  A theory is presented for the onset of shear thickening in colloidal
suspensions of particles, stabilized by an electrostatic repulsion. Based on an
activation model a critical shear stress can be derived for the onset of shear
thickening in dense suspensions for a constant potential and a constant charge
approach of the spheres. Unlike previous models the total interaction potential
is taken into account (sum of attraction and repulsion). The critical shear
stress is related to the maximum of the total interaction potential scaled by
the free volume per particle. A comparison with experimental investigations
shows the applicability of the theory.
