Dr Richard Sear - University of Surrey, UK
How to model nucleation when the nucleation rate does not exist?
Richard P. Sear
University of Surrey, UK
Experimental results for the nucleation of crystalline tin , aspirin crystals  and ice , are not consistent with a nucleation rate that is in the thermodynamic limit. A well defined nucleation rate does not exist in these systems. What is happening 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 supercooling. The droplet-to-droplet variation is caused by the fact that nucleation is occurring at an interface (between the crystallising liquid and a solid impurity) and this interface is rough. This roughness varies randomly from one droplet to another, and the extreme sensitivity of nucleation rates to microscopic surface features means that the rate itself varies from one droplet to another.
I will present experimental results showing this behaviour and then show it can be modelled effectively . This uses ideas from a statistics and other fields of engineering. It allows us to make predictions for how typical nucleation times scale with properties such as volume and level of impurities.
 G.M. Pound and V.K. La Mer, JACS 74 (1952) 2323.
 Y. Diao et al., JACS 133 (2011) 3756.
 R.J. Herbert et al., Atmos. Chem. Phys. Disc. 14 (2014) 1 1, 4191.
 R.P. Sear, Cryst. Growth Design 13 (2013) 1329.