Complex fluids exhibit time-dependent changes in viscosity that have been
ascribed to both thixotropy and aging. However, there is no consensus for which
phenomenon is the origin of which changes. A novel thixotropic model is defined
that incorporates aging. Conditions under which viscosity changes are due to
thixotropy and aging are unambiguously defined. Viscosity changes in a complex
fluid during a period of rest after destructuring exhibit a bifurcation at a
critical volume fraction PHIc2. For volume fractions less than PHIc2, the
viscosity remains finite in the limit t => infinite. For volume fractions above
critical the viscosity grows without limit, so aging occurs at rest. At
constant shear rate there is no bifurcation, whereas under constant shear
stress the model predicts a new bifurcation in the viscosity at a critical
stress sB, identical to the yield stress sy observed under steady conditions.
The divergence of the viscosity for stress s sB is best defined as aging.
However, for s > sB, where the viscosity remains finite, it seems preferable to
use the concepts of restructuring and destructuring, rather than aging and
rejuvenation. Nevertheless, when a stress sA (sB) is applied during aging,
slower aging is predicted and discussed as true rejuvenation. Plastic behaviour
is predicted under steady conditions when s > sB. The Herschel-Bulkley model
fits the flow curve for stresses close to sB, whereas the Bingham model gives a
better fit for s >> sB. Finally, the model's predictions are shown to be
consistent with experimental data from the literature for the transient
behaviour of laponite gels
