Kinetics of ultrasonically induced coalescence within oil/water emulsions: Modeling and experimental studies

A model for the coalescence of the oil phase within aqueous emulsions driven by the application of a low-intensity, resonant ultrasonic field has been developed. Under the application of a resonant ultrasonic field, the density and compressibility difference between the dispersed and continuous phases results in a net force that pushes droplets toward pressure antinodes where coalescence subsequently occurs. A mathematical model that determines the relative transport of two individual droplets under the action of acoustic and other relevant forces was recently published. That approach is utilized here in the development of a population balance model which predicts global coalescence rates and the evolution of the droplet size distribution under the influence of the ultrasonic field. These results are experimentally validated and good qualitative agreement between model predictions and experimental observations of the evolution of the droplet size distribution is observed. Discrepancies between the experimental and modeling results are attributed to spatial non-uniformities in the acoustic field utilized in the experiments and the associated lateral radiation forces. The contribution of these forces to overall coalescence rates is described in terms of an effective strength of the acoustic field within the acoustic chamber.