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the transformation of a readily mobile or viscous fluid into a solid (a body without fluidity) that is elastic, plastic, and brittle. Gelation is characteristic of solutions of macromolecular compounds and of disperse colloidal systems. It is caused by the formation of a three-dimensional structural network (skeleton), which fills the entire volume of the liquid and destroys its mobility. In solutions of polymers such a network is formed from macromolecules linked by intermolecular forces or chemical bonds; in colloidal systems, it is formed from coupled particles of the dispersed phase.
Gelation may be caused by an increase or decrease in temperature, an increase in the concentration of the dissolved or dispersed material, a change in chemical composition of the system as a result of a chemical reaction between its components, or the introduction of specific reagents. Gelation is sometimes reversible— that is, when conditions change, a system may pass repeatedly from the liquid state to the solid state and vice versa. However, in the case of pro-found chemical or physical changes in a system, the process is irreversible.
Gelation is often observed and plays an important role in many natural and manufacturing processes. The liquid layer of a paint, lacquer, glue, or photographic emulsion gelates before finally drying. Gelation occurs upon the introduction of electrolytes into rubber latex, during the hardening of synthetic resins in the manufacture of plastics, during the cooling of gelatin and starch paste solutions, and during the coagulation of blood.