Evolution of rheological properties of highly concentrated emulsions with aging - Emulsion-to-suspension transition

The effect of aging on the rheological properties and physical structure of highly concentrated water-in-oil emulsions with dispersed phase of 82-90 v % is the subject of this study. It was proven by various experimental techniques that aging leads to the emulsion-to-suspension transition. Significant shift of rheological properties to the solid-like behavior is the result of the emulsions aging, which shows itself as an increase of the storage modules with time as well as Newtonian viscosity measured in the upward sweeping shear rate mode. Comparison of flow curves measured in the upward and downward sweeping shear rate modes shows that the rheopectic effect at low stresses is observed for both fresh and aged emulsions. Viscosity measurements in the downward mode demonstrate transition to the elastic-like behavior at low stresses with appearance of strongly pronounced yield stress. Dependencies of the characteristic rheological parameters on aging have been investigated by using different analytical methods such as optical microscope studies, differential scanning calorimetry measurements, and x-ray diffraction. All these methods showed that aging is associated with the process of slow crystallization of the dispersed droplets that comprise of overcooled highly concentrated aqueous solution, which can be considered as the main mechanism for the evolution of the emulsion's rheological properties. In this sense, the observed emulsion-to-suspension transition is the consequence of the phase transition. The kinetics of the transition process is described by the John son-Mehi-Avrami-Kolmogorov equation and correlates with the evolution of the rheological properties of the dispersion. (c) 2006 The Society of Rheology.