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

This paper reports experimental observations on the rheology and microstruc ture of concentrated oil-in-water emulsions stabilised by macromolecular an d low-molecular-weight emulsifiers. From the four different samples that we re tested, certain general trends were identified and the measured linear v iscoelastic and non-linear viscoelastic properties were compared using a ph enomenological factored integral constitutive equation with a continuous re laxation spectrum. The self-consistency of the model was quite good in two cases (vegetable protein and polyoxyethylene glycol nonionic surfactant emu lsifiers) with the general exception of low steady shear rate data where th e model overpredicted experimental stress measurements. It is shown that th e fitting of the experimental data is sensitive to a wide range of interrel ating factors. In addition, optical observations of the sheared materials c onsistently showed low shear rate surface slip and this observation was cor related with the rheology miss match. For some systems the optical microstr ucture studies reveal a range of behaviour for the emulsions dependent on e mulsifier composition. Wall slip, microdomain movement, chaining and change s in droplet size distribution were all observed under different conditions and in some cases it has been possible to correlate these microstructure o bservations with the sample rheology.