March 10, 2015 § Leave a comment
One of the earliest scientific observations you may have performed as a kid may be that of snowflakes. Their delicate morphology, with multiple branches, has a unique appeal to the eye and can easily be observed with magnifying glasses. No wonder that snowflakes already caught the attention of scientists and poets for centuries. In the 17th century already, Johannes Kepler noticed their 6-fold symmetry, as well as their unique nature – not two snowflakes are alike.
For a very long time, the only way to record the shape of snowflakes was drawing. If you ever looked at snowflakes under a magnifying glass, you can easily imagine how difficult it is to draw – notwithstanding that snowflakes tends to have a very short lifespan. In the early 20th century, Wilson A. Bentley was the first one to photograph snowflakes, systematically capturing thousands of unique snowflakes for over 40 years. His collection has proved to be incredibly valuable to investigate their morphology and is also a unique piece of art, if you ask me.
Following on Bentley’s work, Kenneth Libbrecht, at Caltech, is dedicating his carrer to the study of snowflakes. Driven by both passion and science, he developed over the years a unique setup to capture images of natural snowflakes. There is still a lot to learn from snowflakes. Or there is actually not that much we understand about the growth of snowflakes and the physics behind it. One thing me know: when it comes to snowflakes, there is more than the 6-branches morphology that anyone will draw if you ask them. Depending on the conditions (temperature and supersaturation), you can get anything from needle to plates. The morphology of natural snowflakes directly depends on the conditions they encountered in the sky. As such, snowflakes can be seen as little messengers from the clouds, telling a very local climate story.
Systematic investigations, required to understand the physics behind snowflakes, are thus notoriously difficult with natural snowflakes. Physicists have long been trying to grow artificial snowflakes in the lab, under controlled, reproducible conditions. The first attempts to grow such snowflakes used … rabbit hairs ! A well-controlled (at that time), one-dimensional object, suitable to trigger the nucleation of snowflakes.
In a paper published on arXiv last week, Libbrecht describes a very unique microscope, designed to grow snowflakes under controlled conditions, and to record their growth in real time. The pictures are stunning, as usual. I have one of Libbrecht’s book of snowflakes collection on my desk, and peruse through it every once in a while. You should, too.
The most interesting thing to me are the time-lapse observations reported in the paper. By varying the supersaturations and temperature conditions, Libbrecht triggers, in a controlled manner, side branching events, effectively engineering the morphology of snowflakes. Increasing the supersaturation for a brief moment initiate the development of branches at the corners of the growing snowflakes. Several new branches are eventually created from each corner, each of them growing in a synchronized fashion, the conditions being homogeneous at the scale of the snowflake.
This behavior reminded be of the beautiful microscopic flowers reported in Science by Noorduin two years ago where, by varying the CO2 concentration, Noorduin was able to change the growth morphologies of its tiny flowers in a very controlled manner. In both cases, crystal growth occurs under diffusion-limited conditions and may thus share more than meets the eye.
Having a much better control of the conditions, the grown, engineered snowflakes have a much better 6-fold symmetry than their natural counterpart. Growing branches while falling through the windy sky is a tough job. The conditions vary constantly. By the time each snowflake reaches the ground, their infamous 6-fold symmetry is seldom preserved. By deliberately engineering the growth of his snowflakes, Libbrecht obtains new insights into the physics of snowflakes growth, which may certainly be valuable for our understanding of crystal growth. But I can’t help but see the sheer beauty of the highly symmetrical engineered snowflakes, too. Libbrecht may very well be the very first one to grow two identical snowflakes, ruining a long-standing belief that not two snowflakes are alike.