Two-dimensional phase changes in water adsorbed on ice-nucleating substrates

Abstract

The ability of a solid substrate to nucleate ice in super-cooled water depends on the extent to which the layer of water adsorbed on the substrate resembles a two-dimensional (2-D) ice structure. On certain low-energy substrates the adsorbed layer has two possible states—a disordered state stable above its 2-D phase transition temperature T_D, and an ordered ice-like state stable below this temperature. The low temperature state, once formed, is an excellent ice nucleator. The transition from the ordered to the disordered state proceed at T_D but the reverse process does not proceeds until the temperature falls to T_C, a temperature which is many degrees below T_D and which depends on the nature of the substrate. By establishing the value of T_D over a range of pressures it has been possible to interpret the history-dependent properties of a number of substrates in terms of 2-D transitions. The relevance of 2-D phase transitions to the activation of ice-forming nuclei in the upper atmosphere is discussed.