Emulsification of particle loaded drops in simple shear flow

• Particle load of drops influences their breakup in simple shear flow.
• Influence depends on particle interaction with liquid/liquid interface.
• Particles situated in the liquid/liquid interface do not hinder drop breakup.
• Particles situated inside the disperse phase hinder drop breakup.
• Changes in drop breakup behavior can be related to the shear thinning flow properties of the disperse phase.

We investigated the influence of a particle load of disperse phase drops on their emulsification. Newtonian liquids were chosen for this investigation and a surfactant was present in all cases. A Couette-like shear cell was used enabling us to apply simple shear flow. The drop size distribution and drop morphology of the resulting oil-in-water emulsion were characterized. The influence of the wetting characteristics of the particles and the influence of the volume fraction of solids were investigated by comparing the breakup of silicone oil drops containing two types of particles, which showed different wetting behavior with that of pure silicone oils. One type of particles was situated in the liquid-liquid interface, the other type remained mainly wetted in the oil drops. With increasing volume fraction both suspension types showed shear rate dependent rheological behavior due to particle-particle interactions. Emulsification results were interpreted by means of apparent critical capillary numbers and correlated with the measured drop-to-matrix viscosity ratio. To calculate the viscosity ratio, the viscosities of the disperse phases were measured for each applied shear stress. Depending on the wetting characteristics, the particle load of the disperse phase showed different influence on the emulsification result. Particles situated in the liquid-liquid interface did not affect the apparent critical capillary number and therefore the drop breakup in the investigated volume fraction range. This result was explained by the reduction of particle loading during drop deformation and breakup due to the shearing-off of highly filled drops. Therefore the matrix fluid of the disperse phase governs the overall emulsification result. In contrast, particles located inside the disperse phase hindered the drop breakup. In comparison to Newtonian liquids with comparable bulk viscosities, the apparent critical capillary number was higher. It increased with increasing particle volume fraction. Additionally the apparent critical capillary number showed a dependence on the applied stress in case of the particle containing drops. This behavior is assumed to be due to an inhomogeneous viscosity inside the particle-loaded drop. We suggest using an effective drop viscosity to calculate the viscosity ratio in case of this type of system.

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