Coalescence Mechanism of the Explosive Solidification of Supercooled Liquids

Abstract

The explosive solidification of supercooled liquids is analyzed using heating and cooling thermograms of various substances. As proof of the phenomenon of explosive solidification, thermograms in the temperature–time coordinates are presented for tellurium, bismuth, water, and acetic acid. A schematic cooling curve is used to demonstrate the structural changes occurring in a liquid phase and the stages of solidification, namely, the formation of crystal nuclei, their coalescence, and subsequent isothermal solidification. Explosive solidification at the second stage is characterized by a rapid temperature rise from a supercooling region to the melting temperature at the rates exceeding the cooling rate (i.e., heat removal) by 2–3 orders of magnitude. An attempt is made to explain the phenomenon of explosive solidification in terms of the well-known postulates of the cluster-coalescence model of solidification and the theory of chain reactions. The calculations demonstrate that the critical nucleus sizes are comparable with the unit cell sizes of the corresponding crystal lattices and the nucleus formation energy is comparable with the intermolecular bond energies. Based on the calculations of the heat released during nucleus formation and the interfacial surface energy released during the coalescence of crystal nuclei, we advance a hypothesis that crystalline clusters and stable crystal nuclei can serve as the “building blocks” of crystal growth along with molecules. The energy released during the formation of nuclei and their coalescence is shown to be equivalent to electromagnetic radiation quanta and to generate new centers of solidification, multiplication, and coalescence of nuclei. The calculations demonstrate that the energy released during the coalescence of many nuclei is high enough to quickly heat a substance from the supercooling region to the melting temperature. By analogy with the well-known thermal explosion diagram, we plotted and analyzed a similar diagram for the time dependence of the heat release and the heat removal during explosive solidification. The critical values of the beginning of the explosive process and the cooling rate of a liquid phase are found. The final stage of equilibrium solidification after explosive solidification is estimated.