Stress-independent delay time in yielding of dilute colloidal gels

We investigate the yielding under shear for dilute poly(N-isopropyl acrylamide-co-fumaric acid) (PNIPAM-FAc) colloidal gels obtained above the volume phase transition temperature. In this temperature range, the microgel suspensions form colloidal gels due to hydrophobic interparticle interactions under appropriate pH and ionic strength conditions. Step-strain tests revealed that yielding occurs when the applied strain exceeds a specific threshold, requiring a finite, stress-independent delay time (t_D). This is distinct from previous findings on delayed yielding in other colloidal gels, where t_D decreases with increasing stress. In the start-up shear tests, yield strain (γ_y) at a higher strain rate () increases with escalating , while γ_y at lower  remains constant. This characteristic γ_y- relationship is successfully explained by a simple model using the stress-independent t_D value without an adjustable fitting parameter. The distinctive yielding behavior, underscored by a stress-independent t_D, is expected to originate from strain-induced macroscopic phase separation into a dense colloidal gel and water, observable separately from rheological measurements.