What do crystals nucleate on? What is the microscopic mechanism? How can we model nucleation?
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Introduction Nucleation is the process that determines how long one has to wait before a crystal appears in the system, or if a crystal forms at all. Whenever we want a crystalline material, whether it be a metal or a drug, we need nucleation. For this reason, the nucleation of crystals is of interest across much of materials science. Here, we consider universal aspects of how crystals nucleate, because many features of crystal nucleation appear to be common to all, or almost all, crystallizing systems. An example is the essentially universal observation that nucleation is typically heterogeneous: the crystal nucleus forms in contact with a surface. Although specific crystallizing systems are used to illustrate the points here, these points apply very widely in materials science. In particular, we show results for ice, as the nucleation of ice is perhaps the best studied crystallizing system of all. Materials scientists can learn from the excellent and innovative work done by atmospheric scientists on ice nucleation.1,2 Currently, we have very little ability to predict how long nucleation will take (i.e., how long we have to wait before a crystal nucleates) for a given volume of sample at a given supersaturation. We cannot predict if we will have a burst of many small crystals nucleating, or just a few. This is mainly due to a combination of not knowing what
nucleation is occurring on, or what the microscopic mechanism is. We know that nucleation is usually heterogeneous (i.e., that it occurs in contact with something), but we almost never know exactly what that surface is. This lack of knowledge is a major stumbling block in this field. The next two sections offer a review of the progress being made on this problem. In the absence of knowledge of exactly where nucleation is occurring and what the mechanism is, the best approach is to combine quantitative experimental data with simple phenomenological models. These models, such as the Weibull model, are routinely taught and used in materials science in other contexts (brittle fracture),3,4 but surprisingly, are almost never used for nucleation studies. The article discusses how progress could be made by applying these well-known models to this problem.
What do crystals nucleate on? Nucleation is usually heterogeneous: The nucleus of the crystal forms in contact with something. In most cases, this is some form of dirt or impurity in the system, therefore, we have little idea of the properties of the surface where nucleation is occurring. However, it is sometimes possible to observe the early stages of growth of a crystal
Richard Sear, Department of Physics, University of Surrey, UK; [email protected] doi:10.1557/mrs.2016.88
© 2016 Materials Research Society
MRS BULLETIN • VOLUME 41 • MAY 2016 • www.mrs.org/bulletin
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WHAT DO CRYSTALS NUCLEATE ON? WHAT IS THE MICROSCOPIC MECHANISM? HOW CAN WE MODEL NUCLEATION?
(although not nucleation itself), and be able to say with some confidence what surface nucleation occurred at. Gurganus et al.5 imaged crystallizatio
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