Gel

Gels are semi-solid materials in which a liquid phase is immobilized within a three-dimensional network that spans the entire structure. The network can be formed by polymer chains, inorganic or organic colloids, or a combination thereof. Gels are typically categorized by the solvent of the liquid phase: hydrogels contain water, organogels contain organic solvents, and xerogels are dried gels with minimal liquid content. Gels can be reversible (physical gels) or irreversible (chemical gels) depending on the type of crosslinks.

The three main mechanisms to form a gel: chemical crosslinking (covalent bonds between polymer chains), physical crosslinking (hydrogen bonding, ionic interactions, entanglements), or a combination. Crosslink density and network structure determine mechanical properties, swelling capacity, and diffusion through the gel. Gels may swell or shrink in response to solvent composition, temperature, pH, or ionic strength.

Common hydrogel examples include gelatin, agar, alginate, carrageenan, polyacrylamide gels. Silica gels are inorganic networks formed by condensation of silica precursors. Organogels include gels of oils with gelling agents. Hydrogels are widely studied for biomedical and tissue engineering applications, such as drug delivery, contact lenses, wound dressings, and soft robotics.

In food, gels provide structure and texture (gelatin desserts, jam, jelly candies). In industry, gels serve as desiccants (silica gel), thickening agents, or matrices for catalysis or sensors. Gel properties are assessed by rheology, swelling studies, and diffusion measurements; practical behavior depends on history and environmental conditions.