PERSPECTIVE: Analysis of thixotropic timescale^a)

Thixotropy is characterized by a time-dependent rise in viscosity under no or weak flow conditions and a decrease in viscosity over time when subjected to strong flow conditions. The characteristic timescale associated with the thixotropic phenomenon, particularly how the viscosity increases with time, has been termed the thixotropic timescale. Several approaches have been suggested in the literature for estimating the thixotropic timescale. The most prominent approach, however, infers it from a specific form of a kinetic expression for structure parameter evolution. In this paper, we study the various kinds of structural kinetic models. By carefully analyzing the same, we propose a parameter for the thixotropic timescale associated with the most generic form of kinetic expression for structure parameter evolution. We observe that increasing the thixotropic timescale weakens the thixotropic character of a system when the viscosity of the structural kinetic model continuously increases over time and eventually diverges under quiescent conditions. We also propose a new phenomenological measure of the thixotropic timescale: $τ t h i x= ( d ln η / d t ) − 1$⁠, where $η$ is viscosity and t is time. The proposed definition allows a straightforward and unique way to determine the thixotropic timescale through experiments and agrees well with the conventional notion of thixotropy.