Shear-induced crystallisation in binary colloidal suspensions investigated using confocal microscopy

Abstract

Hard sphere colloids crystallise at large volume fractions. However, crystallisation is suppressed in the presence of large polydispersities. We investigate the effect of polydispersity on shear-induced crystallisation using binary suspensions of hard spheres. Depending on the size ratio, the samples fully crystallise, partially crystallise or remain amorphous. Using confocal microscopy, the structural changes are characterised on a single particle level. This allows us to determine the local bond order parameter, the number of nearest neighbours, the Voronoi volume, the local volume fraction and other quantities. Upon the application of shear, only minor changes of these quantities are detected in regions that remain amorphous whereas noticeable changes are observed in regions where shear-induced crystallisation occurs. These changes mainly reduce the effects of the different particle sizes on the particle arrangement. This allows substitutionally disordered crystals to form. However, with increasing size disparity the volume fraction in substitutionally disordered crystals decreases while the volume fraction in amorphous regions slightly increases. Beyond a size ratio of 1.18, the volume fraction of the crystals is smaller than the volume fraction of the amorphous regions. At the same size ratio shear-induced crystallisation ceases. The terminal size ratio of 1.18 is close to the value given by the empirical Hume-Rothery rule for metallic alloys and the corresponding polydispersity (about 0.1) is consistent with the polydispersity beyond which crystallisation is found to be suppressed.