Rheological characterization and modeling studies of highly concentrated emulsions*

Highly concentrated emulsions (HCEs) are high viscosity fluids having complex rheological properties. In this work, rheological properties of water-in-oil HCE, having 93.5% internal phase volume fraction, are characterized using five deformation modes: shear sweep, amplitude sweep, frequency sweep, creep, and relaxation tests. Each test provides significant insights into the complex behavior of HCE resulting from the tight droplet packing, polydispersity, interfacial tension between the two phases and viscosity differences. The rotary rheological tests indicate that the viscosity of HCE is significantly decreased with increase in shear rate, and the HCE typically behaves like a shear-thinning fluid. The oscillatory rheological tests primarily determine the structural stability of the HCE and indicate that at low shear stress values the emulsions behave like an elastic material, and once the shear stress is increased past the yield stress, the viscous effects dominate. The creep and relaxation tests indicate that the HCE is quite stable when the shear stress values are within the yield stress, beyond which it behaves like a viscoelastic material. An attempt is also made to compare the experimental rheological data with theoretical models available in the literature.