Microstructure, Microscale Dynamics and Rheology of Thermoreversible Colloidal Gels

Colloidal suspensions transform between fluid and disordered solid states when parameters such as the colloid volume fraction and the strength and nature of the colloidal interactions are varied. Seemingly subtle changes in the characteristics of the colloids can markedly alter the mechanical rigidity and flow behavior of these soft composite materials. In addition to that, careful control of external processing factors such as stress, temperature, pH, impurity etc. also allows us to access a wide range of microstructures and macroscopic behavior. This sensitivity creates both a scientific challenge and an opportunity for designing suspensions with specific functionalities. In this thesis, we investigate the role of colloidal microstructure and dynamics in shaping the evolution of mechanical behavior in a model colloidal gel over a wide phase space. Taking advantage of newly developed x-ray scattering capabilities and the ability to tune precisely the strength of the particle attractions, we track the evolution in the microscopic organization and mobility of the particles and correlate them with the time-dependent macroscopic mechanical behavior of the suspensions. We find that the rate of gel formation is surprisingly sensitive to the strength of attraction; however, the suspensions proceed through identical intermediate states of microscopic and macroscopic behavior even as the time needed to form a gel varies by orders of magnitude. We propose a model of gel formation in the regime of weak attraction in which network formation is a hierarchical process whose initiation depends on the creation and stability of small clusters in which the particles arrange in particularly favorable configurations. We also describe a non – dimensional variable that suitably captures the similarity in the evolution of the microscopic and macroscopic behavior. Next, the effect of shear in fluidizing these gels and subsequent recovery of structure, dynamics and elastic moduli is also studied using X-ray scattering. It is clearly demonstrated that in these gels that it is the localization length that controls the moduli and its evolution. Finally, we study the scaling relationships which collapse the variation of the elastic moduli, initial rate of growth and yield stress onto universal master plots when scaled with suitable parameters that capture the distance from the gel boundary for unimodal and bimodal gels.