Physiscs of Snow Crystals

Physics of Snow Crystals

Snow crystals, also called snowflakes, are single crystals of ice that grow from water vapour. They form in copious numbers in the atmosphere and are well known for their elaborate, symmetrical patterns. The physics of snow crystal formation is a specific example of the more general problem of how crystals grow and develop, creating complex structures on many length scales in the process. Because crystallization is a basic phase transition and crystals make up the foundation of several major industries, much effort has been expended toward developing a detailed understanding of the physics of crystal nucleation and growth. There is also a considerable literature on pattern formation during solidification. Beyond the intrinsic scientific merit of these problems, the burgeoning commercial interest in the self-assembly of nano-scale devices has reinvigorated our desire to understand just how solidification produces ordered, and sometimes complex, structures from disordered precursors. Structure formation during crystal growth is a rich many-body problem, for which there are few overarching theories and perhaps even fewer uncomplicated experimental systems. In part this reflects the fact that many factors contribute to crystal growth, including both largescale phenomena (e.g. particle and heat transport) and microscopic dynamics (e.g. surface diffusion and chemistry). Different crystals grown under different conditions exhibit a broad

range of morphologies, growth rates and other characteristics, reflecting the variety of physical mechanisms that influence crystal growth. The inherent complexity of the physics has resulted in a rather large gap between the basic tenants of crystal growth theory and the phenomenology

of growing practical crystals.

1.1. Ice as a case study

In many ways, the formation of ice crystals from the vapour phase is an excellent case

study of crystal growth dynamics and pattern formation during solidification. Although