Topographical Control of Crystal Nucleation

Surface topography is here investigated as a route to controlling crystal nucleation. The nucleation from vapor of crystals of neo-pentanol and tetrabromomethane was studied on flat surfaces of glass and mica, and on identical substrates scratched with diamond powders of varying particle size. The result is a study which presents a systematic comparison of the nucleating ability of surfaces with the same chemistry and wettability, but varying surface topography. An increase in nucleation density of up to an order of magnitude was observed on scratched mica surfaces compared to unscratched ones, and there was a decrease in the induction time by up to 60%. Larger diamond particles led to enhanced effects, particularly on the nucleation density. Although the nucleation density and induction time on unscratched glass were similar to those on mica, the surface scratches on glass had no significant effect on nucleation density or induction time. The results suggest that a high density of nanoscale features of the surface topography, such as those produced as the diamond particles fracture the mica, is necessary for an enhancement of nucleation. The apparent length scale of the topographical features on mica is discussed with reference to classical nucleation theory and other models. These results show that both a quantitative reduction in induction time and an increase in nucleation density can be achieved as a result of mechanically produced topographical surface defects, which suggests that the engineering of nanoscale topographical features has real potential for control of heterogeneous nucleation.