The rheological and microstructural characterisation of the non-linear flow behaviour of concentrated oil-in-water emulsions

Abstract

This paper reports experimental observations on the rheology and microstructure of concentrated oil-in-water emulsions stabilised by macromolecular and low-molecular-weight emulsifiers. From the four different samples that were tested, certain general trends were identified and the measured linear viscoelastic and non-linear viscoelastic properties were compared using a phenomenological factored integral constitutive equation with a continuous relaxation spectrum. The self-consistency of the model was quite good in two cases (vegetable protein and polyoxyethylene glycol non-ionic surfactant emulsifiers) with the general exception of low steady shear rate data where the model overpredicted experimental stress measurements. It is shown that the fitting of the experimental data is sensitive to a wide range of inter-relating factors. In addition, optical observations of the sheared materials consistently showed low shear rate surface slip and this observation was correlated with the rheology miss match. For some systems the optical microstructure studies reveal a range of behaviour for the emulsions dependent on emulsifier composition. Wall slip, microdomain movement, chaining and changes in droplet size distribution were all observed under different conditions and in some cases it has been possible to correlate these microstructure observations with the sample rheology.