Freezing heterogeneous nucleation

Reid et al. [ 1.12] described the effect of 1 % addition certain polymers on the heterogeneous nucleation rate at-18 °C the rate was 30 times greater than in distilled, microfiltered water and at -15 °C, the factor was still 10 fold hogher. All added polymers (1 %) influenced the nucleation rate in a more or less temperature-dependent manner. However, the authors could not identify a connection between the polymer structure and nucleation rate. None the less it became clear that the growth of dendritic ice crystals depended on to factors (i) the concentration of the solution (5 % to 30 % sucrose) and (ii) the rate at which the phase boundary water - ice crystals moved. However, the growth was found to be independent of the freezing rate. (Note of the author the freezing rate influences the boundary rate). 

Particulate impurities are needed for heterogeneous nucleation, and these are always present (natural waters tend to start freezing at, say, — 1°C). The formation of air bubbles is also dependent on impurities, some of which may contain a tiny air pocket (see Section 14.4) some natural ice is opaque, due to the numerous small bubbles it contains. Other impurities, be they dissolved substances or particulate matter, can greatly affect the size and morphology of the ice crystals. 

Thus far, we have focused on the homogeneous nucleation in colloidal suspensions. However, in the real world, crystallization is usually initiated by heterogeneous nucleation. If ice could only form through homogeneous nucleation, the freezing of water would be a rare phenomenon in countries with moderate climates. 

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