Several destabilsation processes can occur in dispersions, causing changes and affecting their stability. Important destabilisation processes include sedimentation, creaming, coalescence, agglomeration, and aggregation.
Sedimentation and creaming occur due to gravity and the difference in density of the various dispersion components. Denser ("heavier") particles or droplets sink to the bottom, forming sediments and contributing to phase separation. The two processes are shown schematically in Figure 2.
Less dense ("lighter") components rise in the continuous phase, a process known as creaming. Creaming occurs, for example, in dispersions containing oil droplets immersed in water. The speed of sedimentation or creaming essentially depends on the difference in density between the disperse and continuous phases, the particle or droplet size and the viscosity of the continuous phase.
Coalescence is a process in which droplets or particles of the disperse phase merge with each other (see Figure 3). This is due to the so-called Brownian motion, which causes collisions between dispersed droplets or particles. During those collisions, the dispersed particles fuse and form a larger compound. The initial particles are then indistinguishable from each another. This process leads to a gradual increase in particle or droplet size and can subsequently accelerate destabilisation processes such as sedimentation.
Agglomeration and aggregation are similar processes and are often used interchangeably in the literature, because they are difficult to distinguish experimentally. In agglomeration, dispersed particles or droplets form loose clusters through weak attractive forces such as van der Waals forces, while remaining identifiable as individual components (see Figure 3). These clusters can typically be broken up by mechanical influences such as shaking the dispersion.
In aggregation, multiple droplets or particles form clusters that are held together by stronger forces such as hydrogen bonding, making them harder to separate (see Figure 3). However, there is no complete fusion as in coalescence. Both agglomeration and aggregation can be promoted by a higher salt content in the solution, which weakens the repulsive electrostatic forces between the components.