Paper  |
Water is not easy to freeze. Purified droplets of 10 mm diameter, a typical size found in clouds, can be cooled to nearly -40 °C. and depending upon the conditions, can form cubic ice instead of the more thermodynamically stable hexagonal form. A likely super cooling record is for water clusters of 5 nm diameter that do not freeze until -70 °C. Addition of judiciously selected foreign substances can raise the freezing point of water considerably. Today suspensions of bacterial fragments of Pseudomonas Syringae are mixed in with water sprayed over ski slopes to favor production of artificial snow at temperatures only slightly below 0 °C. The choice of AgI as an ice nucleation (IN) agent by Bernard Vonnegut was made because its crystalline lattice constant was a near match to that of the basal face of ice. And AgI in the form of a smoke is still used for cloud seeding operations.
The mechanism of IN, laid out in classical terms, is elegantly described by Fletcher and a survey of effective (and ineffective) agents that catalyze the freezing of water is reviewed by Pruppacher and Klett. More recently, simulations have explored freezing at the molecular level.
Despite the vast literature on ice freezing, no uniform picture of the IN process has been put forward. However factors that might favor an effective IN agent have been suggested. Among them are 1) A near match of the IN agent lattice constant with one of the ice lattice constants, 2) The IN agent surface should be hydrophilic, and 3) The IN agent surface should have defects.
To explore these factors we have chosen alpha-Al2O3 (sapphire) as a model substrate for our IN studies. High purity crystals are readily available. The alpha-Al2O3 (0001) face has an hexagonal lattice whose constant is within a few percent of that of the basal face of ice. The (0001) face can be annealed into a smooth form and characterized by atomic force microscopy (AFM). The face can be chemically treated to make it hydrophilic. Finally alpha-Al2O3 can be produced in forms rich in defects. |
