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Complex fluids exhibit time-dependent changes in viscosity that have beenascribed to both thixotropy and aging. However, there is no consensus for whichphenomenon is the origin of which changes. A novel thixotropic model is definedthat incorporates aging. Conditions under which viscosity changes are due tothixotropy and aging are unambiguously defined. Viscosity changes in a complexfluid during a period of rest after destructuring exhibit a bifurcation at acritical volume fraction PHIc2. For volume fractions less than PHIc2, theviscosity remains finite in the limit t => infinite. For volume fractions abovecritical the viscosity grows without limit, so aging occurs at rest. Atconstant shear rate there is no bifurcation, whereas under constant shearstress the model predicts a new bifurcation in the viscosity at a criticalstress 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 touse the concepts of restructuring and destructuring, rather than aging andrejuvenation. Nevertheless, when a stress sA (sB) is applied during aging,slower aging is predicted and discussed as true rejuvenation. Plastic behaviouris predicted under steady conditions when s > sB. The Herschel-Bulkley modelfits the flow curve for stresses close to sB, whereas the Bingham model gives abetter fit for s >> sB. Finally, the model's predictions are shown to beconsistent with experimental data from the literature for the transientbehaviour of laponite gels.

Aging, rejuvenation and thixotropy in complex fluids. Time-dependence of the viscosity at rest and under constant shear rate or shear stress