Immiscible fluid mixing coalescence

This paper is divided into two main, interconnected parts—breakup and coalescence of immiscible fluids, and aggregation and fragmentation of solids in viscous liquids—preceded by a brief introduction to mixing, this being focused primarily on stretching and self-similarity. 

The treatment of mixing of immiscible fluids starts with a description of breakup and coalescence in homogeneous flows. Classical concepts are briefly reviewed and special attention is given to recent advances—satellite formation and self-similarity. A general model, capable of handling breakup and coalescence while taking into account stretching distributions and satellite formation, is described. 

A series of articles was published by Utracki et al. [318-322] on the modeling of mixing of immiscible fluids in a twin screw extruder. The fourth paper in the series [321] incorporates several refinements of the earlier model, one of the most important refinements being the incorporation of the effect of coalescence. The model considers two breakup mechanisms, both based on the micro-rheology. One breakup mechanism is the drop fibrillation and disintegration into fine droplets when the Weber number is greater than four times the critical Weber number. The second mechanism is drop splitting that occurs when the Weber number is below four times the critical Weber number. 

The only way significant amounts of immiscible fluids can be mixed together is if the interfacial layer surrounding the dispersed droplets is occupied by an adsorbed layer of molecules that keep the droplets from coalescing. Figure 1.1 shows the importance of the interfacial layer in emulsion systems for the two main classes of surface-active molecules, surfactants and proteins, that stabilize them. Low molecular weight surfactants, lipids, and emulsifiers self-assemble at interfaces with the appropriate part of the molecule associating with the appropriate hydrophilic or hydrophobic phases. Proteins, on the other hand. 

It should be noted that this technique is not the same as external mixing of separately atomized droplets coalescing within the chamber. One reported work in the literature utilized droplets atomized from separate nozzles that were then allowed to collide and coalesce within the spray dryer. Janssens et al. [19] have shown that this approach did not improve the solubility and the extent of dissolution of Intraconazole and Kollicoat IR particles the droplets behave as individual droplets without significant mixing and coalescence within the spray dryer. In the 3- and 4-fluid nozzle technique, the mixing of the two immiscible occurs at the position just prior to atomization under very high shear. 

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