Rheological and Structural Changes During the Mixing of Suspensions and Emulsions

The rheological effects occuring when an emulsion is mixed with a suspension can be detected using a helical ribbon stirrer. The stirrer rotates in a rheometer cup and is connected to the torque sensor of the rheometer. During mixing, microstructural changes which are related to the interactions between the continuous fluid phase and the disperse droplets (or disperse solid particles) as well as between droplets and particles will have a strong impact on the measured apparent viscosity. Phase interactions and surfactant influences can thereby be described in terms of their effect on the viscosity of the mixture. The viscosity trend during the mixing time is strongly dependent from the disperse phase concentration of the suspension, the polarity of the suspension particles and from the nature of the emulsifiers used. When a structural equilibrium is reached, the apparent viscosity as measured with the stirrer can be recalculated using the Rieger-Novak and the Metzner-Otto approach. This allows for a comparison of the apparent viscosities as determined by means of the stirrer and viscosities as determined in uniaxial shear in the concentric cylinder geometry. As microscopic observations demonstrate, the final state of the mixture of emulsion and suspension is the state with the energy minimum of the interfacial system, i.e. the total interfacial area is minimised by forming water containing agglomerates in a continuous oil phase