Role of interparticle attraction in the yielding response of microgel suspensions

We use rheological measurements to examine the yielding behavior of a microgel system spanning the range from soft jammed glassy suspensions dominated by inter-particle repulsion to colloidal gels produced by attractive interactions. Under repulsive conditions, the suspensions display a prototypical soft glassy yielding response in which the shear loss modulus exhibits a single peak on increasing strain during the crossover from elastic to viscous behavior. By contrast, under fully attractive conditions the colloidal gel displays a more complex yielding, with two distinct peaks in the loss modulus in the vicinity of the yield strain. It is apparent that the gels yield initially by network rupture, followed by shear induced densification which leads to the formation of compact clusters. We show that the second peak in the loss modulus is consistent with the subsequent breakup of these dense clusters. We quantitatively map the steady progression from simple glassy yielding to the more complex gel response on increasing attraction strength by the evolution of peak locations, magnitudes and frequency dependencies. Notably, the peak locations diverge as the network becomes more fragile and spatially heterogeneous with increasing attraction strength. There is little frequency dependence in the peak positions, but the amplitude of the second yielding peak shows a non-monotonic dependence with a maximum near 5 rad s(-1). Time-resolved measurements and decreasing strain sweeps highlight pronounced differences in the reversibility of the network rupture and cluster breakup processes. Correspondingly, the linear viscoelastic properties of the gel are strongly dependent on mechanical history whereas the glass exhibits no such dependence.