Georgia Ioannou, Ph.D. candidate of the fluids lab won the best paper award in the 12th PESXM conference for the poster:
“DYNAMICS OF ELASTO-VISCO-PLASTIC MATERIALS IN STRONG EXTENSION”, by G. Ioannou, S. Varchanis, Y. Dimakopoulos, J. Τsamopoulos
In this work, we examine the flow of elasto-visco-plastic materials in the optimized-shape cross-slot extensional rheometer (OSCER). The shape of this geometry has been optimized numerically in order to generate a wide region of homogeneous elongational flow. Haward et al. demonstrated experimentally using PEO solutions, that such an ideal flow can remain steady for a wide range of extension rates in the OSCER. Based on their results for viscoelastic materials in the OSCER, we study the dynamics of elasto-visco-plastic materials in this purely extensional flow field. Such an investigation of yield-stress materials in elongational flows is necessary as indicated by recent experiments of Zhang et al. In their work, noticeable differences have been revealed with respect to the extensional behavior of complex yield-stress materials (e.g. the ratio of the extension yield-stress to the shear yield-stress is larger, by a factor of 1.5, than expected from the standard theory). In order to investigate the dynamics of elasto-visco-plastic materials in strong extension, we have simulated the flow of a 0.08% Carbopol® -940 aqueous solution using the Saramito/Herschel-Bulkley constitutive equation  in the OSCER geometry. Performing a wide-range parametric analysis of the dynamics of the flow, we determine when steady-state or oscillatory solutions arise. Surprisingly, we find that even at high extension rates the system can reach a steady state at which strong extension dominates a wide region around the stagnation point. However, when the flow rate is furtherly increased, we identify a critical value of the flow rate that drives the system to a periodic state.