Anomalous effects in the flow behaviour of oil-in-water emulsions

In this study the rheological data obtained for various differently concentrated oil-in-water emulsions from viscometers of different design and geometry are analysed and compared. The two types of viscometers used are a Fann coaxial cyclinder viscometer with different rotor-bob combinations and a Bohlin constant-stress rheometer with a cone-and-plate geometry. The results obtained from the two instruments for the same emulsions show large discrepancies. For example, the cone-and-plate measurements show strong shear-thickening (dilatant) behaviour in emulsions although no such effect is seen in the coaxial cylinder viscometer. The observed discrepancies in the measurements are explained in terms of the “creaming effect” in emulsions. While a small amount of creaming has little or no effect on the viscosity measurements in a coaxial cylinder geometry, it can have a large effect on the measurements in a cone-and-plate geometry.