Extensional rheometry through hyperbolic contraction

Abstract: To understand fluid rheology, properties such as consistency and viscosity are determined in both shear and extensional flow. Many commonly-used materials and formulations exhibit complex rheological properties and in order to determine the behaviour as well as predict and govern their properties, effective and reliable characterisation of these fluids is important. Rheological properties are used as quality control parameters, since they have an impact at all stages of material processing, across multiple disciplines, from formulation development, stability during processing to final product performance and stability. Furthermore, rheological properties can vary depending upon external conditions, and can be correlated with the product microstructure. Various techniques are available for characterising viscoelastic liquids in shear, but extensional rheological properties are experimentally more difficult to determine and thus not many techniques are available commercially. In this work, a measuring technique for extensional rheology based on hyperbolic contraction flow is developed and evaluated through numerical and experimental studies. The evaluation was performed using numerical simulations through a hybrid finite element/finite volume scheme to evaluate the behaviour of the flow through a hyperbolic contraction nozzle. Several configurations, differing in contraction angle, were considered, and contraction-expansion configurations were also evaluated. The hyperbolical shape was found to give an almost constant strain rate throughout the measuring regime in contrast to the other configurations, and the influence of shear from the wall was minimised. The use of a corrected pressure drop, called the excess pressure drop, as a measure of extensional properties has been evaluated and proved to strongly depend on extensional viscosity. By changing the configuration to a contraction-expansion configuration, the shear contribution to the excess pressure drop could be neglected. This was also verified experimentally. The numerical results were compared with experimental measurements of the pressure drop using a set of model test fluids with various rheological properties, a Newtonian, a Boger and a shear-thinning fluid. By matching the shear viscosity of the model fluids, it was possible to address the influence of elasticity and shear thinning. Both numerical and experimental results demonstrated increasing excess pressure drops over the Newtonian reference line with increasing deformation rates for both the hyperbolic contraction configuration and the contraction-expansion configuration. This opposed to previously reported results on the sharp 4:1:4 configurations, which show pressure drops under the Newtonian reference line. Finally, the hyperbolic contraction device was utilised to develop edible model fluids differing in rheological properties with the aim of determining the rheological influence on swallowing. Fluid elasticity was overall found to be beneficial for safe and easy swallowing.