The Water to Ice Transition: Implications for Living Cells

When liquid water is cooled, a temperature is reached at which the solid phase, ice, becomes stable. That is to say that an ice crystal, when added to the cold water, will not melt. If the liquid is below that fusion temperature, the ice crystal will, in fact, grow; water molecules will leave the jostling, random walk of the liquid phase and join the fixed lattice of the ice crystal. When the pure liquid is cooled below the fusion temperature without any external ice being added, however, the solid phase does not necessarily appear. There is a temperature range in which the liquid is metastable; the presence of a seed crystal (or other nucleating enhancement) is required to initiate the phase transition. In this region there are clusters of water molecules joining together to form networks that extend the correlation length between water molecules beyond the scale of a few molecules. As these networks grow, the number of possible paths for continued growth increases exponentially,but the correlation length also decays exponentially because of collisions that break the network. Only the latter phenomenon is dependent on temperature, so there is a critical temperature at which the multiplicity of growth paths can overwhelm the collision-based fractures. Below this temperature a nucleation event can occur whereby the length scale of a particular network exceeds the length scale of the destructive collisions that break these extended clusters. The growing network becomes an ice crystal.