The effect of magnetic particles covering the droplets on the heating rate of Pickering emulsions in the AC magnetic field

Abstract

Emulsions stabilized with magnetic particles have been reported as useful for numerous applications, including thermal medical therapies where they can be used as heating materials. It is well-known that magnetic particles placed in the alternating magnetic field can act as heating agents. The generation of thermal energy is owing to magnetic energy dissipation caused by Néel and Brown relaxation, and hysteresis loss. These mechanisms depend on the properties of the applied magnetic field and the characterization of magnetic particles used as well. But, when residing on droplet surfaces e.g. as stabilizers in Pickering emulsions, magnetic particles somehow can be found trapped and their movement can be limited. Also, the change in Pickering droplet sizes and the increasing solidity of particle shell, e.g. through the coalescence events of droplets, can influence how magnetic energy is dissipated in such multi-phase system.In our work, we investigate the efficiency of magnetic heating of oil-in-oil emulsions stabilized with magnetite particles of various sizes. The results indicate that the temperature rise in such systems varied for different concentrations of particles used, but also for the different macroscopic appearance of emulsions, i.e. coarse emulsions, not fully-covered by particles and final emulsions with stable droplets demonstrated different heating efficiency when placed in the alternating magnetic field. The emulsion droplets with different particle coverage were obtained either by using only ultrasonic homogenization or using ultrasound and a subsequent coalescence under the electric field. The presented study can help in the development of applications of magnetite-stabilized droplets in heating, e.g. when magnetic Pickering droplets are used as thermal agents in hyperthermia treatment or as templates for colloidal capsules.