Ice Filaments Grown in a Gradient of Vapour Pressure

Abstract

It has been observed that when ice crystals are nucleated on fine threads in a low-pressure diffusion chamber, they grow preferentially upward and sublimate downward leaving, between crystals and threads, thin filaments and sheets of ice.
The features and growth process of these structures are presently studied.
Filaments and edges of sheets have a typical shape, formed by a nearly periodical series of “beads” and “necks ”. During growth, new beads appear at the base of the filaments, while all the structure looks as if it were rising up against the vapour flux. The phenome-non is explained by considering an alternate process of condensation upward and evaporation downward of the ice surface, due to the uni-directional vapour flux.
It is shown that the vapour density in the region where filaments form is near to equi-librium, and the typical shape of “beads” and “necks” is related to the compensating effect of the different equilibrium vapour pressure at their surfaces, due to their different curvature.
The interpretation of the present phenomenon may be applied to explain the formationof ice filaments observed in more general conditions.