Phase behavior, microstructure and mechanics of colloid-polymer mixtures

In this thesis we present a comprehensive systematic study characterizing the depletion driven changes to the suspension phase behavior, structure and flow properties as a function of the strength and range of interparticle interactions. In particular, we establish links between the suspension microstructure and the mechanical response of gels formed via short-range depletion attractions. First we highlight the failure of extant classical models to capture the observed phase behavior over a wide range of polymer-to-particle size asymmetries, particle volume fraction and solvent quality. Accounting for polymerpolymer interactions and polymer chain conformational degrees of freedom, the Polymer Reference Interaction Site Model (PRISM) is shown to capture all the subtle changes in the phase behavior of colloid-polymer mixtures in different solvents, over the entire region of phase space. Small-angle scattering techniques are used to determine the suspension microstructure and excellent quantitative agreement is demonstrated with PRISM in the equilibrium fluid. As expected, equilibrium theory predictions fail in the gel state accompanying complete structural arrest over all measurable length scales. The suppression (enhancement) of fluctuations on intermediate (mesoscopic) length scales is understood in terms of a heterogeneous structure resulting from the presence of dense compact clusters and the attendant creation of meandering, randomly distributed voids. Finally, a detailed rheological study of depletion flocculated gels and suspensions are presented. A naïve mode coupling theory is applied with the full 2-component PRISM input to predict the transient gelation boundaries and particle bond-bond elasticities. Quantitative agreement with experimental elastic shear moduli are seen when we account for the heterogeneous nature of the gel structure.