Particle Aggregation

Particle aggregation refers to formation of clusters in a colloidal suspension and represents the most frequent mechanism leading to destabilization of colloidal systems. During this process, which normally occurs within short periods of time (seconds to hours), particles dispersed in the liquid phase stick to each other, and spontaneously form irregular particle clusters, flocs, or aggregates. This phenomenon is also referred to as coagulation or flocculation and such a suspension is also called unstable. Particle aggregation can be induced by adding salts or an other chemical referred to as coagulant or flocculant. Some people refer to specifically to flocculation when aggregation is induced by addition of polymers or polyelectrolytes, while coagulation is a more widely used term.

Particle aggregation is normally an irreversible process. Once particle aggregates have formed, they will not easily disrupt. In the course of aggregation, the aggregates will grow in size, and as a consequence they may settle to the bottom of the container, which is referred to as sedimentation). Alternatively, a colloidal gel may form in concentrated suspensions which changes its rheological properties. The reverse process whereby particle aggregates are disrupted and dispersed as individual particles is referred to as peptization hardly occurs spontaneously, but may occur under stirring or shear.

Colloidal particles may also remain dispersed in liquids for long periods of time (days to years). This phenomenon is referred to as colloidal stability and such a suspension said to be stable. Stable suspensions are often obtained at low salt concentrations or by addition of chemicals referred to as stabilizers or stabilizing agents.

Similar aggregation processes occurs in other dispersed systems too. In emulsions, they may also be coupled to droplet coalescence, and not only lead to sedimentation but also to creaming. In aerosols, air-born particles may equally aggregate and form larger clusters (e.g., soot).