Evaluation of theoretical viscosity models for concentrated emulsions at low capillary numbers

The theoretical models for the viscosity of concentrated emulsions are revi ewed. All models predict that the relative viscosity of an emulsion, at low capillary numbers, is a function of dispersed-phase volume fraction and vi scosity ratio (ratio of dispersed-phase viscosity to continuous-phase visco sity). The predicted values of relative viscosity from different theoretica l models are compared with a large amount of experimental data. The cell mo del of Yaron and Gal-Or [Rheol. Acta 11 (1972) 241] predicts the relative v iscosities of emulsions reasonably well over a wide range of dispersed-phas e volume fraction and viscosity ratio. The cell model of Choi and Schowalte r [Phys. Fluids 18 (1975) 420] overpredicts the relative viscosities, espec ially at high values of dispersed-phase volume fraction. The Phan-Thien and Pham model [J. Non-Newtonian Fluid Mech. 72 (1997) 305] underpredicts the relative viscosities of emulsions at high values of dispersed-phase volume fraction. (C) 2001 Elsevier Science B.V. All rights reserved.