Heterogeneous nucleation and crystal growth on curved surfaces observed by real-space imaging

We present a real-space imaging study of homogeneous and heterogeneous crystal nucleation and growth in colloidal suspensions of slightly charged and polydisperse particles. Heterogeneous crystallization is observed close to curved surfaces with radii of curvature, R, in the range from 4 to 40 particle diameters, d. Close to a curved surface, we find crystal nucleation and growth to be suppressed for R less than or similar to 10d. For R greater than or similar to 15d, fast crystal growth is observed similar to that on a flat wall (R = infinity). We use the purely topological method of shortest path rings to determine the orientation of the crystal on the length scale of the nearest neighbor distance. Crystal nuclei forming close to a curved surface are oriented analogous to crystal growth on a flat wall with hexagonal planes parallel to the wall. While the smallest nuclei appear to be unaffected by the surface, larger nuclei are found to be suppressed for radii of curvature R less than or similar to 10d. The critical nucleus size in the vicinity of a curved surface is found to be about the same as for homogeneous nucleation.