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Fundamental Nucleation Studies

Heterogeneous nucleation
According to classical nucleation theory (CNT) heterogeneous nucleation rates are always higher than those of nucleation in bulk, and the barrier to nucleation vanishes when the condensing or depositing substance wets the substrate completely (has a zero contact angle). CNT predicts that nucleation rates are further increased in topographical defects such as pits and grooves.

Nucleation density (left) of neo-pentanol crystals depositing from vapour on unscratched mica and (right) mica scratched with 10 nm diamond powder, both at a relative saturation S = 1.5 (± 0.1).

These predictions are borne out by scattered observations in the literature, and may be shown by simple experiments such as the ones illustrated above and below.

Research in this area aims to increase our fundamental knowledge of nucleation events, and to enable the use of topographical features to control factors such as nucleation rates and polymorph selectivity. Such capabilities would be of great value to practical applications involving, e.g., advanced materials, pharmaceuticals and thin-film devices.

Ice crystals depositing at – 60 °C in 100 nm × 10 nm deep grooves milled 20 mm apart on a Si wafer.

Growth of camphor crystals from vapour at about S = 1.7 along a step edge on a muscovite mica surface. Note the morphology of the hexagonal camphor crystals.

It is now clear that many systems do not behave as postulated by CNT, e.g. nucleation of protein crystals from solution often occurs from denser, metastable clusters, and related effects have been found for smaller solute molecules. Similar ideas have emerged from computer simulations of crystals nucleating from solution, and simulations have also suggested that crystals may form from vapour with liquid droplets as intermediates.

This type of behaviour, which leads to greatly enhanced rates of homogeneous nucleation, has been described as two-step mechanisms of nucleation. A related phenomenon, first suggested 50 years ago for atmospheric ice nucleation on aerosols, involves the condensation of supercooled liquid in surface pits and grooves followed by crystal nucleation in the undercooled liquid.

The schematic on the right shows an example of two-step nucleation: Crystal deposition from vapour is accelerated by nucleation and crystal growth from a condensate of supercooled liquid around the contact point of a small glass bead pressed against a microscope slide.

A thermodynamically analogous mechanism may operate during precipitation of amorphous precursor phases on surfaces, and two-step mechanisms are likely to be common in heterogeneous nucleation. A better appreciation of the detailed mechanisms can only lead to increasing practical significance of these routes to crystal nucleation and growth.

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