Shear-induced microstructural changes in flocculated suspensions of fumed silica

Suspensions of fumed silica exhibit a wide range of rheological properties, depending on the type of microstructure present. At high silica concentrations, the rheological behavior is ‘‘gel-like,’’ due to the formation of a network consisting of interconnected silica flocs. When large amplitude oscillatory preshear is applied on these systems, the network linkages are disrupted, resulting in the formation of isolated flocs. In this study, we focus on the extent to which the network is restored on cessation of preshear. By applying small amplitude oscillations we can study the development of the elastic modulus (G′) with time, following disruptive shear. We find that the restoration of the network after preshear is instantaneous; however, G′ recovers to different levels depending on the amplitude of the imposed preshear strain. Contrary to expectations, larger preshear strains (which cause a greater degree of microstructural disruption) do not always lead to lower levels of recovered G′. For strains greater than a critical value, the recovered G′ progressively increases with increasing preshear strain. This anomalous behavior of the elastic modulus is explained in terms of microstructural rearrangements that may occur during oscillatory preshear.