Quantitative Evaluation of Experimental Results an the Heterogeneous Freezing Nucleation of Supercooled Liquids

Freezing experiments using large numbers of small drops are frequently used for the study of both homogeneous and heterogeneous nucleation of water and of other substances. For heterogeneous nucleation, the spread in the observed freezing temperatures of drops has been shown to arise from the presence of nuclei of different activities in the sample. In the past no quantitative assessment of the nucleus content could be given independent of the drop sizes used. It is shown in this paper that from the observed freezing temperatures of the drops one can derive both a differential and a cumulative nucleus spectrum. The differential spectrum represents the concentrations of nuclei which are active at specific temperatures and the cumulative spectrum represents the concentrations of nuclei active at all temperatures warmer than the selected temperature. The accuracies of the derived spectra were examined by Monte Carlo simulation and are shown to be such that the concentrations are reliable to within factors of 2–4. The dependence of the average freezing temperature on drop volume is shown in general to be determined by the shape of the nucleus spectrum but is approximately exponential for many spectra.