(475j) The Role of Surfactant Mass Transport in the Formation of Microscale Droplets

Surfactants play a significant role in the formation of emulsion droplets in microfluidic devices, altering the droplet size for a given set of flow conditions and the transitions between droplet breakup modes. At the right concentrations, tipstreaming is observed, in which thin, sustained threads are drawn that segregate into micron-scale droplets. We have characterized this phenomenon in detail as a function of fluid properties and flow kinematics. However, we still have only a phenomenological understanding of the role of the surfactant itself in the tipstreaming process. Experiments and recent literature suggest that the timescales for mass transport of surfactant to the interface including diffusion, adsorption, and desorption can all be significant in determining the local surface concentration and interfacial tension, and therefore the ability to stabilize thin threads. In this talk, we present microfluidic tipstreaming experiments in which the length of the hydrophobic tail of the surfactant molecule is varied in a homologous series, nominally altering the desorption rate constant significantly more than the other transport coefficients. We interpret the results of these experiments in the context of scaling arguments and an advection-diffusion model for soluble amphiphiles that includes the effects of diffusion, adsorption, and desorption in micron-scale geometries.