Is thixotropy important in small-scale, steady-state flow modeling?

Many particle suspensions are characterized as thixotropic-viscous materials and are present in different industrial processes. In most cases, the flow of these materials is modeled using a generalized Newtonian model that assumes the viscosity to be a function solely of the local deformation rate. However, particle structure of a suspension, and therefore its viscosity, does change instantaneously with the stress (or deformation rate) level. At a constant imposed stress (or shear rate), the microstructure evolves until reaching an equilibrium value. Liquid particles flowing through regions of a complex flow experience different levels of stress. Assuming that the viscosity at each point of the flow is the steady-state viscosity described by a generalized Newtonian model may lead to an inaccurate flow description. The flow of a thixotropic-viscous liquid through a constricted capillary is analyzed here using two rheological models: a generalized Newtonian model and a thixotropic model that takes into account the liquid time-dependent response. The resulting set of fully coupled, non-linear equations is solved by the Galerkin and SUPG Finite Element Method. The results show that the use of a simpler generalized Newtonian model to describe thixotropic-viscous materials, such as some particle suspensions, can lead to very large errors on the predicted flow behavior, highlighting the need for a more complete model that takes into account time-dependency of the flowing liquid in a certain range of flow parameters.