Emulsification of highly concentrated emulsions—A criterion of shear stability

The problem of shear stability of highly concentrated (dense) emulsions has been discussed based on treating these emulsions as viscoplastic media. Their rheological properties were characterized by the linear elastic modulus and the yield stress because these properties are responsible for the rheological properties of emulsion. The shear stability was defined as the characteristic time of the drop size decrease in emulsification. This approach is opposite to the usual characterization of shear stability of emulsions consisting of the application of the capillary number, this parameter was excluded from the discussion. The analysis was based on the experimental data for samples of various concentrations (ranging from 0.79 to 0.92 vol. %), prepared with different surfactants, four types of external phases, refined at different shear rates and using two types of emulsification devices with different types of the flow. For constructing master curves, the elasticity number (El) and the Bingham number (Bn) as the function of the rate of refinement were computed, and the stability was characterized by the dimensionless product (Gamma) of the shear rate and characteristic time of the refining rate which is some analog of the Weissenberg number. This model implies that achievement of critical values of these parameters is necessary for opening further possibility of the drop breakup. It was found that general correlations between El and Gamma and Bn and Gamma do exist. These dependencies are expressed by the empirical scaling power laws with the exponents -0.74 and -0.53, respectively. (C) 2018 The Society of Rheology.