Monodisperse colloidal plates under shear

The structure of a dispersion of monodispersed, plate-shaped colloidal particles has been investigated under shear. The dispersion displays a columnar phase when at rest, and if subjected to shear at low rates (0.1–1 ${\mathrm{s}}^{\mathrm{-}1}$), this structure aligns with the axis of the columns in the flow direction. At low shear rates, the plates within these columns are tilted, with their normals in the compressional quadrant, at 20° to the flow direction in the flow-gradient plane. At high shear rates (∼100 ${\mathrm{s}}^{\mathrm{-}1}$), the dispersion forms a different structure that consists of layers of particles with their plate normals in the gradient direction. The transition between these two shear-induced “phases” is described. Evidence is presented that suggests that at intermediate shear rates there is coexistence between the two phases, implying that there is a shear-induced “phase separation.” As the shear rate is further increased evidence for shear-induced disorder is found. All the shear-induced structures that have been observed relax back to the equilibrium columnar phase over a period of a few hours. At rest after shear at low rates (0.1–1 ${\mathrm{s}}^{\mathrm{-}1}$), the amount of orientational order present in the aligned columnar phase increases, while there is no measurable positional rearrangement. After shear at high rates (67–1000 ${\mathrm{s}}^{\mathrm{-}1}$), the layer phase relaxes into a columnar phase. The structure changes via an intermediate state consisting of planes of particles normal to the vorticity direction. The positional rearrangement occurs at the expense of the orientational order, which increases again after the positional rearrangement is complete. The final orientation of the columnar phase is such that the direction of alignment of the plates does not change upon relaxation.