gelation: one construct, many systems

In polymer science, gelation is the moment a branching, cross-linking liquid stops being a pourable sol and becomes a gel: as bonds form between growing chains, one molecule eventually spans the entire sample, so the system develops a yield stress and a diverging viscosity even though most of it is still liquid. Flory's 1941 Molecular size distribution in three dimensional polymers. I. Gelation gave the statistical theory of this gel point, predicting the critical degree of cross-linking at which an infinite network first appears. The referent is the sol-to-gel transition of a cross-linking polymer, quantified by the gel point and the fraction of material bound into the network, the same connectivity threshold that reappears in food gels and in sol–gel glasses.[1]

Unit

The critical extent of reaction (gel point) at which an infinite network first spans the system

Recovery means

Not applicable

Measured by

Located by the divergence of viscosity and the first appearance of an equilibrium modulus as cross-linking proceeds

Food scientists use the same construct for how gelatin, pectin, starch, and egg or milk proteins turn a fluid into a self-supporting solid: dispersed biopolymers associate into a continuous three-dimensional network that immobilises the water, giving custards, jams, yoghurt, and jellies their set texture. Clark and Ross-Murphy's 1987 Structural and mechanical properties of biopolymer gels treats these as physical gels whose onset and stiffness follow the same network-connectivity logic as synthetic polymers. It is the identical sol-to-gel transition, here realised in an edible biopolymer system and measured by gel strength and the setting point rather than by a chemical extent of reaction.[2]

Unit

The concentration or temperature at which a biopolymer dispersion sets into a space-spanning network

Recovery means

Not applicable

Measured by

Assessed by small-deformation rheology (storage modulus), gel strength, and the setting or melting temperature

In materials chemistry the same word names the heart of the sol–gel process: a colloidal sol of hydrolysed metal–oxygen precursors condenses until the particles or oligomers link into a continuous solid skeleton filling the liquid, the point at which the sol becomes a rigid gel that can later be dried and fired into glass or ceramic. Hench and West's 1990 The sol-gel process reviews this route and the gelation step that governs it. It is the same connectivity-threshold construct as in polymers and food, transposed to inorganic oxide networks and measured by the gel time and the onset of elastic rigidity.[3]

Unit

The gel time at which the condensing oxide sol becomes a continuous rigid network

Recovery means

Not applicable

Measured by

Tracked by the gel time, the rise in viscosity, and the emergence of an elastic modulus during condensation