Recent record snowstorms throughout the United States may rekindle in some wandering minds an age-old question: Exactly why are no two snowflakes alike?
The question also continues to flummox some scientists.
"This stuff falls out of the sky and we don't know how it works and we're naturally curious," said Kenneth Libbrecht, professor of physics at California Institute of Technology
Studying snowflakes, Libbrecht points out, is strictly for curiosity's sake. The discovery of why each flake is unique is not expected to have much of an impact on anything.
The effort to learn about snowflakes is kind of like brain candy. Indeed, many great minds have applied themselves to finding out why, such as philosopher Rene Descartes and nuclear physicist Ukichiro Nakaya.
Still, no concrete answer.
This is where the Caltech snow crystal research team comes in.
By growing snow crystals in the laboratory under controlled conditions, the research team discovered that they develop differently depending on the temperature and how wet the air is.
A typical snowflake begins by forming around a speck of dust. From this unassuming beginning it grows into a tiny hexagonal prism, just a few microns in size. The initial symmetry of the snowflake results from the intrinsic molecular structure of ice.
As the crystal grows, it's often blown about in the sky. The air and temperature around the crystal are constantly changing. Snowflakes are extremely sensitive; even a small change in these conditions can lead to different growth patterns.
The final shape of the crystal reflects these growth conditions. The longer the snowflake is blown about in the skies, the more complex the resulting snow crystal.
"No two crystals have the same history so they don't grow in the same way," said Libbrecht.
And, he added, no two have ever been the same, or ever will be.
To date, the Caltech team does not understand how temperature actually affects growth. To understand this process they are carrying out a number of experiments to study the physics of how snow crystals form.
The experiment that Libbrecht likes best uses a half-meter diameter copper tank that is chilled by cooling coils soldered to its body. Two buckets of heated water are placed under the tank. These give off vapor needed to grow crystals.
"We drop tiny flecks of dry ice in the tank which nucleates crystals," Libbrecht said.
The crystals start off very small – some are only a micron in size. As they fall, they grow. The researchers measure this growth.
When the crystals hit the bottom of the tank, they're photographed and measured. By altering the conditions within the tank, researchers can establish what temperature and air conditions produce what kind of crystals.
The tank allows the researchers to set up experiments in a controlled environment. No unexpected variables are allowed to interfere with the crystal's growth. A controlled number of the crystals are produced so they don't bump into each other upsetting their growth patterns.
"It is a very simple experiment but we need to do it very many times under very controlled conditions in order to quantify the results," Libbrecht said.
Another experiment Libbrecht's team carries out involves the creation of "designer" snowflakes.
This consists of a small diffusion chamber of 8-by-12 inches. It produces moist warm air, which diffuses, producing supersaturated – or wet – air.
"We have a wire going into the chamber charged to 2,000 volts. From this we grow thin needles of ice – like No. 2 pencils – hexagonal columns that can get very long," Libbrecht said.
"We can grow these and then turn off the voltage and lower the temperature to minus-15. When we do this, a star plate crystal will grow on the end of the needle; it looks like a flower on a stick."
By moving this plate up and down the researchers can change the temperature and so program plates or columns or arms or side branches – all on command.
"It's not clear whether this will be useful scientific stuff, but they make for an interesting study. When we're feeling artistic, we grow stars."
Libbrecht admits that his team will not have any definitive answers any time soon. "It'll be years rather than months before we have concrete results."
"We're not doing the research for any particular application," Libbrecht admits. "It's a nice challenge to find out why these crystals grow the way they do.... You can learn things from unusual places."
"I'm not seeing anything that I would classify as groundbreaking, but it's nice work. He (Libbrecht) seems to have set up carefully controlled conditions to produce snowflakes and to test a theory in a very controlled way," said Tom Lubensky, professor of physics at University of Pennsylvania.