Recent experimental results for the nucleation of both aspirin crystals in solution [1], and the freezing of water droplets [2], are not consistent with a nucleation rate that is in the thermodynamic limit. Essentially, what is happenining is that although each crystallising droplet has a well defined nucleation rate, this rate varies by orders of magnitude from one nominally identical droplet to another. Then there is no well defined rate for a set of droplets at a given temperature and supersaturation. This highly variable rate is caused by heterogeneous nucleation on impurities that vary from one droplet to the next. I show how to diagnose an undefined nucleation rate, and how to model the probability of nucleation as a function of time [3]. The simplest way to observe an undefined nucleation rate is to plot the fraction of a set of droplets that have crystallised - deviations from an exponential function of time indicate an undefined rate. Then it may often be possible to fit to the data what is called the Weibull distribution of extreme-value statistics. If so then this fit can be used to determine the scaling of the median time to nucleation with volume [3]. I will show that, generically, the larger the deviations from exponential behaviour, the slower the time to nucleation varies with solution volume. [1] Y. Diao et al., J. Am. Chem. Soc. 133 (2011) 3756. [2] B.J. Murray e al., Atmos. Chem. Phys. 11 (2011) 11, 4191. [3] R. P. Sear, Cryst. Growth Design 13 (2013) 1329.

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