Polysaccharide strong and weak gels.

Small deformation oscillatory shear measurements have enabled a distinction to be made between so-called "strong" and "weak" gels, in particular those formed from biologically significant polysaccharides. At small enough strains, both systems give essentially the same mechanical spectrum, with G' > G", and with both moduli largely independent of frequency. However, the deformation dependence of the two classes of materials is very different. Strong gels are essentially strain independent (linearly viscoelastic) for strains of greater than about 0.25, whereas weak gels show such a response only for strains of less than about 0.05. At large deformations strong gels will rupture and fail, and will never "heal" without melting and resetting. Conversely, weak gels will recover and can flow without fracture, giving a power law response, with an exponent approaching -1, so-called "yield stress" behavior. The rheological properties of a strong gel, agarose, derived from the Rhodophyceae (marine algae) and a weak gel xanthan, an exocellular slime exuded by bacteria of the genus Xanthomonas, are measured in vitro, and related to in vivo requirements.