Snowflakes are special. Their immediately recogniseable shape evokes the smell of glögg, the taste of pepparkakor and the spirit of Christmas. But before the snowflake was popularised as a Christmas icon by Wilson ”Snowflake” Bentley, the delicate and intricate structures had captured the imagination of physicists who wanted to understand how they formed. They were smart and meticulous about it their work, examining thousands of ”wild” snowflakes as well as trying to grow them in the lab. I’d imagine that their peers thought they were either crazy or juvenile which is the only reason I can think of for the naming their snowflake science, ”dendritic growth during diffusion-limited solidification”. Way to make something not sound Christmassy.
But crystal growth is a big deal, and not only in physics. The ability to form crystals from proteins gave rise the field of structural biology and is an essential tool in modern research. Finding out how crystals form can ultimately lead to understanding more about the interactions of molecules in biology which we know is important.
Two parameters are critical for snowflake formation, temperature and supersaturation. A supersaturated solution or vapor contains enough solute to push it beyond saturation. It’s like my ambitiously overfilled shopping bag. It’s more full than it should be and desperate for some kind of relief. For my shopping bag, relief came as the single sharp edge of a tin of beans creating a tiny hole in the bag which instantly propagated a fatal tear (true story). For supersaturated solutions, relief can come when a single object, often a crystal is added acting as a nucleation point which then propagates a chain reaction of crystal formation.
So what’s the effect of these parameters on snow crystals? Well at temperatures close to zero, snowflakes will form flat plates, exactly like the christmas decorations. As you increase the supersaturation, the plates will become thinner. However, when you decrease the temperature by a few degrees and approach -5, something strange happens. The snow flakes become columns. With low supersaturation, the columns are short and fat and at high supersaturations, crystals form as multiple needle-like clusters. Decreasing the temperature even more (-15 degrees celsius) reverts the crystals back to the plate form.
It’s fascinating to think that just by changing these two parameters you can produce wildly different and complex 3D structures. It gives me a whole new respect for crystalographers and the next time I go outside in the snow, I’m probably going to take a magnifying glass.