Determining the overlap concentration of semi-flexible polyelectrolyte solutions using dripping-onto-substrate rheometry

In a salt-free solution, semi-flexible polyelectrolytes such as xanthan gum adopt a rigid conformation, while the addition of salt to the solution makes them increasingly flexible. This conformational change of xanthan gum from stiff to flexible reduces polymer persistence length and leads to dramatic changes in both shear and extensional flows. In shearing deformation, the addition of salt decreases shear thinning. In a transient extensional flow such as that generated by dripping-onto-substrate (DoS) rheometry, addition of salt decreases the filament breakup time and the transient extensional viscosity. At a fixed salt concentration, increasing polymer concentration results in a transition in capillary thinning dynamics from inelastic filament breakup to a long-lived elastic filament beyond critical concentration. This transition marks the onset of an elastocapillary response and provides a signature of the overlap concentration, at which polymer chains begin to interact with each other. Compared to traditional methods, DoS requires fewer tests and is highly sensitive to changes in the polymer dynamics. It also provides overlap concentration values in agreement with the scaling theory for semi-flexible polymers, making it suitable for rapid rheological characterization of semi-flexible polymer solutions.