Crystallization controlled by the geometry of a surface.

Crystallization is of great importance in fields as diverse as atmospheric chemistry and the pharmaceutical industry. It starts with nucleation, the formation of a microscopic crystal. Here we study, via computer simulation, the heterogeneous nucleation of a crystal in wedge-shaped grooves in a surface. We find that nucleation in these grooves is many orders of magnitude faster than on a flat surface. We also find that there is an optimum wedge angle at which nucleation is fastest. Our observation that nucleation in grooves dominates that on flat surfaces not only explains the effectiveness of scratching a surface to induce crystallization but also allows us to see the microscopic behavior that underlies this common technique. We also show how it may also be possible to control the crystal form, the polymorph, by controlling the groove geometry to favor nucleation of the desired polymorph.