(413b) Microstructural Origins of Two-Step Yielding and Strain-Induced Strengthening in Dilute Colloidal Gels

Colloidal gels are particulate soft materials traditionally utilized in the food, ceramics, and cosmetics industries as well as a number of emerging technologies including direct write assembly. In such applications, colloidal gels are often subjected to transient flow during their processing and handling, and their end-use properties are governed by the coupling between their microstructure and transient nonlinear rheology. In this work we investigate the nonlinear rheology of dilute depletion-induced colloidal gels as model systems, and report that they can undergo yielding via a two-step process, with mechanisms fundamentally different from dense suspensions. We attribute the two-step yielding in dilute gels to interparticle bond rotation and bond breakage at the respective yield points. Implications of this behavior for transient deformation in step-stress experiments are examined. These include remarkable creep profiles at intermediate applied stress values, highlighted by strain-induced anisotropic hardening and flow arrest at very large accumulated strains. Quantitative time-resolved confocal microscopy in conjunction with a custom-built shear cell is used to investigate the microstructural origins of this behavior. The results are relevant to the processing or final consumer use of particulate suspensions with attractive interparticle interactions, and will be discussed in light of a balance between the globally imposed stresses and the thermodynamic interparticle forces.