Rheological consequences of microstructural transitions in colloidal crystals

The flow properties of a suspension of 146 nm diameter charge, stabilized particles at a volume fraction of 0.33 are detailed, and links are made with small-angle neutron scattering structure determinations. At rest the suspension is ordered, having hexagonally close-packed planes lying parallel to the rheometer wall that show weak registration in the shear gradient direction. The suspension shows a distinct dynamic yield stress and two regions in which the stress is a decreasing function of shear rate. Between these regions, the suspension sustains steady-state deformation with a time-averaged microstructure consisting of a strained crystal in which the hexagonally close-packed layers remain intact. In the second stress minimum, the suspension microstructure is polycrystalline, while at higher stresses a sliding-layer microstructure is observed. Metastable viscosities (shear rates) occur in constant-stress experiments in the regions near the local stress maxima. Neither shear melting nor shear thickening was observed at shear rates up to 1.1 X 10 4 s-l.