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 concentrat ed, nonflocculated polydisperse emulsion is presented. The model inclu des the effects of hydrodynamic hindrance and thermal diffusion. Conce ntration profiles for the total volume fraction and for an individual particle size, along with the size distribution, are presented. Numeri cal results are also presented for ultrasound velocity as a function o f sample height, since ultrasound is being increasingly used experimen tally to observe the creaming process in concentrated colloidal system s. The velocity is calculated from the Urick equation and also from si ngle- and multiple-scattering theory. Results are presented for two id ealized emulsion systems, 20% hexadecane in water, and 10% sunflower o il in water using a log-normal size distribution centered on a diamete r of 0.72 mum. It is concluded that the effects of scattering are cons iderable in these emulsions, especially in the concentrated cream laye r. The Urick equation is insufficient to interpret ultrasound velocity data in these systems. Strategies for minimizing and accounting for t he errors due to scattering effects are given. The model predicts that a considerable variation in average particle size occurs within the c ream layer, and that this significantly affects the ultrasound velocit y profile. (C) 1994 Academic Press, Inc.