Leading research by Dr Christenson gives insight into crystal formation

Research by Dr Hugo Christenson, School of Physics and Astronomy, gives in-depth insight into why crystals first form in topographical defects, like cracks and crevices.

How topography affects crystal formation has been the topic of extensive scientific research. It has long been known that crystal nucleation - the first process in the formation of a crystal - almost always occurs on a surface. Ice crystals in cirrus clouds in the upper atmosphere will form on a small aerosol particle rather than in the air itself - but the physical and chemical properties of surfaces that are conducive to crystal nucleation are still not clearly understood.

Dr Christenson, School of Physics and Astronomy, has conducted recent research that seeks a clearer picture of the topographical properties that promote nucleation, which is the focus of his research.

Dr Christenson has studied various aspects of phase change dynamics to investigate what variables affect crystal nucleation. Drawing on the Classical Nucleation Theory (CNT) - which describes how physical attributes, such as defects or crevices on a surface, foster nucleation - Dr Christenson, as a result of his extensive research, has devised several experiments to study precisely the effect of these physical features.

His research associate Dr James Campbell has contributed significantly to the design of the experiments and carried out most of the experimental work.

Demonstrating the dependency of nucleation

Using three different experimental approaches, Dr Christenson has demonstrated the dependency of nucleation on surface defects. In agreement with CNT expectations, the cracks on a surface reduce the height of the energy barrier to nucleation. Put simply, these cracks allow molecules to congregate for longer than they would be able to on a flat surface, increasing the likelihood that they will nucleate.

Dr Christenson’s research shows that mechanically produced surface defects can increase both the rate and density of crystal formation. His research has now extended to include crystal nucleation from solutions and pure liquids with results that correspond to his work on vapours - which has increased our basic understanding of how crystals form.

Importantly, it also suggests that the engineering of nanoscale topographical features has real potential for control of heterogeneous nucleation, although this remains a significant challenge. Careful implementation of crystallisation techniques would allow for applications in industry (for example, chemical vapour deposition) and medical diagnostics (understanding how kidney stones form) as well as others.

Increasing the rate and density of crystal formation

The results from Dr Christenson’s research shows that mechanically produced surface defects can increase both the rate and density of crystal formation. The three approaches Dr Christenson untertook are as follows:

  • Controlling the surface defects: Dr Christenson designed a test to investigate the idea that defects on a surface could help control crystal nucleation. For this experiment, the nucleation from vapour of two organic substances (neo-pentanol and carbon tetrabromide) was studied on flat surfaces of glass and mica using optical microscopy, as well as on identical surfaces that had been scratched with diamond powders. The results from this experiment clearly demonstrated that nucleation increased on the scratched surfaces. In this experiment, induction time - measured from first exposure to the vapour to the first appearance of a crystal visible in the microscope - was also analysed. 

  • Naturally present defects: In an alternative experiment to investigate the same problem, Dr Christenson and his team studied nucleation of four different crystals (carbon tetrabromide, camphor, norbornane, and hexachloroethane) from vapour on mica. Instead of manufacturing surface features, naturally present defects on the mica surface were studied. All four compounds were allowed to nucleate repeatedly on a number of mica substrates. All four were seen to nucleate preferentially on the same types of sites which were typically characterised by an acute wedge geometry. 

  • Deep pockets: In a third experiment, the formation of ice and organic crystals were observed in mica. Well-defined topographical features with sharp acute wedges, referred to as 'pockets', which naturally occur when Muscovite mica is cleaved, were studied. The results of this experiment showed that crystals were more likely to form inside and from the corners of the pockets. Although the defect features prepared on the mica surface for this test were in the order of micrometres in size it can be inferred that the nucleation would be attracted to the acute crevices even in naturally occurring or manufactured nano-scale sites.

About Dr Christenson

Dr Christenson's background in surface science has inspired work on crystal nucleation from vapour, on the influence of topography on nucleation, and crystallisation of biominerals via amorphous precursors. Further work concerns surface patterning with crystals during dewetting, and nucleation and crystallisation studies of organic compounds from vapour and the melt.

Dr Christenson’s collaborative research has shown how amorphous calcium carbonate (ACC), the precursor phase to the biominerals calcite and aragonite, may be stabilised by confinement alone, without the need for specific interactions with biomolecules. In the presence of polymers like polyaspartic acid ACC will enter submicron pores via capillary action. When the ACC eventually crystallises to calcite, single-crystal nanowires of aspect ratios as high as 100 are produced.

Further information

For more information about Dr Christenson's research into how topography affects crystal formation, view Research Features' spotlight on Dr Christenson's work.

For further details about Dr Christenson's research, view his staff profile.

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