Non-Newtonian Effects in Dilute Solutions Due to Heterogeneity
Dilute polymer solutions often show a remarkable and unexpected non-Newtonian flow behavior, which can only be explained by assuming hydrodynamic interaction between the macro-molecules.
Such interaction may occur in dilute solutions, if the molecules are sufficiently stretched by the flow but also in solutions which are heterogeneous and consists of long strings of concentrated solution dispersed in the solvent.
In the following it is shown that such strings may be present in “fresh” polymer solutions, which are dilute only on the average. The strings may be made visible by various means. Experimental observations are explained by assuming a fluid model based on heterogeneity due to the presence of strings of concentrated solutions. The non-Newtonian flow characteristics will depend on the dimensions and concentration of the strings which, in their turn, depend on the manner of preparation of the solution. From theoretical considerations the similarity between the flow of dilute and concentrated solutions can also be explained.
Experimental results of the flow of sodium oleate (soap) solutions through a porous medium are presented, showing an apparent onset, maximal and subsequent decreasing non-Newtonian effect with increasing strain rate. These results for low Reynolds number flows of sodium oleate solutions are similar to those of macro-molecular solutions, just as their drag reduction characteristics in turbulent shear flow are similar. The non-Newtonian effects of sodium oleate solutions are thought to be due to the stringlike structure formed by their miscelles, another indirect support of the proposed model.