66 FUNDAMENTALS OF SUBMICROSCOPIC MORPHOLOGY 



deviations from Hagen-Poiseuille's law are observed in the case of 

 cellulose of polymerization degrees exceeding roo. Since the anomaly 

 of flow is caused by the mutual positions of the colloid particles, it 

 has been designated as structural viscosity (Ostwald, 1925; Philip- 

 POFF, 1955). 



c. Gel Structure 



Gelj'rawe. If the coherence between the individual colloid particles 

 becomes still more pronounced than in the "gel solution", gels are 

 formed with a more or less fixed shape and distinctly elastic properties. 

 Of course there exist all kinds of gradations from the gel solutions, 

 in which the elastic coherence of the particles can only be proved by 

 testing them for structural viscosity, and the real gels whose units are 

 more or less fixed in their mutual positions. The gels that become 

 liquid on shaking and soHdify again at rest, form a typical intermediate 

 stage. This remarkable phenomenon is due to the same effect as the 

 decreasing viscosity at increasing pressure; it is called thixotropy (cf. 

 Freundlich, 1942). Colloid silicic acid and gelatin, for instance, can 

 occur as thixotropic gels at suitable concentrations. 



If spherical colloid particles cluster together to a gel, the result is a 

 rather compact gel and from Fig. 5 1 a it is obvious that such a structure 



can onlv be formed at a 

 relatively high concent- 

 ration of the solute. 

 With sols containing 

 long chain molecules, 

 however, a fixed mutual 

 position, i.e., a structure 

 is attained much more 

 easily. At concentrations 

 as low as a few per cent, 

 of a long chain high 

 polymer, the chain molecules can combine into a loose meshwork, as 

 represented by Fig. 51. Such a colloid already possesses a structure, 

 although a very loose one, which may still easily undergo some plastic 

 deformation. It also possesses a certain elasticity, because the places 

 where the chains are interlaced can be regarded as fixed points. As will 

 be shown, these can be due to various kinds of forces. Since in biolo- 



Fig. 51. Submicroscopic stnacture formation a) with 

 spherical, ^) with filiform particles. 



