Stability and rheological properties of hydrophobic fumed silica suspensions in mineral oil

Hydrophobic fumed silica powders were suspended in mineral oil with varying silica volume fraction ϕ. These suspensions were classified into sol, pre-gel, and gel states with increasing ϕ. The hydrophobic fumed silica suspension caused gelation at a higher ϕ than the hydrophilic fumed silica suspension because of the lower surface silanol density of the former suspension compared to the latter suspension. Shear stresses and dynamic moduli of the hydrophobic fumed silica suspensions were measured in terms of ϕ. Plots of the steady-state shear stress (τ) against shear rate () of the hydrophobic fumed silica suspensions in a sol state below and above = 10 s−1 followed a power law, , where n is an exponent. The n value for < 10 s−1 is larger than that for > 10 s−1, irrespective of ϕ. Dynamic moduli in the pre-gel and gel states were dependent on ϕ. At a fixed ϕ, the storage modulus G′ in the linear region was larger than the loss modulus G″; above the linear region, the suspensions showed strong strain overshoot.