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In Part I of this paper [Haward et al. submitted (2023)], we presented a newthree-dimensional microfluidic device (the optimized uniaxial and biaxialextensional rheometer, OUBER) for generating near-homogeneous uniaxial andbiaxial elongational flows. In this Part II of the paper, we employ the OUBERdevice to examine the uniaxial and biaxial extensional rheology of some modeldilute polymer solutions. We also compare the results with measurements madeunder planar extension in the optimized-shape cross-slot extensional rheometer[or OSCER, Haward et al. Phys. Rev. Lett. (2012)]. In each case (uniaxial,planar and biaxial extension), we use micro-particle image velocimetry tomeasure the extension rate as a function of the imposed flow rate, and wemeasure the excess pressure drop across each device in order to estimate thetensile stress difference generated in the fluid. We present a new analysis,based on solving the macroscopic power balance for flow through each device, torefine the estimate of the tensile stress difference obtained from the measuredpressure drop. Based on this analysis, we find that for our most dilute polymersample, which is "ultradilute", the extensional viscosity is well described bythe finitely extensible non-linear elastic dumbbell model. In this limit, thebiaxial extensional viscosity at high Weissenberg numbers (Wi) is half that ofthe uniaxial and planar extensional viscosities. At higher polymerconcentrations, the experimental measurements deviate from the modelpredictions, which is attributed to the onset of intermolecular interactions aspolymers unravel in the extensional flows. Of practical significance (andfundamental interest), elastic instability occurs at a significantly lower Wiin uniaxial extensional flow than in either biaxial or planar extensional flow,limiting the utility of this flow type for extensional viscosity measurement.

journal of rheology

Extensional rheometry of mobile fluids. Part II: Comparison between the  uniaxial, planar and biaxial extensional rheology of dilute polymer solutions  using numerically-optimized stagnation point microfluidic devices