Creaming and oscillation rheology of weakly flocculated concentrated emulsions

Abstract

Addition of the non-adsorbing polymer hydroxyethylcellulose (HEC) to oil-in-water emulsions induces flocculation via the depletion mechanism. In the present work, we report results on the delay time prior to creaming of hexadecane-in-water emulsions, flocculated by HEC. In addition, we report on oscillation rheology measurements of a comparable, density-matched system. We compare the two in the search for rheological signatures of delay, and for insight into the delay mechanism. Our results suggest that the commonly used low-frequency relaxation time does not map simply to delay time. We do, however, find that rheology can tell which of our compositions will exhibit delay, and which will not. In addition, those compositions having the greatest shear modulus have the longest delay. A simple model, in which the delay is attributed to a slow creaming stage over a very small distance, produces an acceptable correlation with the product of complex viscosity and the strain at the end of linearity. Thus the area under the linear portion of the complex viscosity–strain plot may provide a useful marker of delay time. The unexpected observation of a liquid-like regime at high oscillation frequency merits further investigation.