On drop dispersion and mixing performance in a stirred vessel: effect of surfactant solubility

The effect of surfactant solubility on drop dispersion and flow phenomena accompanying the mixing process in a stirred vessel equipped with a pitched blade turbine is studied. A wide range of surfactant solubility is explored by varying the value of surfactant desorption coefficient by three orders of magnitude. Massively-parallel, high-fidelity, three-dimensional large eddy simulations of O/W emulsification are carried out, coupled with the use of a hybrid level-set/front-tracking algorithm. We also resolve surfactant transport in the bulk phase and on the interface, which enables us to elucidate the delicate interplay between interfacial dynamics and surfactant transport in realistic mixing scenarios. We compare the flow field generated in the systems with surfactants of different solubility, followed by an investigation of the modification of emulsion formation. We correlate the captured physics with mixing performance metrics of interest to industrial practitioners, such as droplet count and their size distribution. Our results reveal that highly soluble surfactant 'escape' from the interface at the onset of impeller rotation so fast that the interface in the corresponding system shares similar dynamics with that observed in the surfactant-free case. Conversely, an interface covered with poorly soluble surfactant is prone to experience a high degree of stretching and elongation prior to dispersion. Mixing performance in terms of interfacial area and droplet count is also improved dramatically when surfactant of low solubility is used. Furthermore, we show that Marangoni stresses modulate the interfacial dynamics associated with capillary-driven ligament breakup and surfactant-induced tip-streaming.