384902 Structural Origins and Nonlinear Mechanics during Yielding of a Heterogeneous Colloidal Gel

Wednesday, November 19, 2014: 1:15 PM
Marquis Ballroom A (Marriott Marquis Atlanta)
Matthew E. Helgeson1, Juntae Kim1, Dimitri Merger2 and Manfred Wilhelm2, (1)Chemical Engineering, University of California, Santa Barbara, Santa Barbara, CA, (2)Karlsruhe Institute of Technology, Karlsruhe, Germany

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.

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