Control of Crystal Nucleation on Surfaces

Crystallisation is a widespread phenomenon in nature and technology. Indeed, snowflakes, gemstones, table salt, boiler scale, metals and many advanced materials used for example in nanotechnology or advanced batteries are all crystalline. The ability to control when, where and how crystals form is therefore essential to areas as diverse as climate modelling, pharmaceutical formulation, the semiconductor industry and the design of dental and medical prostheses. At present, our ability to direct and control crystallisation is often very poor, so any improvement would be of far-reaching significance.

The majority of strategies used to control crystallization from solution relies on the use of soluble additives or on changing the reaction conditions. Here, we propose to develop a different and potentially quite general technique. Rather than varying the solution chemistry - which is specific to each different crystal - we will use the topography of the surfaces on which crystals grow to control their formation.

The formation of crystals requires that a substantial energy barrier be surmounted and is hence usually a rare event - liquids such as water may undergo sizeable supercooling before they freeze. The energy barrier is reduced, and crystallization rendered easier if it occurs on a surface rather than in bulk - during a frosty night ice crystals form on surfaces but not in the air away from any objects. Theoretical considerations show that crystallization will become even more favourable if there are pits or grooves in a surface, and there are many observations that attest to the validity of these ideas. There have, however, been very few systematic studies of the effect of topography on crystallization. Consequently, we have almost no understanding of the type of defects that best favour crystallisation, and to what extent they may depend on the nature of the surface and the nature of the crystallising substance. Part of the problem is that the surface defects which are likely to have a large effect are small - of the order of 5 nanometres or less, which is the typical size of a crystal nucleus. Clearly, features on such a length scale cannot be reproducibly created by methods such as scratching or abrading a surface.

Thanks to recent advances in surface engineering techniques we are now in a position to design and manufacture surfaces that contain defects with well-defined geometries and also with dimensions small enough to enable a detailed assessment of their effect on crystallisation. Indeed, our proposal is supported by preliminary data showing that this strategy works! We have succeeded in showing that surfaces in which nanoscale grooves have been deliberately cut nucleate crystals almost exclusively in the grooves, with negligible growth on the flat areas in between.

We now propose to study nucleation of crystals from vapour and from solution using surfaces with a range of different nanoscale surface features such as pits, grooves and trenches with undercuts. We will relate the density and rate of crystallisation of different substances to the type of feature and try to use these findings to establish criteria for the design of good nucleants, or crystallisation promoters. The breadth of the experiments will allow us to establish the extent to which optimisation of nucleation by surface features differs between vapour, liquid and solution and the differences between systems of simple organic molecules, water, and inorganic salts (electrolytes).

This project will therefore ultimately provide us with a novel and potentially quite general approach to control crystallization in a huge number of applications, ranging from the engineering of thin films for solar cell devices to the promotion of natural bone growth on implant surfaces, or to the prevention of crystallization in kettles, oil wells or in building materials (prevention of weathering).

This project centres on the topic of crystal nucleation control, which is of great significance to UK industry and society. Crystallization is fundamental to a vast array of technological processes and natural phenomena as diverse and significant as the production of pharmaceuticals, foodstuffs and personal care products, the synthesis of nanomaterials, the formation of biomaterials and biominerals such as bones and teeth, the precipitation of ice in the atmosphere and the prevention of scale. Within these areas it is essential to be able to (1) produce crystals of defined sizes, shapes and polymorphs, (2) to control the location of crystals on surfaces and (3) to prevent crystallization on surfaces. Our project will address these requirements by developing a novel, and potentially quite general approach to controlling nucleation, which is based on the use of surface topographical features rather than solution chemistry and additives.

This work offers the possibility of controlling polymorph, which is defined at the moment of nucleation, and other properties such as crystal size, which is related to nucleation density. Better control of crystallisation can be important for the yield of a product, for obtaining it in a form more suitable for packaging and transport, and for obtaining a product of higher activity, as in, for example, pharmaceuticals. Crystallization on the surface of a bone or dental implant often determines whether it will be accepted or rejected. Better control of protein crystallisation is crucial for structure determination of these by X-ray diffraction.

It may also be desirable to avoid or minimise nucleation, as in the prevention of scale formation in water pipes and boilers and the inhibition of crystallisation and freezing damage in building materials, historical and archaeological artefacts. In other cases, nucleation must occur at higher temperatures to avoid freezing damage, e.g. in biological tissues during cryopreservation, or during of freezing of foodstuffs. This is the essence of the link with our project partner Asymptote Ltd., described further in the "Pathways to Impact". Clearly, knowledge of the topographical features that can enhance nucleation can provide the basis for the development of surface treatments that best retard nucleation. There is therefore potential for significant impact on the UK economy in areas ranging from healthcare to everyday technologies to protection of the national heritage.

This project will provide excellent training for the two PDRAs, who will gain an excellent training in the valuable research skills of surface lithographic methods, crystallization techniques and analytical skills. Activities such as writing publications, presenting conference talks, establishing contact with other research groups, investigating commercialisation potential, participating in staff education and outreach work will provide outstanding opportunities for professional development.

Finally, the wider community will benefit through a programme of outreach work, in which all of the researchers will participate. The aesthetic appeal of crystals will ensure the success of such activities. Crystallisation is a phenomenon which can readily be displayed in real time to an audience with the help of microscopes and computer projection, and the investigators have experience in such demonstrations. Leeds hosts the annual "Leeds Festival of Science" where local schools take part in educational workshops related to science and engineering, and we will design a workshop relating to crystal growth, and its relevance to everyday life. We will also prepare a "Schools Talk" based on this material. In addition, the investigators will use the Leeds Centre for Crystallization to offer summer placements in our university laboratories to undergraduate and sixth-form students. This will lead to better contacts with schools, and the possibility of widening the participation of pupils.