Second Newtonian region of polymer solutions
Chemical and Thermal Process Engineering
Technical Thermodynamics
Final Report Abstract
We could extend the knowledge about the rheological behavior of polymer solutions to high shear rates, covering now the entire viscosity function up to the infinite-shear viscosity plateau. The infinite-shear viscosity plateau showed viscosities close to that of the solvent. Even for high concentrations, only four times higher values could be reached at most. For polyelectrolytes, we could identify two different concentration regimes in the infiniteshear viscosity plateau corresponding to dilute and semidilute solutions. The crossover concentration between them is considerably higher than in the first Newtonian branch, which can be related to the orientation of the polyelectrolytes. This indicates that concepts developed for the scaling laws of the zero viscosity may be transferred to the infinite-shear viscosity plateau. For the onset of the infinite-shear viscosity plateau, we find only a weak dependence on the shear rates. At the high shear rates studied, we could also study the flowinduced normal-stress differences, to the best of the author’s knowledge inaccessible up to now. They increase with an exponent of about 1 in the infinite-shear viscosity plateau, indicating that they are rather due to collisions between the polymers than due to viscoelastic polymer deformation. This suggests that polyelectrolyte solutions behave at high shear rates similar to nematic liquid crystals or fiber suspensions. For neutral polymers, the measurements were particularly demanding. The infinite-shear viscosity plateau is only slightly larger than the solvent viscosity. Hence, as a rule of thumb, even the viscosity of entangled solutions reduces to the level of the solvent viscosity. Resolving the concentration dependence indicates a behavior corresponding to dilute solutions. Although the normal forces were rather small and could only be detected at the highest concentrations, we found a relation between normal-stress differences and shear rate that had been proposed for the limit case of infinite shear rates. Although the polymers were strongly sheared, we could show that mechanical degradation did not take place. In this project, we studied a parameter range so far inaccessible. Hence, we encountered some surprises: One of them was that the infinite-shear viscosity plateau could not be reached at the accessible shear-rate range for some of the samples that we had planned to study. For λ-DNA, for instance, deviations from the power-law behavior at low shear rates into the infinite-shear viscosity plateau extended over a shear-rate interval of almost three orders of magnitude. Another surprise was the strong shift of the onset of the infinite-shear viscosity plateau when adding salt. This impeded quantifying the impact of salt on xanthan solutions. Evaluating the normal force at narrow gaps, the viscosity particularly of neutral polymer solutions and avoiding evaporation was delicate and required more dedication than initially foreseen. The results were also surprising: Initially, it was unclear whether there are different regimes for the concentration dependence of the infinite-shear viscosity plateau and whether concepts developed for the zero viscosity near thermodynamic equilibrium might be suitable far from thermodynamic equilibrium in the infinite-shear viscosity plateau. In addition, normalstress differences were unknown at these high shear rates. We had not expected to find such a low increase of the infinite-viscosity plateau particularly for neutral polymers. Evaluating the measurements, we could derive a picture of the polymer solutions at the infiniteshear viscosity plateau, where, for instance, we had not expected that polyelectrolyte solutions can be described in terms of nematic liquid crystals in this parameter range.
Publications
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Narrow-gap rotational rheometer, 5th International Conference on Experimental Fluid Mechanics – ICEFM 2018 Munich, Germany (2018)
H. Dakhil, A. Wierschem
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Infinite-shear viscosity plateau of salt-free aqueous xanthan solutions, Journal of Rheology 63, 63-69 (2019)
H. Dakhil, D. Auhl, A. Wierschem
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The shear rheology of λ-DNA solutions at high shear rates, Lasermethoden in der Strömungsmesstechnik, 27. Fachtagung 2019, eds: A. Delgado et al., Karlsruhe, 32.1 – 32.7 (2019)
H. Dakhil, S. Steiner, S. Basu, A. Soller, S. Pan, N. Germann, B. Kappes, A. Wierschem
