An AFM investigation of the mechanism of secondary nucleation induced by contact

Although secondary nucleation is the main source of new crystal nuclei in an industrial crystallizer, the precise source of contact nuclei has not yet been determined. To address the various theories presented in the literature, we have adopted the dual approach of using microscopy to characterize, at their earliest stages of growth, the secondary nuclei that are produced by contact and also the surface topography of the primary crystals before and after contact. AFM studies of a range of crystals, including isomorphous alums, combined with scanning and transmission electron microscopy experiments indicate that, at least for the case of very gentle contact, transfer of crystallographic information can occur through an ordered solute layer in a supersaturated solution without the need for the transfer of solid materials from the primary crystal by microattrition. Knowledge about this mechanism has the potential to improve the quality of product from industrial crystallizers, through control of crystal morphology and size distribution.