Structural Origins and Nonlinear Mechanics during Yielding of a Heterogeneous Colloidal Gel

We investigate the yielding of a colloidal gel comprised of nanoscale oil droplets in water driven by thermo-reversible interdroplet attractions. The system forms a heterogeneous structure that is best described as a two phase system at the micron scale containing droplet-rich domains of fractal clusters and droplet-poor domains. By combining large amplitude oscillatory shear (LAOS) measurements with simultaneous ultra-small angle neutron scattering (rheo-USANS), we characterize both the nonlinear mechanical processes and strain-dependent microstructural changes through the yielding transition. We find that the material undergoes a broad yielding process in which the nonlinearity evolves over an order of magnitude in strain amplitude between the initial yield point and flow. By analyzing the intracycle response as a sequence of physical processes, we monitor several parameters throughout the nonlinear yielding process, including the residual elasticity, yield stress and recoverable strain of the network. Frequency-dependent measurements show significant rate-dependence of the yielding process, which is driven by poroelastic effects. Correlating these results with structural parameters extracted from rheo-USANS data reveals that the material passes through a “top-down” cascade of structural breakdown. First, the droplet-lean domains consolidate into large voids, which saturate near the initial yield point. Second, at higher amplitudes, cluster-cluster correlations become increasingly homogenous, suggesting a de-percolation of cluster-cluster bonds as the ultimate process determining the transition to flow. We note that all significant structural changes occur on the µm-scale, suggesting that large-scale rearrangements of thousands of particles, rather than the immediate rearrangement of particle-particle bonds, are responsible for the yielding behavior of heterogeneous colloidal gels.