Microrheology and macrorheology to probe aging of a colloidal glass after volume fraction jumps and shear-melting perturbations

In a complex glass-forming liquid, when the relaxation time approaches the range of 1-1,000 s, the corresponding range of the temperature or volume fraction is defined as the glass transition region. Physical aging and structural recovery behaviors of molecular glasses have been widely studied through temperature jumps and in a typical experiment the temperature of a material system is jumped from an equilibrium state to an out of equilibrium state. There are three aging signatures in molecular glasses studied and cataloged by Kovacs in the 1960s, i.e., intrinsic isotherm, memory (crossover) effect, and asymmetry of approach. Colloidal systems are thought to be good models to study the phase and dynamic behaviors of molecular systems due to time scale and size range. However, different from the molecular systems, for colloidal dispersions, the key parameter controlling the phase and dynamic behaviors is the effective volume fraction (φeff) and not the temperature. As the effective volume fraction of an ideal monodisperse hard-sphere system approaches 0.58, the colloidal particles are surrounded by their neighbors, thus forming cages, and the colloidal system displays structural arrest and falls into an out of equilibrium state, defined as a colloidal glass. Aging behaviors of colloidal dispersions, e.g., gels and glasses, have generally only been investigated after shear-melting perturbations at a fixed volume fraction or mass concentration, unlike the situation in molecular glasses where temperature jump perturbations have been widely used in the study of aging dynamics. There is limited work using volume fraction jumps to study the aging behaviors of colloidal glasses, similar to the temperature jumps in molecular systems. Poly (N-isopropylacrylamide) (PNIPAM), as a thermoresponsive polymeric colloidal particle, undergoes a size or volume change as a function of external temperature in water. This thermosensitive behavior provides a potential mean to examine structural recovery similar to the Kovacs experiments in molecular glasses, e.g., temperature jumps, to study the aging dynamics of a colloidal glass after a temperature induced volume fraction jumps. In the present thesis, core-shell thermoresponsive polymeric colloidal particles were synthesized and characterized. The aging dynamics of colloidal glasses made with these PS-PNIPAM-AA colloidal particles were investigated after shear-melting perturbations and volume fraction jumps are investigated. The aging behavior after the two types of perturbation were compared and similarities and differences were found. This thermoresponsive colloidal dispersion was also used to study the three Kovacs aging signatures in volume fraction jump histories and the results were compared with those seen in molecular glasses subjected to temperature jump histories. In addition, the aging results obtained here for the colloidal glasses using conventional macrorheology were compared with the results obtained from microrheology (multi-speckle diffusing wave spectroscopy, DWS). Similarities and differences were also found and discussed.