Surface instability and mass transfer during the bonding of ice spheres

Abstract

The morphological changes occurring in a vertically aligned, one-dimensional ice sphere array under quasi-isothermal conditions were investigated as a geometrically simplified model of snow aggregates, in order to understand the mechanisms operating during the bonding of ice crystals. Time-lapse three-dimensional images of the specimen were obtained using computed X-ray micro-tomography, in order to characterize the time-dependent structural evolution of the specimen. Fine-scale structural features were examined immediately after the time-lapse observations using a scanning electron microscope equipped with a cryo-system. Porous necks between adjacent ice spheres were observed to develop via the growth of small protrusions on the surface of the ice spheres. The instability of the specimen surface, i.e. the initially smooth surface breaking down into small protrusions, was analysed using the model proposed by Mullins and Sekerka. The analysis indicates the strong influence of the wavelength of the protrusions on surface stability and the important role of vapour transfer during the bonding process of the ice spheres. The grain boundaries that formed between the protrusions and the small mass build-ups on the surfaces of the ice spheres provide evidence for grain boundary migration.