The Mechanism of Heterogeneous Ice Nucleation Revisited
Peter Conrad
Indiana State University

Contrary to popular belief, water must be cooled well below 0°C before freezing will occur. The formation of ice at temperatures approaching 0°C requires the presence of so-called nucleating agents. The properties of effective nucleating agents are not well understood. In 1947, Bernard Vonnegut proposed that substances whose lattice constants match those of one of the faces of ice would make effective nucleating agents. The past fifty years of research on heterogeneous nucleation has seen repeated tests of this claim, with little success. Initially our research focused on barium fluoride, whose (111) face provides an almost exact match for the basal face of hexagonal ice. While barium fluoride would appear to be an ideal nucleating agent, we have found that barium fluoride provides only a modest increase in the nucleation threshold for ice. Furthermore, we have found that thin films of ice on barium fluoride are unstable. These results have prompted a reevaluation of the conditions that must be met by an effective nucleating agent. In particular, interactions between the surface of hydrophilic substrates (such as barium fluoride) and water molecules disrupt the precise arrangement of bonds needed for ice formation. Nonetheless, hydrophilic substrates, ranging from inorganic crystals to steroids, have been shown to dramatically increase the nucleation threshold for ice. Explaining these results will require the development of a general mechanism for the heterogeneous nucleation of ice. I propose a series of experiments designed to elucidate the effects of substrate properties, including surface chemistry, lattice geometry, and defect density, on the nucleation process. It is hoped that the results of this research will lead to the development of a general mechanism for the heterogeneous nucleation of ice.