Rheological behavior of high internal phase water-in-oil emulsions: Effects of droplet size, phase mass fractions, salt concentration and aging

The rheological properties of high internal phase emulsions (HIPEs), comprising polydisperse aqueous droplets in oil, have been characterized as a function of emulsification time, salt concentration, phase mass fractions and aging. The droplet size distribution and structural details of the emulsion samples were obtained using cryogenic-scanning electron microscopy (cryo-SEM) and optical microscopy. For rheological characterisation, amplitude sweep tests performed on HIPE samples with a high mass fraction of dispersed phase (93.5 wt%) show that the strain behavior, especially the yield strain (epsilon(y)) and crossover strain (epsilon(c)), are almost independent of the droplet size and polydispersity. However, emulsions with smaller droplets have higher yield stress (tau(y)) and storage moduli (G') values; explanations for these observations, based on the physical properties of the systems are suggested. Furthermore, it is observed that, for constant mass fractions of oil and aqueous phases, the strain behavior is also independent of the salt concentration in the dispersed phase. Our findings indicate that, independently of the salt concentration, the energy requirement for the emulsion to start flowing is greater when smaller droplets are present. Aging studies, performed over a period of 6 months, show no significant change in the rheological properties of the HIPEs. Experimental rheology data is compared to the Princen and Kiss model, and with modified Mougel model (Ind. Eng. Chem. Res. 2011, 50, 10359-10365), giving insight into the critical effects of non-ideality induced by polydispersity in thickened emulsions. (C) 2017 Elsevier Ltd. All rights reserved.