(646h) Rheology of Dilute Emulsions of Surfactant-Covered Ferrofluid Droplets

Magnetic emulsions are a class of two-phase mixtures consisting of ferrofluid droplets suspended in a non-magnetic, immiscible liquid. This class of complex fluids represents a promising alternative to ordinary materials (e.g., regular oil-in-water emulsions or particle suspensions) in technological and industrial applications at the intersection of materials processing, biophysics, and engineering. The macroscopic mechanical response of magnetic emulsions reveals complex physical phenomena at the microscopic level in response to imposed bulk stresses. In this talk, we interrogate the combined effects of surface-active moieties and imposed velocity and magnetic fields on the emulsion bulk rheology. The dynamics of surfactant-covered droplets with structured interfaces is discussed based on the effects of surfactant coverage factor, flow-induced surface convection, and magnetic field on droplet shape. Our analysis considers a single two-dimensional insoluble-surfactant-covered droplet of a superparamagnetic ferrofluid suspended in an immiscible, non-magnetizable liquid confined in a channel between parallel plates. The system is simultaneously subjected to a simple shear flow and an externally applied uniform magnetic field. The methodology combines the level-set method to capture the interface implicitly and the closest point method to solve the surfactant transport equation. Our results show that the rheological behavior of the emulsion is strongly altered by the presence of surfactants, especially in the limit of purely advective regimes. In contrast to the classical problem of surfactant-covered droplets in non-magnetic emulsions, where the maximum concentration of surfactants occurs at the droplet tips, the maximum concentration is shifted about the tip location in magnetic emulsions depending on the field direction relative to the impose flow field. The surfactant distribution along the droplet interface promotes significant variations in the emulsion rheology with marked signatures of shear-thinning and shear thickening behaviors. We further show how local variations of surface tension relate to asymmetries in the emulsion stress tensor via the definition of a rotation viscosity and an induced magnetic torque.