Coagulation

Coagulation is the process by which a liquid becomes more viscous or a colloidal suspension destabilizes to form a solid, gel, or visible floc. The term is used in several disciplines, notably in biology to describe blood clotting and in chemistry and environmental engineering to describe particle aggregation. Coagulation generally involves changes in electrical charges or intermolecular interactions that lead to a connected solid phase.

Biological coagulation, or hemostasis, stops bleeding after vascular injury. Platelets form a temporary plug, and plasma coagulation factors activate a cascade that culminates in the conversion of fibrinogen to fibrin by thrombin, creating a stable fibrin mesh that reinforces the plug. The cascade has intrinsic and extrinsic initiation routes that converge on a common pathway and is tightly regulated by anticoagulants and fibrinolytic enzymes.

Chemical coagulation of suspensions uses coagulants, such as aluminum or iron salts, to neutralize particle surface charges, reducing repulsion and promoting aggregation into larger flocs. This is followed by flocculation and sedimentation or filtration. Coagulation is central to water and wastewater treatment and to mineral and pigment processing. Effectiveness depends on pH, coagulant dose, mixing, and the nature of the colloid, and is often assessed by turbidity and zeta potential.

In food and materials processing, coagulation refers to protein gelation or firming through heat, acid, or salt, such as when eggs set or milk curdles. Controlled coagulation is also used to aid separation, clarify liquids, or build textures in products.