Effect of formulation parameters and process on the structural properties of concentrated Pickering emulsions

Concentrated non-conventional anti-Bancroft Pickering direct oil-in-water emulsions stabilized with partially hydrophobic silica particles are addressed. The dispersed-phase fraction of oils is varied between 0.1 to 0.65. A special focus is put on the emulsions formulated at a dispersed-phase fraction of 0.65. The effects of formulation parameters (1 wt% and 4 wt% of silica particles) and emulsification processes (sonicator and rotor-stator shearing devices) on a particle's repartition and organization, and on the resulting rheological behavior of the emulsions are investigated. Emulsions are mainly characterized by droplet size distribution measurements, confocal microscopy images, partitioning of the particles in the emulsion via a mass-balance approach, and rheology. The rheological structural properties of the emulsions were probed via the study of the dependency of the viscosity on fraction of the dispersed-phase. A modified model of the rheological behavior based on the minimum energy dissipation energy principle, (Quemada's model), describes fairly well the experimental data. In particular, the two fitting parameters, η _s and ϕ _c, are sensitive to the repartition of the particles in the emulsions resulting in variation of the stirring process and the amount of silica. We exploit here the 2 parameter-model adjustment which provides a rheological signature of the samples associated to the repartition of the particles inside the system that leads to an evolution of their rheological characteristics. This approach is applied here by playing on the process (rotor-stator and sonication) without changing the physico-chemical parameters. This rheological signature corresponding to the repartition of the particles inside the system is confirmed by other characterization means including confocal laser scanning microscopy and mass-balance approaches.