvature effects are important is developed, which will aid in the design of microfluidic studies involving adsorbing species.