On topography and crystal nucleation

The influence of topography on crystal nucleation is investigated, mostly with regards to nucleation from vapour but also regarding nucleation from the melt and from solution. Two mechanisms are discussed for acute features to promote nucleation: a thermodynamic reduction in the free energy barrier provided by a favourable geometry, and the formation of a confined condensate of a metastable phase which then transitions to the crystalline phase. Organic compounds nucleating from vapour are used to demonstrate the efficacy of scratches and mineral steps as preferable nucleation sites. A study of various compounds on mica demonstrates that highly acute features provide the most favourable nucleation sites. High-magnification study of the growth of crystals from such features reveals the presence of small condensates prior to the appearance of bulk crystals, and the growth of these is studied quantitatively. Ice is shown to have a similar nucleation site preference to the organic compounds, although no condensate was seen prior to bulk nucleation. Well-defined 100 nm-wide trenches were milled into silicon substrates and shown to have a great ability to direct ice nucleation. The freezing of picolitre droplets of water was studied on smooth and roughened silicon, glass and mica substrates, and the roughening was shown to have little to no effect. Calcium carbonate growth and its dependence on topography was investigated, and found to be broadly anomalous and greatly dependent on surface chemistry