(532b) Analysis of Droplet Coalescence in Emulsions Subjected to Acoustic Fields

Separation and recovery of the oil phase from emulsions has practical applications in a variety of situations, including environmental remediation, pharmaceutical manufacture, and food processing. In our laboratory, we have developed methods for such separations by applying a low-intensity, resonant ultrasonic field within a rectangular chamber, which is optionally filled with a highly porous mesh. The density and compressibility difference between the dispersed droplets and the continuous phase results in a net force on the droplets. The drops migrate to the pressure antinodes of the standing wave field under the influence of this primary acoustic force. Subsequent coalescence of the droplets takes place due to interdroplet forces. Efficient coalescence and transport of the oil droplets was experimentally demonstrated and oil separation efficiencies as high as 90% were observed in laboratory-scale devices.

The present work focuses on obtaining fundamental understanding of the relevant physical and chemical phenomena that underlie the transport and coalescence of droplets observed in these experiments. A microscopic mathematical model is developed which includes pertinent body forces (buoyancy and the primary acoustic force) and interdroplet effects (van der Waals forces, hydrodynamic interactions and secondary acoustic forces) that govern the phenomena. The model predicts the relative motion of a pair of droplets, given various parameters like the strength and frequency of the sound field, the initial positions of the two interacting drops, their sizes, plus the viscosity and density of both the continuous and dispersed phases. The results from this model are used to compute the collision rate for a given droplet pair in acoustic field. This leads to the development of a macroscopic population balance model to predict the evolution of drop-sizes in a given drop population coalescing under the influence of acoustic field. Such predictions can then be validated by performing experiments to track the droplet size distribution of an emulsion subjected to an acoustic field.