ABSTRACT
In a number of key industries, including the pharmaceuticals, photographic, paper, and food industries, gels and gelling processes have for many years occupied a position of prime importance, fulfilling many functions from controlled release, surface coatings, and consumed products. Although many definitions have been suggested and attributed to gels in the literature [1]−[3], it is accepted that gels are notoriously difficult to define as a class of materials. They represent a state of matter somehow intermediate between solid and liquid, and they are usually considered as showing solid character overall, although some features of a liquid may also be present. Even though a gel may consist of 99.9% liquid, it can still behave as a solid. Under normal conditions of observation, gels are able to store the work employed in their deformation, and to recover their original shape, and it is this property of “elasticity” that is the essence of a solid. Generally, a simple gel consists of a solid network, which is interspersed (swollen) by a liquid. This network, in the case of biopolymer gels, is built of biological macromolecules and the liquid is usually water. Biopolymer gels may range from so-called weak-gel systems, actually just structured liquids which break down under shear to flow like viscous fluids, to rigid solids, but they are typically soft and resilient or simply jelly-like. In what has later proved to be one of the most quoted articles on gels and gelation, Paul Flory in 1974 proposed in his introduction to a “Faraday Discussion on Gels and Gelation” a classification of gels established on the following basis:
1. Well-ordered lamellar structures, including gel mesophases 2. Covalent polymeric networks, completely disordered 3. Polymer networks formed through physical aggregation, predominantly disordered but
with regions of order 4. Particulate disordered structures
In this chapter, these gels will be called, respectively, Flory I, II, III, and IV. A more recent definition of a gel has been proposed from the rheological point of view by Almdal and coworkers in 1993 [4]. This states that a gel is a soft, solid, or solid-like material, which consists of two or more components, one of which is a liquid, present in substantial quantity. The gel shows a flat mechanical spectrum in an oscillatory-shear experiment. This requires that G′ exhibits a plateau extending to times of the order of
seconds, whereas G″ is considerably smaller. This follows earlier suggestions, for example, by Winter and Chambon [5] that the gel point, the discontinuity when sol → gel, can be associated with a common power-law dependence of G′ and G″.
