Rheological changes in emulsions on aging: IV. O/W Emulsions at Intermediate and Low Rates of Shear

Abstract

Rheological changes in O/W emulsions stabilized by Aerosol O.T. or sodium oleate have been studied during aging by low shear viscosity and creep compliance tests at a constant low shear stress. Globule coalescence does not produce a continuous fall from the outset in the apparent relative viscosity at any particular rate of shear, and in the swelling factor of the flocculates calculated from these data, nor in the rheological parameters calculated from creep compliance-time data, viz., instantaneous elastic modulus, elastic moduli, and viscosities associated with retarded elasticity, and the Newtonian viscosity, as was observed previously in studies on concentrated W/O emulsions. Instead three distinct phases of change are observed. Initially all parameters increase for several days until maximum values are reached. These may be maintained for a few days, after which all parameters decrease sharply with aging time to values which subsequently change slowly with time. Throughout these changes the density of globule packing appears to be greater than the theoretical value for ordered close packing of rigid spheres. This is attributed to the deformable character of the globule surfaces, which results in their flattening at an increasing number of points as the flocculates grow and the number of contacts between each globule and its neighbors increases. Following optimum packing of globules in the flocculates the rate-determining step is globule coalescence. It relieves the pressure on the globules, thus reducing the degree of distortion. In addition, the fall in the number of globules per unit volume weakens the flocculates by rupture of linkages and reduces the volume of continuous phase which is immobilized within them.