Images of shear-induced phase separation in a dispersion of hard nanoscale discs

Optical microscopy has been used to study the flow behaviour of a dispersion of plate-like particles. The sample of hexagonal discs of nickel hydroxide stabilised with a short-chain, charged polymer was investigated over a range of shear rates up to 100 s(-1) with a polarising, transmitted light microscope. Direct evidence for a shear-induced phase transition with a change in alignment of the particles is seen. At low shear rates (below 1 s(-1)), the discs aligned with normals in the flow direction while at high shear rates (above 18 s(-1)) the discs were aligned with their normals in the gradient direction. Between these two shear rates a striped texture consisting of both the low and the high shear rate structures was observed. These results are compared with previous studies using theological measurements, scattering and diffraction. (C) 2001 Elsevier Science Ltd. All rights reserved.