Modeling of concentration profiles and ultrasound velocity profiles in a creaming emulsion: importance of scattering effects

A computer model which simulates the creaming behavior of a concentrated, nonflocculated polydisperse emulsion is presented. The model includes the effects of hydrodynamic hindrance and thermal diffusion. Concentration profiles for the total volume fraction and for an individual particle size, along with the size distribution, are presented. Numerical results are also presented for ultrasound velocity as a function of sample height, since ultrasound is being increasingly used experimentally to observe the creaming process in concentrated colloidal systems. The velocity is calculated from the Urick equation and also from single- and multiple-scattering theory. Results are presented for two idealized emulsion systems, 20% hexadecane in water, and 10% sunflower oil in water using a log-normal size distribution centered on a diameter of 0.72 μm. It is concluded that the effects of scattering are considerable in these emulsions, especially in the concentrated cream layer. The Urick equation is insufficient to interpret ultrasound velocity data in these systems. Strategies for minimizing and accounting for the errors due to scattering effects are given. The model predicts that a considerable variation in average particle size occurs within the cream layer, and that this significantly affects the ultrasound velocity profile. © 1994 Academic Press. All rights reserved.