Concentration regimes in salt-free aqueous xanthan solutions under shear

Abstract

The concentration regimes in polymer and polyelectrolyte solutions can be identified by scaling laws for the relation between specific zero-shear viscosity and concentration. As recently shown, the same is true for the infinite-shear viscosity plateau. The shear-thinning range is usually accessed by focusing on the viscosity functions for the respective concentration regime. For the first time, we extend here the concept of scaling laws for the viscosity for different concentration regimes to the entire shear-rate range. To this end, we study salt-free aqueous xanthan solutions, where we find power-law dependencies of the specific viscosity on concentration throughout the entire shear-rate range. We distinguish six different concentration regimes. Apart from those already known for the zero-shear viscosity of polyelectrolyte solutions, we identify two other regimes at high concentrations: We find for viscoplastic materials a linear regime at low shear rates and another regime at higher shear rates. The fact that the regimes extend smoothly from the zero-shear regime into finite shear rates, i.e., away from thermodynamic equilibrium, shows that indicators such as critical concentrations remain valid at finite shear rates. This allows us to follow the shift of relevant interaction mechanisms with shear rate. Within some regimes, the power-law exponents change smoothly with shear rate, particularly when deviating from the zero-shear viscosity plateau before the power-law regime of the viscosity function. Some regimes merge as their power-law exponents approach each other, indicating the disappearance of supramolecular structures with shear.