Rheological changes in emulsions on aging: III. At very low rates of shear

Abstract

Creep compliance-time studies on water-in-Nujol emulsions (30%, 50%, 65% wt/wt disperse phase) stabilized by 1.5% (wt/wt) sorbitan monooleate at a shear stress of 4.9 dynes/cm. 2 indicate that their rheological parameters change drastically during the first hours of aging. The experimental data suggest that the elastic moduli almost disappear, and that the viscosity moduli decrease by as much as one order of magnitude or more. These changes are believed to be associated primarily with the rapid coagulation of globules with diameters not exceeding 0.5 μ. Following rapid flocculation globules associate as dense cores which are linked together by projection of less densely packed globules. With increasing volume concentration of water the cores become larger and the projections shorter. Coalescence of globules during aging produces a more open flocculate structure. The attraction forces between globules in close contact are modified substantially by the adsorbed layers of emulsifying agent so that the attraction potential changes slowly with decreasing distance of globular separation. Nevertheless, flocculated globules are bound together by significant attraction forces, so it is concluded that either the repulsion potential in W/O emulsions is smaller than previously thought, or the adsorbed layer of emulsifying agent reduces the attraction potential by less than calculation suggests.