De la rhéologie des systèmes aqueux à base de gommes

On the Rheology of Aqueous Systems on the Basis of Gums. The water „solutions” of macromolecular carbohydrates (arabic gum, alginate, guaran, starches) often show a non‐Newtonian behaviour. Therefore it is impossible to give only one viscosity value for the definition of the rheological behaviour; viscosity changes with the different rheological conditions. But it is possible to speak of an „apparent viscosity” for a given shear rate D i or stress r i . To characterize the rheological behaviour it is necessary to establish the relationship between shear rate and shear stress. Sometimes, for limited decades of shear rates, the behaviour may be described by a power law = τ = K D a : sometimes the Cross relation \documentclass{article}\pagestyle{empty}\begin{document}$ \overline \eta = \eta _\infty + \frac{{\eta _0 - \eta _\infty}}{{1 + \alpha D^{2/3}}} $\end{document} has to be applied; but it may also occur, that one can only give the curve log τ = f (log D). To obtain absolute values it is necessary to apply a correcting factor at the actual values given by Newtonian approximation. Thus it is possible to study the influence of temperature, concentration and ageing on the rheological parameters K, a, η or the graph log τ = f (log D). From the results it seems that the behaviour of this kind of systems is rather like the behaviour of emulsions, than like that of true solutions. Perhaps that is the reason why, the process for the preparation of these systems has to be well defined and rigorously followed in order to obtain reproducible results. To give a sense to rheological studies in the laboratory, it is important to simulate the exact industrial conditions of preparations and of the rheological stresses of these systems. This calls for an exact knowledge of the industrial rheological stress; up to now this knowledge is very rare in most cases.