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

It is known that ice growing freely from supercooled water has a morphologi cal transition at T = - 2.7 degrees C, from a flat dendritic structure at h igher temperatures to a twelve-sided double-pyramid structure at lower temp eratures. The double-pyramid structure, which can be described as two hollo w six-sided pyramids joined at their spices, is built from dendrites growin g in well-defined growth directions which are noncrystallographic in the pl anes normal to the basal plane while their projections on the basal plane r etain the hexagonal symmetry. Similar structures have been reported in othe r 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 dep endence happens to be zero at the freezing point for regular water (H2O), w e use heavy water (D2O), and achieve considerably greater sensitivity. The free growth experiments performed with heavy ice reveal that their morpholo gical behavior is similar to regular ice, as well as their velocities and t he angle between the pyramids as a function of supercooling. The temperatur e measurements showed that the interaction between the two sides of the pyr amid via the temperature field is weak. This leads to the conclusion that t he solution for the growth mode of the dendrites should be found in the sin gle dendrite level. Explanations of this phenomenon are discussed in the li ght of recent advances in dendritic growth theory - in particular the conce pt of microscopic solvability - combined with the behavior of the surface t ension and the kinetic effect as a function of crystallographic orientation . It can be shown that growth in a low symmetry direction leads to an asymm etrically growing crystal and to asymmetry in the observed temperature held . (C) 1999 Elsevier Science B.V. All rights reserved.