Characterization of Preferred Crystal Nucleation Sites on Mica Surfaces

Rates of heterogeneous nucleation can be greatly increased not only through control of the chemistry of a surface, but also of its topography. Following previous work in which we showed that scratching a mica surface significantly enhances crystal nucleation from vapor, we here use a new experimental approach to understand better the effect of topography on crystal nucleation. The compounds carbon tetrabromide, camphor, norbornane, and hexachloroethane were deposited from vapor onto mica sheets containing various surface defects, and their nucleation was studied using optical microscopy. Following subsequent evaporation of the crystals, examination of the sites where these had nucleated with a scanning electron microscope enabled the nature of each material’s preferred nucleation sites to be determined, and all four compounds appeared to exhibit a strong preference for sites characterized by delamination of the layered mica structure. Indeed, comparison of the four compounds on the same mica substrates showed that they all favored the same nucleation sites. These observations are attributed to the presence of an acute wedge geometry at the delamination lines, which can provide a thermodynamic reduction in the free energy barrier to nucleation directly from vapor; alternatively, the results are also consistent with a two-step nucleation mechanism via a liquid capillary condensate.