The Microstructural Origins of Gel Rheology in Dense Mixtures of Colloids and Adsorbing Polymer

We investigate the link between the microstructure, dynamics, and rheological properties in dense mixtures of charge-stabilized silica colloids and oppositely-charged poly(ethylene imine) (PEI) polymer in a refractive index matched solution of dimethylsulfoxide and water. Such systems form self-supporting gels over a finite range of polymer concentrations, due to polymer-induced interparticle bridging. Dynamic rheological measurements show a reentrant behavior with increasing polymer concentration, in which both the storage modulus and yield stress of the gel initially grow, but go through a maximum and subsequently drop with rising polymer concentrations. Analysis of the spatial configuration and dynamics of the particles by quantitative confocal microscopy reveals the microstructural origins of this behavior. In the first regime where gel stiffness increases with polymer concentration, the particle dynamics are remarkably heterogeneous, due to the coexistence of an arrested gel phase with freely diffusing particles and small clusters. We segregate the particle populations based on their mobility and quantify the microstructure of the arrested phase. Both the overall particle dynamics and gel stiffness are found to correlate with the effective volume fraction of the arrested phase in this regime. However, in the range of polymer concentrations studied, the reentrant rheology is not captured by the particle dynamics. Instead, it is accompanied by subtle changes in the microstructure of the arrested phase and possibly in the strength of interparticle bonds. The implications of this behavior for complex fluids engineering of colloid/polymer mixtures are discussed.