It is generally considered to be a difficult work to make analysis of rheological constants of the colloidal substances from the microscopic point of view on account of their complicated nature. For the interpretation of their mechanical properties from the technological viewpoint, however, there are many cases in which their phenomenological observation is sufficient. Measurement has been made, for example, of the dynamic viscoelasticity of the paste of rice starch from various provinces. Obvious relation has been taken notice of between the elasticity of starch gel and the taste or flavor of the corresponding rice. There is another example demonstrating the efficacy of rheological measurement by which creep viscosity of some candies has been estimated and compared with their flow and tackiness in practical use.
Most dispersed systems are known to behave as non-linear materials; carbon-black suspensions or starch pastes show strong tendency of non-Newtonian viscosity, thixotropy or non-linear viscoelasticity. There are some cases, however, in which typical linear behavior is observed. D.J. Shaw (1963) reports that ice cream behaves as a typical linear material; its creep compliance curve is perfectly independent of stress, as long as the strain is kept small, in spite of the fact that it has a composite structure of ice grains, fat particles and a network of protein. Shaw assumes that the factor responsible for its creep behavior is not ice nor fat crystals but the network of denatured protein, and that the linear behavior comes from the rubber-like elasticity of the network.
In the present article the author proposes to discuss two problems, i.e., the problem of structure formation in dispersed systems and that of mechanical stability of colloidal dispersions.
(1) Structure formation in dispersed systems.
The concept of structure formation in colloidal systems has long been one of the working hypotheses in colloid chemistry along with the concept of solvation or hydration.
Structure formation is obviously demonstrated in some cases. M.J. Forster and D.J. Mead (1951) measured electrical conductivity of carbon-black suspension in oil in the rotating-cylinder vessel, and observed marked decrease of conductivity in case of flow and its rapid recovery by settling.
There are cases, however, in which this concept is not successfully applicable. The filler reinforcement in rubber, for example, that was once interpreted in connection with the structure by carbon-black particles in rubber matrix, is now explained as a result of a strong quasi-covalent linking between the polymer molecules and the pigment particles.
The author has been engaged in the visco-elasticity measurement of dispersed solid systems of some pigments in asphalts and pitches. The result shows that these systems behave as a comparatively linear materials in mechanical properties. They are thermo-rheologically simple, and moreover, the so-called shift law, analogous to the time-temperature reduction law, holds in the effect of pigment concentration on the creep compliance. The concept of structure formation is applicable to this case, and the nature of the structure can be discussed.
(2) Mechanical stability of colloidal systems.
Some colloidal dispersions rapidly lose their stability by strong agitation, even in laminar flow shear, and coagulate and precipitate.
The problem was originally treated by Smoluchowski, and recently modified and satisfactorily applied to the shear flow coagulation of polystyrene latex by D.L. Swift and S.K. Friedlander (1964).
