The museum's snowflake machine had to be regulated to a fraction of a
degree F, otherwise the result was plates, not "flakes". I have heard
that in air, tiny changes in temperature and humidity cause very different
styles of crystal growth. I suspect that these changes actually control
the RATE of crystal growth, and as rate increases, all sorts of nonlinear
effects turn on. We can get polyhedra at low rates, fractal branching and
percolation structures at medium rates, and needle shapes at high rates.
This is identical to frost growth, and the growth of polyhedra versus
needles if we grow crystals from solution.
If a snowflake falls through air having badly-mixed "swirls" of various
temp and humidity, the style of growth of the boundaries of each flake
would shift wildly back and forth. To guarantee that two flakes have
identical structure, they would have to fall two-abreast and maintain
their separation. The shape of a snowflake seems to be a complicated
"serial data" record of its growth rate's history.
If we wanted to artificially create identical snowflakes, we would simply
need to start with similar seed crystals, gently blow cold moist air upon
them, and then "play back" a sequence of humidity and temperature
variations in the stream of air. Might the pressure variations of loud
sound waves even have an influence? Does the earth's natural e-field have
any impact on the crystal shapes? I wonder how well the patterns at the
edge of the growing flakes could be controlled. Maybe draw little
pictures and writing made from programmable ice needle structures? Don't
go there. The end result could only be a pulsed microwave beam which
causes all the flakes in a snowstorm to be emblazoned with "Nike" or
"Drink Coke".
If naturally-grown snowflakes could fall in air of perfectly uniform
temperature and humidity, they might be all identical. Or maybe not,
since each flake leaves a laminar wake behind it which has had some water
vapor removed, and has been slightly heated. Perhaps some of the
complexity of snowflakes come about when they pass through the maze of
"warm dry" wakes of their fellow flakes which preceded them.
Another museum thingy: a company called NEW CURIOUSITY SHOP in CA sells a
device called a "space spider": http://www.newcurshop.com/Products/Crystal.htm It is a double-walled
transparent refrigerator which maintains the proper temp/humidity
conditions for frost growth of the ice-needle type. When it operates, the
entire interior fills with nearly invisible ice needles of many inches
length. They are like spider webs, and can be seen when light reflects
from their sides. What amazed me was their length. How can something the
size of a strand of spiderweb grow outwards from a horizontal surface?
The stress inside these ice crystals must be staggering.