TEMPERATURE-DEPENDENCE OF THE GROWTH FORM OF NEGATIVE CRYSTAL IN AN ICE SINGLE-CRYSTAL AND EVAPORATION KINETICS FOR ITS SURFACES

Negative crystals, which are holes bounded by crystallographic faces, were formed in ice single crystals by such a method as the evaporating water vapor being evacuated through a thin injection needle pierced into the ice. ''Growth processes'' of negative crystals were observed in situ, and evaporation rates of each surface bounding the negative crystals were measured. The evaporation steps were also observed in situ on both the {0001} and the {1010BAR} faces of negative crystals. The negative crystals were always observed as hexagonal prisms at temperatures below -2-degrees-C, and they showed habit changes twice in a sequence of plate (-5-degrees-C) --> prism (-10-degrees-C) --> plate with decreasing temperatures, in the same manner as snow crystals (i.e., positive ice crystals grown from water vapor in the atmosphere). At temperatures above -2-degrees-C, they changed into spheres truncated only by {0001} faces because of the roughening transition of {1010BAR} faces. Subsaturation in the negative crystal was estimated under the assumption of the viscous flow of water vapor through the needle. As a result, the evaporation rates were not linear with subsaturation and the evaporation coefficients were in the order of 0.01-0.03. It means that the edges of the negative crystal do not work as the predominant step source.