(98g) Microstructural Processes During Yielding of Nanoemulsion Gels

The transition to flow in colloidal gels is known to depend significantly on their network microstructure. However, little is understood about the differences in yielding between gels with the homogeneous structure typically found in dilute suspensions, and those with heterogeneity characteristic of more concentrated systems. In this work, we use nanoemulsion “organohydrogels” [Helgeson et al., Nature Materials, 2012, 11(4): 344-352] as a model system to explore these differences, whereby the interdroplet attractions can be dynamically tuned through a thermoresponsive polymer, allowing for the formation of gels with either homogeneous or heterogeneous cluster-scale microstructure. Large-amplitude oscillatory shear (LAOS) measurements reveal that the non-linear response shows an abrupt transition from elastoplastic strain softening to viscoplastic strain thinning at the yield point, whereas for heterogeneous gels a new behavior emerges where effective strain hardening appears and then disappears during a broad two-stage yielding transition . To probe the microstructural origins of this behavior, we present studies of rheo-small and ultra-small angle neutron scattering during yielding which provide detailed structural information on length scales ranging from the primary particle size to several microns. We find that, in general, yielding arises from a “top-down” cascade of microstructural processes, beginning with large-scale rupture of weak cluster bonds near the yield point, further cluster bond rupture and suspension beyond the yield point, and finally densification and breakage of clusters en route to terminal flow. The broad yielding transition in heterogeneous gels is related to the progressive melting of heterogeneous cluster-cluster correlations. These results suggest more attention should be given to the influence of large-scale microstructure on the yielding of attractive suspensions.