(51f) Structural Dynamics and Kinetics of Rheological Aging of a Model Thermoreversible Colloidal Gel Following a Thermal Quench

Colloidal gels are an important component of industrial and consumer products ranging from personal care, food and pharmaceuticals to cement, asphalt and mining tailings. Based on the ubiquitous nature of the colloidal gels, it is of fundamental interest to understand the molecular mechanisms underlying the macroscopic changes in the colloidal systems. The rheology of colloidal gels typically exhibits aging in the form of a time dependency resulting from the internal dynamic evolution of the system. This is not surprising as the arrest of the colloidal gel in out-of-equilibrium state leads to a complex microstructural evolution whereby the free energy decreases as a function of time and the degree of quenching (e.g., via concentration or temperature). In particular, systems quenched below the spinodal undergo simultaneous phase separation and vitrification. Consequently, the process and mechanism of aging can be challenging to understand, which limits the prediction of the macroscopic behavior in colloidal systems. Therefore, careful studies on model systems can provide critically important guidance for understanding the behavior of more complex, real-world formulations.

In this work, we present new results for the characterization of aging in a well-studied model system of adhesive hard spheres (AHS) quenched below the spinodal. The system is comprised of moderately concentrated suspension of octadecyl coated silica in n-tetradecane undergoing thermoreversible gelation. A combined light scattering and rheology study is carried out to capture the evolution of microstructure upon thermal quench in the model thermoreversible system. For deeper quenches, a unique and new phenomenon is reported – namely after an initial rise in the modulus, a reproducible drop in the modulus is observed, followed by a plateau in the modulus value. This drop can be gradual or sudden depending on the quench depth. The anomalous change in dynamic moduli upon deep thermal quench is consistent with the AHS dispersion microphase separating into colloid-rich and colloid-lean phase. This transition at a fixed quench depth is highly sensitive to non-equilibrium parameters including the rate of thermal quench and extent of rejuvenation. We study the kinetics of transition from homogeneous gel to heterogeneous gel upon deep thermal quench over a wide range of temperature ramp rates used to achieve the target quenched state. Furthermore, we investigate the effect of moderate and large amplitudes on the kinetics of transition to spatially heterogeneous structure by spinodal decomposition. Our rheological characterization at different amplitudes, temperature ramp rates and preshear history shows intriguing effects of non-equilibrium parameters on the kinetics of aging behavior of colloidal gels. Furthermore, in this research we aim to establish a relationship between the mechanical aging, as measured by rheology, the microstructure, as measured by scattering study and various quench protocols in these gels. This study of phenomenon of aging in model colloidal gel system is of both fundamental importance and industrial relevance in determining the shelf-life of products based on colloidal gels.