The effect of volume fraction and impeller speed on the structure and drop size in aqueous/aqueous dispersions

Abstract

The mean drop size and the structure of two-phase aqueous/aqueous dispersions, one-phase sodium alginate-rich of viscosity ∼0.25 Pa s and the other sodium caseinate-rich of viscosity ∼0.022 Pa s , have been measured in an unbaffled vessel fitted with a helical screw impeller. The measurements were carried out over a range of volume fractions and at Reynolds numbers in the range from laminar to low transitional. In addition, the interfacial tension between the two phases has been measured in situ using a recently developed drop retraction technique, which, for the first time, has been successfully applied at a high volume fraction of the dispersed phase. At low volume fractions of the viscous phase (viscosity ratio, λ= μ d/ μ c≈10), drops of that phase are seen much as in equivalent aqueous/oil dispersions but the functionality between the drop size and impeller speed is different. As the volume fraction of the viscous phase increases, the structure first changes to a striated one, something never seen in “pure” oil/aqueous dispersions. The striated structure also evolves into complex (droplets-in-drops) in samples withdrawn from the vessel and within the vessel when stirring is stopped. This implies that the system is in a phase inversion region, but contrary to oil/water dispersions, there is not a rapid switch from one phase being continuous to the other, i.e. the phase inversion region appears to be very stable in time. On a further increase of the volume fraction of the viscous phase, phase inversion occurs when stirring but a striated structure continues to exist, i.e. there is no dramatic change of structure as found with aqueous/oil dispersions undergoing phase inversion. However, when a sample is withdrawn or the impeller is stopped, the complex droplets-in-drops formation no longer appears and only a simple dispersed structure develops. Only at very low speeds and volume fractions of the low viscosity dispersed phase, i.e., λ∼0.1, do drops re-appear in the vessel when stirring. Overall, it can be concluded that there is a very significant difference in the behavior of oil/aqueous and aqueous/aqueous dispersions.