(226c) Effect of Surfactants On Drop Breakup In Complex and Evolving Flows

In high-energy input liquid-liquid contactors, surfactants are employed to stabilize emulsions and dispersions.  While the role of stabilizing agents in preventing coalescence and promoting product life is relatively well studied, their effect on drop deformation and breakup is not well quantified.  Since drops deform rapidly in the ‘dispersion region’, the rate at which surfactant is transported to the drop-fluid interface plays an important role in determining ultimate drop size.  Consider a drop contained within a surrounding matrix fluid containing surfactant.  As the drop deforms, gradients of surfactant concentration are created along the newly formed interface that cause an elastic restoring force, or Marangoni stress, that resists further stretching and breakup.  At the same time, surfactant diffuses from the bulk of the matrix fluid to the interface to relieve the Marangoni stress.  A competition between the time scales for stretching and diffusion determines the drop’s ultimate fate.  The goal of our work is to develop a fundamental link between dynamic interfacial phenomena and mechanical emulsification; that is, in processes far from thermodynamic equilibrium.  To this end, we focus on model systems and dilute dispersions of oils in aqueous surfactants, where the bulk surfactant concentration remains constant, thereby facilitating an understanding of the underlying physicochemical phenomena.

We begin by presenting our attempts to quantify the role of surfactants via measurement and analysis of interfacial properties.  We then report the results of carefully controlled experiments to measure and quantify the equilibrium drop size distribution produced by the complex turbulent flow within a batch rotor-stator mixer.  Next, we report experimental results for the discharge of a laminar, axisymmetric oil jet into an otherwise quiescent aqueous surfactant solution, in which the time evolution of the jet instabilities and breakup have been monitored via high speed imaging.  The jet studies reveal important details that allow a more in-depth interpretation of the mixer data.  Mechanistic analysis of the relevant physical phenomena allows development of correlations to predict both jet behavior and mixer performance.  Limitations of the present studies and directions for future work are also discussed.