(459b) Using Bulk Convection to Approach Kinetic-Limited Surfactant Dynamics

The impact of transport of surfactants to fluid-fluid interfaces is complex to assess and model, as many processes are in the regime where kinetics, diffusion and convection all contribute to the overall process.  We have been developing methods to separate these contributions to allow for the measurement of fundamental transport coefficients via dynamic surface tension of liquid-fluid interfaces.  Using the principle that the rate of diffusion depends on curvature, we previously developed a microtensiometer to accurately measure surfactant dynamics at spherical microscale gas-liquid and liquid-liquid interfaces.  In the present study, we introduce a low Reynolds number flow in the bulk solution to further increase the rate of diffusion.  Dynamic surface tension is measured as a function of Peclet number and the results are put into the context of a simplified convection-diffusion model.  Although a transition from diffusion to kinetic-limited transport is not observed experimentally for the surfactants considered, lower bounds on the adsorption and desorption rate constants are determined.  These lower bound values are much larger than the previously reported rate constants.  This experimental tool and analysis allows the governing mechanisms of surfactant transport at liquid-fluid interfaces to be quantified with and without convection.  The addition of flow near an interface is effective in decreasing the length scale for diffusion, and shifting the relevant timescales.  The results show that the detailed nature of the flow field does not need to be controlled as long as the local Reynolds number is low.  This provides a simple method to control surfactant transport near interfaces.