Strain softening of concentrated cohesive particulate suspensions prior to yield

We study the viscoelastic solid properties of cohesive particulate suspensions using creep and constant rate tests in a vane-in-large-cup geometry. A cup-to-vane diameter ratio larger than 4 is used to ensure that wall effects are minimized. In both the creep and constant rate tests, the modulus becomes nonlinear at strains consistent with scaled interparticle bond distances. Yielding and subsequent flow do not occur until strains of order 1, corresponding to a cage melting or particle crowding failure mechanism. In between the bond and cage melting strains, the modulus shows power-law softening with an index of approximately-0.8, corresponding to progressive bond breakage. The observed behavior in creep correlates with constant rate experiments and demonstrates that strain softening of the modulus prior to yielding is an important component of the rheology of particulate gels and suspensions. The assumption of linear behavior up to yielding appears to be inappropriate. We observe that the transient creep behavior initially shows power-law or Andrade creep that transforms to an exponential decay at long times. For creep stresses that show time-dependent yield, the break time decreases exponentially with stress.