Structure and dynamics of colloid-polymer solutions

Colloid-polymer solutions are of critical importance in a diverse range of technological and scientific applications. The novel physical behavior of colloid-polymer solutions can be tuned by varying the polymer and colloid concentrations, the size ratio of polymer radius of gyration to colloid radius, and the flexibility of polymer backbone chain. With this perspective, the goal of this thesis is to understand the relationship between interactions, structure, and stability of colloid-polymer solutions, which enables us to control the macroscopic properties, such as the rheology, phase behavior, and processing characteristics of these materials. In this thesis, we explore a number of physical regimes of colloid-polymer solutions, including filled semiflexible polymer solutions and surfactant vesicle suspensions with non-adsorbing polymers.;First, we investigate the interactions and fluid structure of colloidal particles dispersed in semiflexible F-actin solutions using diffusing wave spectroscopy microrheology. We find two distinct regimes of the colloid dynamics as a function of filament length, reflecting the transition from dilute to entangled regimes. In the dilute regime, the colloid dynamics are in good agreement with the theoretical model of rod-like and semiflexible polymer solutions. In the entangled regime, we determine the length scale of structural perturbation near the colloids using the hydrodynamic viscoelastic shell model. We further find the structural perturbation is consistent with a formation of depletion layer near the colloids based on the comparison with the PRISM (polymer reference interaction site model).;Second, we study the microstructure, phase behavior, and stability of surfactant vesicles suspended in non-adsorbing poly(diallyl dimethyl ammonium chloride) (polyDADMAC) solutions. As the polymer concentration increases, density gradients build up, and an interface develops between a highly turbid vesicle-rich phase and a polymer-rich phase. Increasing the polymer concentration further forms a gel, which subsequently collapses. However, unlike monodisperse hard sphere colloidal gels, the depletion-induced vesicle gel collapses faster with increasing polymer concentration. This anomalous collapse behavior is explained based on the evolution of porous microstructure. The microstructure begins to rearrange immediately after the polymer is added. Furthermore, as the polymer concentration increases, large pores are observed, which likely facilitates solvent backflow. The characteristic length scale of the pores is obtained by quantitatively analyzing confocal microscopy images. We find a correlation between the characteristic pore area and the initial permeability using Buscall and White's theory of the sedimentation of gels.;Next, we characterize the collapse behavior of depletion-induced vesicle gels beyond the initial rising behavior, in terms of process and formulation variables. Overall, the collapse occurs smoothly to an equilibrium height, forming a weak gel at the top of the container. The final sample height increases with increasing vesicle volume fraction, while it decreases with increasing polymer concentration, resulting in a denser structure. Using the theory of poroelastic consolidation, we extract the characteristic time scale for a gel collapse. The effects of initial sample height, permeability, and elasticity on the consolidation of the vesicle network are investigated, which is in good agreement with the theory. The ability of the gel to resist the compression is quantified by calculating the compressional modulus which increases as the final vesicle volume fraction increases. We also find the ratio of the compressional to the shear modulus is nearly constant.;Based on the AO depletion model, as the polymer concentration increases, corresponding interparticle attractions are expected to be stronger and slow the dynamics of the evolution of the system. In order to understand the microscopic mechanism responsible for this intriguing difference, we investigate a number of important aspects of the vesicle dispersion, which may play a role in the anomalous collapse behavior of vesicle gels. First, using laser tweezers and atomic force microscopy, we find that the interactions between relatively soft vesicles are consistent with depletion and there is no evidence of other long-range interactions. Second, the polydispersity of vesicle dispersion is quantified as sigma ≈ 55% using light scattering and confocal microscopy. From the sedimentation experiment of monodisperse cationic colloid suspended in polyDADMAC solutions, polydisperse vesicle size distribution appears to be significantly related to the anomalous dependence of depletion force on the collapse behavior. Finally, the structural correlation is obtained by calculating the average intensity fluctuation from confocal microscopy images. The long-range heterogeneity is observed in the vesicle network with polymer concentration, which can be accounted for by immobile bicontinuous networks.