If every encounter between two droplets led to aggregation, then emulsions would not remain stable long enough to be practically useful. To prevent droplets from flocculating during a collision, it is necessary to have a sufficiently high repulsive energy barrier to stop them from coming too close together (Chapter 3). The height of this energy barrier determines the likelihood that a collision will lead to flocculation (i.e., the collision efficiency). The collision efficiency (E) has a value between 0 (no flocculation) to 1 (every collision leads to flocculation)* and depends on the hydrodynamic and colloidal interactions between the droplets. The flocculation rate therefore depends on the precise nature of the interactions between the
* In practice, E can have a value which is slightly higher than 1 because droplet collisions are accelerated when there is a strong attraction between the droplets.
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