Kinetics of low Reynolds number creaming and coalescence in droplet dispersions

Abstract

AbstractThe effect of size distribution and concentration of rapeseed oil drops on the kinetics of creaming, and interdrop and interfacial coalescence in surfactant‐free dispersions, in which the droplet Reynolds number for creaming is very small, is investigated. The concentration profiles of oil drops within the dispersion, measured using a radioactive γ‐ray attenuation technique, show that the low Reynolds number of creaming small drops causes the dispersion to remain essentially loosely packed with drops during most of the time period. The creaming velocity and coalescence rate increase initially with time because of an increase in drop size by interdrop coalescence and decrease subsequently as larger drops cream out of the dispersion. Drops in concentrated dispersions, in which their mean diameter and concentration are large, grow in size faster, resulting in creaming and interfacial coalescence rates that are larger than those for dilute dispersions. An analytical model is presented that describes well not only the present experimental data on concentrated dispersions but also those published on very dilute dispersions. The results obtained by the model are in good agreement with those presented by other authors using the population balance model involving extensive numerical computations. The present model also enables the determination of coalescence times of single drops with their bulk homophase using experimental data on dispersions. These coalescence times, which decrease with increasing drop diameter, agree well with those predicted by the published equation for drainage of the continuous phase film between a spherical drop and its bulk homophase. The droplet capillary numbers are found to be very small, indicating that the drops are spherical in shape. © 2004 American Institute of Chemical Engineers AIChE J, 51: 149–161, 2005