Fracture and yielding motifs in colloidal gel

Colloidal gels are comprised of a majority fluid phase but exhibit solid-like mechanical properties as the colloids assemble into a percolating network structure. When subjected to external stresses or strains, these materials undergo a yield transition in which the material no longer elastically deforms but viscously flows. This yielding transition may be continuous across the entire sample or spatially localized, resulting in shear bands separated by a fracture boundary. Here, we use a novel rheological protocol – serial creep divergence (SCD) – to accomplish two primary tasks: (1) identify the motif by which gels transition from elastic deformation to viscous flow and (2) evaluate the role of interparticle interactions on the kinetics of yielding and recovery. First, we find that the creep response exhibits a robust and reproducible bifurcation for gels exhibiting fracture but a stress-dependent superposition for gels which yield continuously. These qualitative differences in the creep response allows us to determine whether gels flow through fracture or yielding. Second, we quantify the kinetics by which gels rebuild their internal structure after failure and demonstrate that the nature of these interactions controls the recovery response. Specifically, materials with permanent covalent bonds or entropic repulsions do not exhibit any time-dependent properties post-yielding. By contrast, materials in which the underlying gel structure is controlled by short-range attractions or physical entanglements exhibit significant recovery of their creep response which can be interpreted through a diffusion-aggregation model across the shear band interface. These findings demonstrate that specific physicochemical interactions can significantly alter the nonlinear response of gels despite possessing comparable linear moduli.