  Thixotropy is characterized by an increase in viscosity when a material is
subjected to no flow (quiescent) or weak flow conditions and a decrease in
viscosity when it is 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. In
the literature, several approaches have been suggested 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, and by
carrying out a careful analysis of the same, we propose a parameter for the
thixotropic timescale that is associated with the most generic form of the
kinetic expression for structure parameter evolution. We observe that when the
viscosity of the structural kinetic model undergoes continuous increase with
time and eventually diverges under quiescent conditions, which we believe is
the most practical scenario, our analysis suggests that increasing the
thixotropic timescale weakens the thixotropic character of a system. We also
propose a new phenomenological measure of the thixotropic timescale: ${{\tau
}_{thix}}={{\left( d\text{ln}\eta /dt \right)}^{-1}}$, where $\eta $ is
viscosity and $t$ is time. The proposed definition allows a straightforward and
unique way to determine thixotropic timescale through experiments and agrees
well with the conventional notion of thixotropy.
