Shear thickening of dilute suspensions of fractal silica aggregates

The increase in viscosity under shear known as shear thickening (ST) is an inherent property of a wide variety of dense particulate suspensions. Recent studies indicate that ST systems formed by fractal particles are promising candidates for various practical applications. However, ST in fractal systems remains poorly explored. Here we experimentally study the ST behavior in suspensions of hydrophilic fumed silica (FS) particles in glycerol. Remarkably, unlike non-fractal systems, we observe a strong dependence of the onset stress for ST on the volume fraction of fractal objects and a reversible weakening of the ST response that depends strongly on the particle volume fraction as well as the properties of the FS system. Using in-situ boundary imaging, we map out the spatio-temporal flow properties during ST for different FS systems. We find that the fractal nature and structural properties like the internal branching of the particles can qualitatively explain the complex ST phase diagram of these systems.