The Basic Mechanisms of Drop Coalescence

The small difference between densities of drops and the ambient liquid, as well as small size of drops in the emulsion, result in a low sedimentation rate of drops in gravitational field. Thus the main challenge in the process of emulsion separation is to increase the drop size. This problem can be addressed by intensifying the coalescence of drops. The factors utilized to enhance the rate of drop integration may include the apphcation of electric field and turbulization of the flow. Before we proceed to describe these effects, consider in general the process of drop coalescence in the emulsion. 

The characteristics of mutual approach of drops, and consequently, of their collisions, depend on the hydrodynamic regime of emulsion motion. Consider first the coalescence of drops settling under gravity in a quiescent liquid. Such kind of coalescence is called the gravitational coalescence. 

Drops of different sizes are settling under gravity with different velocities. As a result, larger drops overtake smaller ones, and their collision becomes possible. Each drop has its own trajectory, whose determination should be our primary goal if we want to calculate the collision frequency of drops. In most cases, the volume concentration of drops is assumed to be low, so it is possible to consider only the relative motion of two drops. The analysis will be carried out in a spherical system of coordinates attached to the center of the larger drop (Fig. 11.2). 

In this coordinate system, the flow of the ambient liquid moves relative to the larger drop. On a large distance from the drop, the velocity can be assumed constant and equal to the sedimentation velocity of the drop. Another drop of smaller size moves relative to the larger drop together with the flow, goes around it, and either touches it or passes by. Due to their small sizes, the motion of drops can be 

All sets of trajectories of smaller drops relative to the larger ones can be divided into two classes trajectories which do not end at the surface of the large drop, and trajectories resulting in a collision of drops (Fig. 11.3). 

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