Heterogeneous nucleation of colloidal melts under the influence of shearing fields

Large, oriented single crystals may be obtained from shear melts of colloidal particles after nucleation at the container walls. We are here interested in the processes occurring during the initial phase of their formation. Using different microscopic and scattering techniques we here studied highly charged suspensions of spherical particles, dispersed in low salt or deionized 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 induced precursors of heterogeneous nuclei consist of wall based, oriented, registered or freely sliding layers with in plane hexagonal symmetry. Cessation of shear initiates a complex heterogeneous nucleation process. If the layer structures are space filling, they register to form a meta-stable randomly stacked close packed hexagonal crystal. In double wall confinement the transformation to the equilibrium body centred cubic structure occurs on long timescales via nucleation and subsequent lateral growth. For non-space filling, wall based layer structures we find indications of competition between the decay of the layers in favour of the shear melt and their stabilization through registering and subsequent coverage by an epitaxially growing wall crystal. From quantitative growth curve measurements we obtain the initial wall crystal thickness d(0), which may serve as a lower bound to the extension of the layer structures under shear. We observe a pronounced dependence of d(0) on both former shear conditions and meta-stability of the melt.