What is a gel?

Definition of a gel

Gels are three-dimensional networks made up of molecules, polymers, particles, colloids, etc., that are connected with each other by the specific parts on them such as functional groups and associative groups. The connected parts are called cross-links. Gels usually contain many solvent molecules inside their networks, and hence they are close to liquid in composition, but show solid-like mechanical properties due to the existence of the cross-links [1–4].

Although this statement can be adopted as a formal definition of gels, there are many exceptional ones that do not fall neatly into this categorization. For instance, colloidal suspensions exhibit gel-like rheological behavior at high densities. Entanglements of long rigid fibrillar molecules or an assembly of molecules mutually hinder their motion, and lead to gel-like rheology due to jamming of the rigid segments. In such materials, there are no direct cross-links, but geometrical or topological constraints play similar roles to the cross-links, although they are delocalized. Therefore, to define gels by connectivity only is not sufficient to include these materials.