  We report experimental and computational observations of dynamic contact
networks for colloidal suspensions undergoing shear thickening. The dense
suspensions are comprised of sterically stabilized poly(methyl methacrylate)
hard sphere colloids that are spherically symmetric and have varied surface
roughness. Confocal rheometry and dissipative particle dynamics simulations
show that the shear thickening strength scales exponentially with the scaled
deficit contact number and the scaled jamming distance. Rough colloids, which
experience additional tangential and rolling constraints, require an average of
1.5 - 2 fewer particle contacts as compared to smooth colloids, in order to
generate the same shear thickening strength. This is because the surface
roughness enhances geometric friction in a way that the rough colloids do not
experience a large change in the free volume near the jamming point. In
contrast, smooth colloids must undergo significant reduction in the free volume
to support an equivalent shear stress. The available free volume for different
colloid roughness is related to the deficiency from the maximum number of
nearest neighbors at jamming under shear. Our results further suggest that the
force per contact is different for particles with different morphologies.
