3D microfluidics for extensional rheometry

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In FY2022, experiments were completed measuring the pressure drop and velocity field for viscoelastic polyacrylamide solutions (at dilute concentrations of 0.005 to 0.04 wt%) in a planar extensional flow device. Pressure drop and velocity field measurements have also been completed in the new uniaxial and biaxial extensional flow device developed during FY2021.A new data analysis has been developed to estimate the extensional viscosity of Newtonian and viscoelastic fluids. This ensures that the expected (constant) extensional viscosity is obtained for Newtonian fluid, and also for the polymer solutions at small flow rates. At high flow rates the extensional viscosity of the polymer solutions increases up to a plateau value that depends on the mode of extension. For the dilute solutions tested, the plateau values in uniaxial and planar extension are equal to each other and double that found for biaxial extension. This is in agreement with the finitely-extensible non-linear elastic dumbbell model, which is often used to describe such fluids in theory. Two journal papers have been prepared based on the work conducted so far in FY2021 and FY2022, which are both currently under review in the Journal of Rheology.Furthermore, experiments with viscoelastic polymer solutions revealed some interesting instability phenomena at high flow rates, which are worthy of a more detailed investigation. Our new data analysis method for estimation of the extensional viscosity also merits some additional experimental verification. These topics will be the focus of research in FY2023.