Measurement of average drop size in aqueous mixtures of Na-alginate and Na-caseinate by linear oscillatory tests

The linear viscoelastic behaviour of two-phase immiscible systems is characterised by an increased elasticity at low frequencies, which can be quantitatively related to the average size of the dispersed phase droplets and to the interfacial tension between the two phases (Palierne, J. F. (1990). Linear rheology of viscoelastic emulsions with interfacial tension. Rheologica Acta, 29(3), 204-214). In the case of aqueous biopolymer mixtures, however, the application of classical emulsion models, such as the one of Palierne, is not trivial, due to sedimentation of the dispersed phase and to the extremely low values of interfacial tension which are typical of these systems. In this work, we investigated the application of Palierne model to aqueous mixtures of Na-alginate and Na-caseinate. The volume average drop radius was calculated by fitting the Palierne model to experimental data of elastic modulus vs frequency right after cessation of shear flow. A comparison with direct drop size measurements was performed by shearing the mixtures in a parallel plate apparatus coupled with an optical microscope. In this apparatus the mixtures were subjected to the same flow history as in the rheometer and drop size was measured by image analysis. Good agreement was found in all the four mixtures investigated, covering two volume fractions of dispersed phase, i.e. 5% and 10%, and two values of viscosity ratio (drop viscosity / continuous phase viscosity), i.e. 0.34 and 3.0.