  Dynamic particle-scale numerical simulations are used to show that the shear
thickening observed in dense colloidal, or Brownian, suspensions is of a
similar nature to that observed in non-colloidal suspensions, i.e., a
stress-induced transition from a flow of lubricated near-contacting particles
to a flow of a frictionally contacting network of particles. Abrupt (or
discontinuous) shear thickening is found to be a geometric rather than
hydrodynamic phenomenon; it stems from the strong sensitivity of the jamming
volume fraction to the nature of contact forces between suspended particles.
The thickening obtained in a colloidal suspension of purely hard frictional
spheres is qualitatively similar to experimental observations. However, the
agreement cannot be made quantitative with only hydrodynamics, frictional
contacts and Brownian forces. Therefore the role of a short-range repulsive
potential mimicking the stabilization of actual suspensions on the thickening
is studied. The effects of Brownian and repulsive forces on the onset stress
can be combined in an additive manner. The simulations including Brownian and
stabilizing forces show excellent agreement with experimental data for the
viscosity $\eta$ and the second normal stress difference $N_2$.
