The double-pyramid structure of dendritic ice growing from supercooled water

摘要

It is known that ice growing freely from supercooled water has a morphological transition at T=-2.7 deg C, from a flat dendrite structure at higher temperatures to a twelve-sided double-pyramid structure at lower temperatures. The double-pyramid structure, which can be described as two hollow six-sided pyramids joined at their apieces, is built from dendrites growing in well-defined growth directions which are noncrystallographic in the planes normal to the basal plane while their projections on the basal plane retain the hexagonal symmetry. Similar structures have been reported in other hexagonal materials. In order to understand the growth mechanism better, we measured the temperature field in the water around the growing crystals by using the temperature dependence of its refractive index. Since this dependence happens to be zero at the freezing point for regular water (H_2O_, we use heavy water (D_2O_, and achieve considerably grater sensitivity. The free growth experiments performed with heavy ice reveal that their morphological behavior is similar to regular ice, as well as their velocities and the angle between the pyramids ass a function of supercooling. The similar to regular ice, as well as their velocities and the angle between the pyramids as a function of supercooling. The temperate measurements showed that the interaction between the two sides of the pyramind via the temperature field is weak. This leads to the conclusion that the solution for the growth mode of the dendrites should be found in the single dendrite level. Explanations of this phenomenon are di