Coupled nonhomogeneous flows and flow-enhanced concentration fluctuations during startup shear of entangled polymer solutions

The flow behavior of entangled polymer solutions in simple shear flow is often assumed to be uniform, both in terms of the velocity gradient and polymer concentration. However, there is growing evidence of nonuniform banded transient or steady state flows of entangled polymer solutions. The present work considers a distinct phenomenon, whereby transient banding is concomitant with the presence of local shear-enhanced concentration fluctuations. Experimental observations are made using combined rheological measurements with simultaneous particle tracking velocimetry and microscopy (rheomicroscopy) during startup shear in Taylor-Couette flow of entangled polymer solutions of polystyrene in a marginal solvent, dioctyl phthalate, at various entanglement numbers (Z). At high Z, the flow develops transient, nonhomogeneous banded velocity profiles over a wide range of imposed shear rates that are inverted from those expected for Taylor-Couette flow, which then relax to a nearly uniform shear rate at steady state. Rheomicroscopy reveals that these transient banded states are accompanied by strong shear-enhanced concentration fluctuations localized to the region of lowest local shear rate. We hypothesize that such finite-amplitude concentration fluctuations lead to increased dissipation that contributes a higher local effective fluid viscosity that, in turn, could produce the observed flow nonuniformity. In spite of the nonuniformity of the flow and the concentration fluctuations, the measured apparent rheology of the fluids is in qualitative agreement with both prior experimental studies and model predictions based on a Rolie-Poly model under the assumption that the fluid remains homogeneous, suggesting that this behavior is only observable through spatially resolved measurements of fluid flow and concentration.