Coalescence in Creaming Emulsions. Existence of a Pure Coalescence Zone

Stored emulsions undergo various destabilizing processes including creaming and coalescence of drops. Many experimental studies have been carried out in the past to quantify drop coalescence; however, the presence of two distinct types of coalescence mechanismssone due to the relative motion between the drops and the other due to their permanent proximity to each other in the creamshas not been recognized. Furthermore, the effects of creaming in these investigations could not be eliminated in ways that do not introduce new modes of coalescence or alter the existing ones. The population balance equation for an emulsion in a column in which creaming, Brownian diffusion, and coalescence of drops occur simultaneously is analyzed. The analysis reveals the existence of a dynamic zone at the top of the column in which the net effect of creaming and Brownian diffusion of drops is eliminated. Thus, while the drops cream, diffuse, and coalesce as in an actual emulsion, the measurements from this zone allow the effects of drop coalescence to be isolated from other destabilizing processes. Based on this finding, a new methodology to investigate coalescence is proposed. Experimental support for the proposed theory is also provided. If the size distribution in the uniform zone evolves in a self-similar manner, it is shown that the techniques already available in the literature can be used directly to estimate the coalescence frequency kernel.