Roseanna Zia, Cornell University
On-demand fluids: to yield or not to yield?
Our recent studies of yield of colloidal gels under shear show that yield occurs under precise conditions and then in distinct stages. Under fixed stress, yield follows a finite delay period of slow solid-like creep. Post yield, the gel fluidizes and may undergo long-time viscous flow or, in some cases, may re-solidify. Under imposed strain rate, the transition from equilibrium to long-time flow is characterized by one or more stress overshoots, signifying a yield process here as well. These rheological changes are accompanied by evolution in morphology and dynamics of the gel network which we show contradict prior models of catastrophic network failure. Similar regimes have been observed in gels subjected to gravitational forcing; the gel initially supports its own weight, or perhaps undergoes slow, weak compaction. This may be followed by a sudden transition to rapid compaction or sedimentation. Various models have been put forth to explain these behaviors based on structural evolution, but this detail is difficult to observe in experiment. In our most recent work we have examined the detailed microstructural evolution and rheology of reversible colloidal gels as they deform under gravity, identifying the critical buoyant force at which yield occurs and the role played by ongoing gel coarsening. Comparisons of the yield process in the three modes—step shear stress, sudden onset of strain rate, and gravitational loading—reveal similarities and differences that shed light not only on the “on-demand” fluid behavior of these smart materials, but also on the more fundamental process of phase separation of kinetically arrested states of soft matter.