Heterogeneous nucleation of colloidal melts under the influence of shearing fields

Abstract

Large, oriented single crystals may be obtained from shear melts of colloidal particles afternucleation at the container walls. We are here interested in the processes occurring duringthe initial phase of their formation. Using different microscopic and scattering techniqueswe here studied highly charged suspensions of spherical particles, dispersed in low salt ordeionized water, in single and double wall confinement, during and after cessation of shear.While the equilibrium phase of our colloidal solids is body centred cubic, the shear inducedprecursors of heterogeneous nuclei consist of wall based, oriented, registered or freely slidinglayers with in plane hexagonal symmetry. Cessation of shear initiates a complexheterogeneous nucleation process. If the layer structures are space filling, they register toform a meta-stable randomly stacked close packed hexagonal crystal. In double wallconfinement the transformation to the equilibrium body centred cubic structure occurson long timescales via nucleation and subsequent lateral growth. For non-spacefilling, wall based layer structures we find indications of competition between thedecay of the layers in favour of the shear melt and their stabilization throughregistering and subsequent coverage by an epitaxially growing wall crystal. Fromquantitative growth curve measurements we obtain the initial wall crystal thicknessd0, which may serve as a lower bound to the extension of the layerstructures under shear. We observe a pronounced dependence ofd0 on both former shear conditions and meta-stability of the melt.