Images of shear-induced phase separation in a dispersion of hard nanoscalediscs

Optical microscopy has been used to study the flow behaviour of a dispersio n of plate-like particles. The sample of hexagonal discs of nickel hydroxid e stabilised with a short-chain, charged polymer was investigated over a ra nge of shear rates up to 100 s(-1) with a polarising, transmitted light mic roscope. 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 r ates (above 18 s(-1)) the discs were aligned with their normals in the grad ient direction. Between these two shear rates a striped texture consisting of both the low and the high shear rate structures was observed. These resu lts are compared with previous studies using theological measurements, scat tering and diffraction. (C) 2001 Elsevier Science Ltd. All rights reserved.