  Thixotropy is a phenomenon related to time dependent change in viscosity in
presence or absence of flow. The yield stress, on the other hand, represents
the minimum value of stress above which steady flow can be sustained. In
addition, the yield stress of a material may also change as a function of time.
Both these characteristic features in a material strongly influence the steady
state flow curve of the same. This study aims to understand the interrelation
between thixotropy, yield stress and their relation with the flow curve. In
this regard, we study five thixotropic materials that show yield stress. The
relaxation time of all the five systems shows power-law dependence on aging
time with behaviors ranging from weaker than linear, linear to stronger than
linear. Furthermore, the elastic modulus and yield stress has been observed to
be constant for some systems while time dependent for the others. We also
analyze the experimental behavior through a viscoelastic thixotropic structural
kinetic model that predicts the observed experimental behavior of constant as
well as time-dependent yield stress quite well. These findings indicate that a
non-monotonic steady-state flow curve in a structural kinetic formalism
necessarily leads to time-dependent yield stress, while constant yield stress
is predicted by a monotonic steady-state flow curve with stress plateau in the
limit of low shear rates. The present work, therefore, shows that thixotropic
materials may exhibit either monotonic or non-monotonic flow curves.
Consequently, thixotropic materials may show no yield stress, constant yield
stress or time-dependent yield stress.
