Rheological Behavior of Colloidal Silica Dispersion: Irreversible Aging and Thixotropy

In this work, we study the rheological behavior of a colloidal dispersion of charge-screened nanoparticles of silica suspended in aqueous media that exhibits soft solid-like consistency. We observe that the system shows various characteristics of physical aging wherein it undergoes time evolution of rheological properties such as elastic modulus, relaxation time, and yield stress subsequent to shear melting of the same. Notably, the relaxation time increases more strongly than linearly with time, which is suggestive of hyperaging dynamics. When considered along with the time-dependent yield stress, this behavior indicates the steady-state shear stress-shear rate flow curve to be nonmonotonic with a negative slope in a lower shear rate region. Performing shear melting on this system at a later date since the preparation of the dispersion (rest time) results in higher viscosity as well as yield stress, and the corresponding evolution of the elastic modulus shifts to lower times. This implies that physical aging in the studied silica dispersion, while reversible over short time scales (on the order of hours), becomes irreversible over longer durations (days) because of the inability of strong shear to break interparticle bonds that have strengthened over long durations. We also develop a thixotropic structural kinetic model within a time-dependent Maxwell framework that captures the experimentally observed rheological behavior well.