6. Molecular level structures of ice surfaces
Though both theoretical and experimental results commonly indicate the occurrence of surface melting on ice surfaces, it is very difficult to discuss the microscopic surface structures of ice. Recent development of super computers has allowed us to simulate ice surfaces and interfaces[35,36,37]. Nada and Furukawa [37] recently carried out the computer simulations for the system including ice surfaces or ice/water interfaces, using a method of molecular dynamics, which is a very powerful method to examine the microscopic structures of water molecule arrangements and the dynamic properties of the surfaces. In this simulation, it is possible to follow up both the lateral and rotational movements of each water molecules packed in a simulation box, over the passage of time in several hundreds ps (1ps=10－12s).
Figs. 8a and 8b show the snap shots of water molecules around the surfaces with the orientations of {0001} and {1010}, respectively, which are projected along the directions parallel to the surfaces. Red and white spheres indicate the oxygen atoms and the hydrogen atoms, respectively. Water molecules included in the crystal are arranged on the lattice points of ice crystal structure. The water molecules near the surface, however, are not arranged on the crystalline lattice points and the disordered layer were observed on both the {0001} and {1010} faces. It should be emphasized that the thickness of disordered layers appeared in these simulations is completely different from each other. We can conclude that the anisotropic surface melting was reproduced in the computer on the molecular scale for the first time. Using these simulation results, we can analyze not only the surface structures but also the dynamic properties of qll.
In conclusion, the habit change of snow crystal should occur in the relation to the anisotropic surface structures of ice crystal. We believe that the model proposed originally by Kuroda[24] is basically reasonable. But we have to remind that many discrepancies are still remained among the theoretical basis, the experimental results and the simulation results.