Normal stress differences in thixotropic elastoviscoplastic fluids

TEVP (Thixotropic elastoviscoplastic) fluids exhibit viscosity changes with time due to their microstructure that breaks under shear and rebuilds upon rest. Various model systems are proposed in the literature for studying this phenomenon, and constitutive models are developed for quantifying the response. While most models proposed in the literature are one-dimensional in nature, a few three-dimensional models are available. However, these models are studied for shear stress response only due to limited normal stress difference measurements. In the present work, rheological measurements are conducted on a model filled polymer melt system using PP (parallel plate), CP (cone and plate), and CPP (cone partitioned plate) geometries to obtain shear stress, first and second normal stress differences (N1, N2) under steady and transient shear protocols. While several issues and shortcomings are identified, some solid findings emerge. The response of N1, N2 with shear rate is qualitatively similar to that of shear stress in steady shear, with N1 being positive and N2 being negative. This is consistent with observations on various polymer melts and solutions. Two TEVP models are considered for quantifying the material response under various steady and transient shear protocols. While both models predict the shear stress response closely, qualitative deviations are observed for N1, while N2 is zero. Both qualitative and quantitative differences are observed for shear stress, N1 and N2 response in transient protocols, highlighting the importance of using a variety of tests and the need for further improvements in modeling, to assess the physical origin of TEVP response.