Sidebranching in Dendritic Solidification.

Snowflakes often grow by sending six dendrites or branched fingers out into the surrounding vapour. Similar treelike structures form in many metal alloys as the solid metal solidifies from the liquid melt. Recent research has focused on understanding the basic physics involved in the growth of a single dendrite; asking how these complex patterns can form from such a simple system as a liquid which is freezing. One problem that remains concerns the origin of the sidebranches which grow from the sides of the dendrite. I suggest that, under certain circumstances, sidebranching may result from the selective amplification of noisy fluctuations in the tip region and that thermal noise may be adequate. This suggestion is based on numerical and analytical studies of the two dimensional boundary-layer model. Recent experiments of A. Dougherty et al. lend support to this picture of noise-induced sidebranching.