Rheological Investigations on the Creaming of Depletion-Flocculated Emulsions

摘要

Preventing creaming or sedimentation by the addition of thickenersnis an important industrial challenge. We study the effect of the addition ofna “free” nonadsorbing polymer (xanthan gum) on the stability against creamingnof sterically stabilized O/W emulsions. Therefore, we analyze our samples usingnmicroscopy and rheological measurements. At low xanthan concentrations, thenemulsions cream. However, above a certain concentration a three-dimensionalnnetwork of droplets is formed, which can prevent creaming. We attribute thenformation of this structure to depletion attraction. The rheological behavior of annemulsion that is macroscopically stable should be elastic, while it should benviscous for a creaming emulsion. In order to distinguish between stable and unstablensamples, we measure their relaxation time by mechanical rheology and findna good correlation to the visual observation. However, the measured relaxationntimes are much shorter than the time-scales, on which we observe creaming. Wenhypothesize that the measured relaxation time is related to the droplet−dropletninteraction. This determines the frequency at which microscopic rearrangements occur, which weaken the network structurenprior to creaming. Based on this interpretation, the relaxation time gives direct access to the microstructural processes involved inncreaming. We therefore suggest using it as a predictive parameter of creaming stability.