Weakly Interacting Colloid Polymer Mixtures

This thesis explores the physics of weakly interacting colloid-polymer mixtures. Colloid-polymer mixtures can consist of two kinds of interactions; depletion and adsorption. Both interactions can lead to colloidal aggregation and formation of space spanning gels that can bear stress. Depending on the range of the attractive interaction, depletion gels bear stress by two mechanisms; bond-stretching and bondbending. Gels formed with long-range interactions bear stress by bond-stretching while gels formed with short-range interactions bear stress by bond-bending. Rheological measurements of the elastic plateau modulus G ′ p and confocal microscopy measurements of the response of the gel to thermal fluctuations support this hypothesis. The phase behavior of the depletion aggregates is also profoundly influenced by the range of the interaction potential. A simple kinetic model that compares time scales of aggregation and bond break-up quantitatively explains the phase behavior for different ranges of the interaction potential. For the case of adsorption, the equilibrium phase of the colloid polymer mixture at rest is that of a Newtonian fluid. However, for certain ratios of colloid and polymer, the system can form a gel on application of shear. The gels that are formed are transient in nature, and revert back to the fluid state over a time scale of minutes to days. We find that the colloid polymer mixtures form ”shake-gels” when the surface