Investigation of the extensional properties of elasto-visco-plastic materials in cross-slot geometries

Abstract

• Flow of an EVP material is studied in the CSER or the OSCER geometries. • The Saramito/HB model is used to account for elastic and plastic effects. • At higher ε ˙ , flow is destabilized either to time-dependent or steady asymmetric flows. • Values for the HB exponent > 0.5 must be avoided with EVP materials. The generation of a wide region of shear-free, strong extensional flow near a stagnation point has been proven to be a great challenge, especially when considering yield-stress fluids. Capillary break up and filament stretching rheometers cannot approximate such flow field when elasto-visco-plastic (EVP) materials are used, because solidification of the material near the solid disks induces strong shear contributions to the flow. To overcome these limitations, we propose the standard (CSER) and optimized (OSCER) cross-slot devices to investigate the extensional properties of yield-stress fluids. To examine and understand the nonlinear dynamics of elasto-visco-plastic materials in strong extension flows, we model the rheology of a typical Carbopol solution with the popular Saramito/Herschel-Bulkley (SRM/HB) model. Using our stabilized finite element method, PEGAFEM-V, we solve the momentum and mass conservation coupled with the constitutive equation in both cross-slot flow geometries for a wide range of flow parameters. At low extension rates, the flow is steady and symmetric, allowing for plastic effects to be evaluated. At higher extension rates, the interplay of material elasticity and plasticity destabilizes the flow leading either to time-dependent or steady asymmetric flows. Our analysis points out that adopting values for the HB exponent > 0.5 must be avoided when modeling typical EVP materials with the SRM/HB model. Finally, by performing a wide parametric analysis, we conclude that these instruments can produce ideal conditions for the characterization of various EVP materials in a wide spectrum of extension rates.