Nucleation and Surface Melting of Ice

James Franck Institute and Department of Chemistry, University of Chicago 5640 South Ellis Avenue, Chicago IL 60637, USA 

The freezing of liquid water to ice is a first-order phase transition that, according to thermodynamics, should take place at 0°C. Kinetics presents a different story, however, and metastable liquid water at temperatures as low as -30°C is well attested when the water is emulsified into small droplets purified bulk samples are readily cooled below -10°C. In a first-order transition, two steps are necessary before a new phase becomes visible nucleation and growth. In most cases, growth of crystals from the melt is rapid and the kinetic barrier to new phase formation is dominated by the first step, nucleation. 

In this chapter I describe theoretical and experimental approaches to the freezing and melting of crystals, with particular emphasis on the ways in which water and ice resemble and differ from simpler materials. 

Edited by John S.Wettlaufer, J. Gregory Dash and Norbert Untersteiner Springer-Verlag Berlin Heidelberg 1999 

Small enough ciystallites surrounded by the melt tend to shrink even in an undercooled liquid large enough crystallites tend to grow because of the greater thermodynamic stability associated with the crystalline phase under these conditions. The crystallite that falls at the boundary between shrinking and growing is called the critical nucleus, because its rate of formation determines the overall rate of the phase transition. 

---