Nucleation Control in Materials and Biology
A. L. Greer
University of Cambridge, Department of Materials Science & Metallurgy
Pembroke Street, Cambridge CB2 3QZ, UK
e-mail:
Changes of phase are of great importance in many fields. This presentation will include examples that range from casting of metals to freeze-tolerance of frogs to neurodegenerative disease. Our focus will be on systems of direct interest in Materials Science, but looking for lessons to learn from Nature. In the classical view, the nucleation of a new phase is a stochastic process, involving fluctuations to surmount an energy barrier. The initial fluctuation is of course small, and so nucleation is rarely observable directly. The classical analyses of nucleation use macroscopic values of phase and interface energies, and have long been recognised as inadequate. This is especially so at large supersaturation, when critical nuclei are so small that the width of the interphase interface is comparable with the nucleus radius. Modern analyses of nucleation, however, can take account of interface width. The study of colloids is starting to allow nucleation processes to be directly observed.
Most phase changes naturally start at very small supersaturation, and recent years have seen progress in understanding the nucleation, which must be exclusively on heterogeneities. We will show that this is best analyzed as an 'athermal' process in which the number of nucleation events is determined by temperature, independent of time. This process is deterministic, not stochastic. The heterogeneities act as templates for the new phase, and we will pay particular attention to this for freezing transitions. Providing a template for a crystalline phase is conceptually straightforward; interestingly, recent work suggests that it is also possible to provide a template for the liquid.
Throughout we consider the prospects for control of nucleation.
Recent references on relevant topics include:
[1] K.F. Kelton and A.L. Greer, Nucleation in Condensed Matter: Applications in Materials and Biology, Elsevier, Oxford (2010).
[2] J. van Meel, R.P. Sear, and D. Frenkel, Phys. Rev. Lett. 105, 205501 (2010).
[3] A.L. Greer, Nature 464, 1137 (2010).
[4] S.A. Reavley and A.L. Greer, Philos. Mag. 88, 561 (2008).
[5] A.L. Greer, Nature Materials 5, 13 (2006).
[6] T.E. Quested and A.L. Greer, Acta Mater. 53, 2683 (2005).