Interfacial and bulk effects in packed microgel yielding

Microgels made from crosslinked polyelectrolytes are used as a rheological modifier of numerous different products. Packed microgels are also used to study the origins of jamming and glassy behavior in soft-sphere model systems. The rheological behavior of microgels has been studied thoroughly at the macroscopic scale; at the particle-scale, jamming, yielding, and glassy-dynamics have been carefully investigated through experiment, theory, and simulation. Yielding of granular materials in general is controlled by interfacial friction between neighboring particles and by particle elasticity. However, in packed microgels, the relative contribution of friction and elastic deformation is not known. Here we show that the macroscopic yielding behaviors in packed microgels is not controlled by gel-gel friction, and instead can be predicted from the classic polyelectrolyte physics scaling laws that control single-microgel elasticity and relaxations. Specifically, we find that the yield stress is the elastic stress associated with single particle deformations during rearrangements and the shear-rate at which microgels transition from a solid to a fluidized state is controlled by the diffusive relaxation of microgel deformation during re-arrangements.