Polyelectrolyte solutions in complex macro- and micro-scale flows*

Studying the behavior of polymer and polyelectrolyte solutions under complex hydrodynamic flow conditions is of interest to a variety of processes, such as extrusion, coating, and flocculation. Often these flow fields include combinations of both shear and extension flows, requiring characterization of the shear and extensional rheology for optimized materials processing. Here, the behavior of polymer solutions in two flow fields will be discussed: Taylor-Couette flows and microfluidic flow-focusing. First, in Taylor-Couette flow, which is flow between two concentric, rotating cylinders, the complex solutions are subjected to a wide variation of hydrodynamic flow states. The addition of non-Newtonian polymer solutions increases the solution’s elasticity, which in turn can modify flow states that are typically dominated by inertial forces. In this study, a cationic polyacrylamide was used to modify the elasticity of the solution, and with varying concentrations of NaCl to alter the ionic strength of the solution. The coil conformation and relaxation times of charged polymers changes depending on the ionic strength, from a more rigid conformation at low ionic strengths to a more flexible conformation at high ionic strengths, resulting in different non-Newtonian responses to shear. The effect of polymer conformation as a result of varying solution ionic strength on TC flows with co- and counter-rotation of the cylinders will be discussed. Second, microfluidic geometries will be used to characterize the extensional properties of low molecular weight, low viscosity polymer and polyelectrolyte solutions. Specifically, filament stretching using a cross-slot microfluidic channel will be used to resolve extensional properties of polyelectrolyte solutions at varying NaCl concentrations.