Effect of emulsion drop-size distribution upon coalescence in simple shear flow: A population balance study

A population balance is used to examine the effect of the shape of the initial drop-size distribution of an emulsion upon its short and long-time evolution in simple shear flow. Initial distributions that are monodisperse, multidisperse, lognormal, bimodal, multimodal, and step functions are considered. At short times, it is shown that the rate of coalescence decreases by up to 25% for step distributions and up to 75% for lognormal distributions as the width of the distribution increases. Bimodal, multidisperse and multimodal distributions show intermediate decreases in the rate of coalescence, between these two values, with increases in the distribution width. Furthermore, it is found that the initial rate of coalescence is strongly dependent upon the presence of large drops. As the number fraction of large droplets within the distribution increases, the rate of coalescence also increases. At long times, all distributions move toward an asymptotic distribution shape in which the frequency of drops decreases algebraically with drop diameter at small drop diameters, and decreases exponentially with drop diameter at large drop diameters. Though portions of each distribution showed the expected asymptotic scaling behavior at long times, each asymptotic distribution nevertheless retains ‘fingerprints’ of the respective initial distribution. Overall, the rate of coalescence for a system is bounded by the initial rate, which is a function of the initial distribution shape, and the asymptotic rate, which is dependent upon the long-time scaling behavior. Finally, it is shown that the resolution with which the drop-size distribution of an emulsion is experimentally measured can have a significant effect upon predicted rates of coalescence.

Evolution of (a) monodisperse and (b) bidisperse drop-size distributions over time due to coalescence. Nt/N0 represents the fraction of initial drops remaining.Figure optionsDownload high-quality image (103 K)Download as PowerPoint slideResearch highlights
► Coalescence rate depends upon shape of drop-size distribution, and is especially sensitive to the fraction of large droplets.
► Fingerprints characteristic of the initial distribution shape remain, even when the number of drops has been reduced by 90%.
► Measurement resolution of the drop-size distribution significantly affects predicted coalescence rates.
► Coalescence rate decreases with the width of the drop-size distribution.