  Dilute oil dispersions of fractal carbon black particles with attractive Van
der Waals interactions display continuous shear thickening followed by shear
thinning at high shear rates. The shear thickening transition occurs at
$\dot\gamma_{c}\approx 10^{2}-10^{3}s^{-1}$ and is driven by hydrodynamic
breakup of clusters. Pre-shearing dispersions at shear rates
$\dot\gamma>\dot\gamma_{c}$ produces enhanced-modulus gels where $G' \sim
\sigma_{pre-shear}^{1.5-2}$ and is directly proportional to the residual stress
in the gel measured at a fixed sample age. The observed data can be accounted
for using a simple scaling model for the breakup of fractal clusters under
shear stress.
