Phase Behavior and Rheology of Colloids with Polymer-Mediated Attractions

Mixtures of colloids and polymers are used in many industrial and commercial applications such as paints, consumer products, and drilling fluids. Addition of polymers to colloidal suspensions can cause attractions between the particles, such as depletion and bridging attractions. These attractions produce complex phase behavior and rheology of the final suspension. In addition to the attractions that arise, the properties of the polymer additives themselves – size, dispersity, charge/interaction with particle – likely also affect the final suspension behavior. In this work, we investigated the effects of these properties of the polymer additives to the phase behavior and rheology of the resulting suspensions. First, we explored the effect of polymer dispersity on the phase behavior of depletion mixtures by using unary and binary mixtures of uniform, small polymers as the depletant in a model colloidal suspension. We found that the phase behavior could be mostly collapsed, irrespective of polymer dispersity, if the polymer concentration was represented as a weighted sum of the two polymers' concentrations in a binary mixture. Then, a new model depletion mixture was developed for measuring stress-dependent rheological properties: shear thickening and first normal stress difference N1. Using this system, we measured the effects of polymer depletant size and dispersity on the rheology of a shear-thickening suspension. The presence of large polymers enhanced the shear thickening of the suspensions and changed the sign of N1 from negative to positive, compared to the nearly hard-sphere suspension. Finally, we explored the effect of polymer adsorption strength on the surface of the particles on the cluster formation and rheology of a model bridging mixture, based on the same model colloidal suspension as the depletion studies. This bridging mixture is a promising model system for future systematic comparisons of the effects of depletion and bridging attractions. The results of this work confirm the importance of studying the effects of the properties of the polymer additives themselves on the final behavior of model colloids with polymer-mediated attractions, and they suggest that this understanding can be used to tune properties of the resulting suspensions.