Understanding the nonlinear flow behavior of entangled fluids including shear thinning (related to stress plateau) or transient stress overshoot upon startup shear remains a fundamental challenge in polymer rheology. For more than five decades, a conventional rheometric measurement was carried out to investigate the nature of these nonlinear viscoelastic properties in entangled fluids by treating them as liquids that could deform homogenously.
Recent particle-tracking-velocimetric (PTV) measurements based on 1,4 polybutadiene (PBD) solutions has found that shear inhomogeneity occurs after stress overshoot when sheared on time scale faster than terminal relaxation time (γ̇appτ > 1.0). Because of experimental difficulties such as edge fracture during shearing, it is challenging to achieve reliable steady state in synthetic entangled polymeric liquids. In this work, we choose DNA solutions as model entangled systems. Since the required DNA concentration is rather low, between 5 to 22 mg/mL, these samples can be sheared for an infinitely long time without experiencing significant edge instability. A wide variety of DNA solutions with various number Z of entanglements per chain and solvents are studied by combination of PTV and rheometric measurements. In the Newtonian regime (γ̇appτ < 1.0), the uniform flow was displayed at all time of shearing for entangled DNA solutions and there is no stress overshoot during measurement. For least entangled DNA solution with Z = 24, transient shear inhomogeneity was presented after stress overshoot, however at long time the velocity field returned to linearity across the gap. For the strongly entangled DNA solutions (Z ≥ 60), the permanent shear banding was observed in both startup shear and creep in the stress plateau (shear thinning) regime. The thickness of high-shear band eventually grows to span the entire gap at the upper end of the plateau region. It was shown that using of glycerol as a solvent minimized interfacial wall slip, permitting shear banding to develop in the bulk at rates that can be studied with the current PTV method. Finally, we found that strong shear banding during startup shear can be avoided by either slow ramp-up from low rate (γ̇ ~ 10-4 < τ-1, s-1) or quenching down from homogenous shear at sufficiently high rate to the specific rate. The final steady shear stress in presence or absence of shear banding is the same exhibiting the limitation of conventional rheometric measurements to depict nonlinear flow behavior of entangled fluids.