Influence of electric field and shear on the rheology of fumed alumina in silicone oil suspensions

We investigate the effect of shear and electric field (E) on the rheology of fumed alumina (Al<inf>2</inf>O<inf>3</inf>) − in − silicone oil suspensions (Al<inf>2</inf>O<inf>3</inf>/silicone oil) at varying Al<inf>2</inf>O<inf>3</inf> nanoparticle loadings of 0.5 wt%–15 wt%. 0.5 wt% and 2.5 wt% Al<inf>2</inf>O<inf>3</inf>/silicone oil suspensions behave as Newtonian fluids and form gel − like suspensions at a nanoparticle loading of 10 wt% and above; attributed to the formation of a particulate network of fumed Al<inf>2</inf>O<inf>3</inf> nanoparticles. Transient step − down experiments with 5 wt% and 15 wt% Al<inf>2</inf>O<inf>3</inf>/silicone oil suspensions indicate shear thinning and thixotropy. We also show that the dynamic rheology of gel − like 15 wt% Al<inf>2</inf>O<inf>3</inf>/silicone oil suspensions is extremely sensitive to the preshearing protocols. Depending on the preshear, partial or complete breakdown of the network structure in 15 wt% Al<inf>2</inf>O<inf>3</inf>/silicone oil suspensions is observed. Once the network structure breaks, the structural recovery doesn't ensue at rest after the cessation of shear. On application of E, Al<inf>2</inf>O<inf>3</inf>/silicone oil suspensions with 0.5 wt%–5 wt% Al<inf>2</inf>O<inf>3</inf> nanoparticle loading exhibit diverging low shear viscosity, frequency independent G′ along with the presence of a yield stress. The electrorheological response of 5 wt% Al<inf>2</inf>O<inf>3</inf>/silicone oil suspensions is independent of the preshearing protocols at an applied E = 1 kV/mm. Furthermore, the shear rheology with applied E does not scale with, instead linear and sub-linear scaling with applied E is observed.