snowflakes growing. I wanted to make one
showing a perfect-looking crystal. Eventually,
I worked out how to grow crystals on a fixed
support and film them. They looked so much
better than crystals that fall from the sky.
They were crisper, sharper. Real snowflakes
have had kind of a hard life, falling through
the atmosphere and banging into other flakes.
They have also begun to evaporate, so the
edges are always a little rounded.
In the lab, though, I can turn knobs
to control the conditions exactly – I can
even change things slightly to make the
growing crystals branch – and so I can get
these designer snowflakes. Then I realised
that if you grow two snowflakes next to each
other in the box and grow them at the same
time, under the same conditions, they come
out very similar. Everybody seems to have
heard this old adage that no two snowflakes
are alike, but then this crazy person is making
snowflakes that are alike.What kinds of questions are still unanswered
when it comes to snowflakes?
My model is very complicated. But it makes
a lot of predictions and I want to test those
out, see what happens and hopefully refine
the model. One of the predictions is that
interesting things might happen to snowflakes
close to their melting point. You might see
what’s called pre-melting, where most of the
snowflake is a rigid crystal, but on the surface
the molecules get disordered. I’ve been trying
to explore that. I’m also trying to make larger
crystals; I’d like to make the world’s largest
snowflake. For no particular reason... there’s
just always something new to try.When you go out in the snow these days,
do you see it in a new light?
I grew up in North Dakota, where it gets very
cold and we have a lot of snow. I used to see
six-pointed star flakes, large ones, but I didn’t
know any of this stuff. Now I know a lot better
what to look for. I’ll go out and have a magnifier
and be looking for different kinds of flakes – like
capped columns, for instance, which are like a
special hybrid of the plate and column types.
I call it snowflake watching. The funny thing
is, my wife is a botanist, and when we go out
together she’s always looking at the different
plants – but they all look like weeds to me. ❚Kenneth Libbrecht
in his snowflake labJoshua Howgego denies rumours
he is a delicate snowflakein miniature. It was discovered in Japan in
the 1930s that these two forms of snowflake
will form at different temperatures. Plates will
form at around -2°C, columns at about -5°C and
then plates again at about -15°C. It’s such a crazy
pattern that it flops back and forth like that.
I really wanted to know why it happens, but
it turned out there was no answer – it was a
complete mystery.
How did you investigate this conundrum?
I decided the way to answer this question was
to systematically grow a lot of snowflakes in
different conditions and measure their growth.
This was 20 years ago, and for several years I
kept hitting problems and made no progress.
I eventually figured out the whole experiment
has to be enclosed in a box. You add in water
vapour, but the conditions, like temperature
and pressure, have to be precisely controlled.
I have all these little rods going into the box
that I use to push things around and turn
things on and off. Then I could grow crystals –
mostly I grew ones that are smaller than the
width of a human hair and then studied them
under a microscope. If they get too large, they
are too complicated in shape to study easily.
You recently published what has been called
a grand unified theory of snowflakes.
People had always thought that if you have
a flat surface in a crystal, it always grows in
the same way under given conditions. What
I found is that in snowflakes, the size of the
surface matters quite a lot. If you imagine a
hexagonal, plate-like snowflake, it has two
wide surfaces and then six much thinner
surfaces around the edge. It turns out that
those thinner surfaces grow much faster
than the broader surfaces, and this creates
a runaway effect where you get thinner and
thinner plates. The fun thing is that this
trend reverses – at certain temperatures, it’s
the wider surfaces that grow faster, and so
you get column-like crystals.
To understand this fully, you need to dig
down into the details about the molecular
structure of the ice crystal surface and how
it changes with temperature on different
surfaces. So far, my model seems to fit all
the data, so it is encouraging that at least
some of the mystery has been solved.Does your work tell us anything about what
snow would be like on other worlds, such as
Saturn’s icy moon Enceladus?
The diffusion of water molecules through
the air affects snowflakes’ growth in a way
that reinforces the molecular effects to create
the thin edges seen in plate-like and hollow-
column crystals. When I grow crystals in a
vacuum, none of this happens. So, yeah, there
would be differences in snow on other planets.
If there’s no atmosphere, you are going to get
blocky crystals, but at high pressure you
would get incredibly thin ones. Of course, the
chemistry of the atmosphere could change
this, too, in ways that are hard to predict.You have also grown two identical snowflakes.
I thought that was supposed to be impossible.
This started when, early on in my snowflake
work, I realised there were no good videos of18/25 December 2021 | New Scientist | 59