Micro-rheological investigations of colloidal systems

In this thesis, the colloidal glass transition and glassy dynamics were investigated using a passive micro-rheological approach, diffusing wave spectroscopy (DWS). As a model soft colloidal system, the structural recovery, including the equilibrium and non-equilibrium dynamics, of a thermosensitive polystyrene (PS)-poly(N-isopropylacrylamide) (PNIPAM) core-shell colloidal latex was studied by a novel concentration-jump protocol (induced by changing temperature) (CHAPTER II). From the non-equilibrium dynamics, we found that the DWS-based micro-rheology observes the system differently from macro-rheology. Based on this finding, the length-scale-dependence of the DWS micro-rheology was studied to further explore the non-equilibrium dynamics and to address the range of utility of DWS as a micro-rheological method (CHAPTER III). Furthermore, to compare with the idea hard-sphere colloidal glass transition phenomena, the softness effect on equilibrium dynamics was investigated for a series of PS-PNIPAM soft colloids with different PNIPAM corona stiffness. A successful two-stage soft-hard equilibrium dynamics mapping revealed the nature of soft colloidal interactions (CHAPTER IV). Beyond micro-rheological investigations on soft colloidal systems, the DWS-based micro-rheological methods for measuring local viscoelastic properties were compared and validated. The generalized Stokes-Einstein-based micro-rheological method was analytically improved for a more accurate application (CHAPTER V).