Kind: captions
Language: en
now i'm gonna turn on 2000 volts
and this is the first step in creating
snowflakes in the lab this is totally
wild
crazy huh the tips of those needles are
like 100 nanometers in diameter that is
so wild
dr ken liebrecht is the snowflake guy i
was the snowflake consultant for the
movie frozen it's okay to conjure
snowflakes out of your fingertips but
they have to be real snowflakes or
people aren't buying it
the u.s post office made snowflake
stamps using my pictures it's not the
kind of thing you normally think of when
you start doing physics that you'd be in
a postage stamp you've written the book
on snowflakes literally so i had like
two successful books in a row so we just
kept making books
until finally they sold zero copy
and then we stopped
sir you're kind of like a snowflake
artist
i call it designer snowflake
[Music]
because yes i am designing this on the
fly i don't have a computer that does
all this for me i just do it by hand so
everyone's a little different
[Music]
ken knows so much about snowflakes he
can design and construct them to his own
specifications
so what's happening now is just growing
and doing its thing at some
it's at minus 13 celsius now but i want
to make some branches i'll just turn
this down to minus 15. then i'm going to
increase the humidity a little bit the
supersaturation and you'll start to see
branches come out there
see you changing those conditions just
caused the plate to kind of stop and
become really i changed the growth
conditions to prefer
branches this little thing right there
that little nub that's the only thing
that touches the sapphire substrate the
rest of this is all growing above this
increases the air flow
those are droplets forming and now i'm
really kicking it in gear
now what i'm going to do is i'm going to
turn that humidity down to zero so the
droplets are starting to recede
and this will stop growing and kind of
start to fasten a little bit
say i want branches again now i'm going
to
really hammer on it so you're giving it
a lot of moisture a lot of moisture now
but you'll see side branches
[Music]
you really start to feel you understand
what's going on when you can say now i'm
going to do this
and then it happens
it's fun i can predict the future
i i like to think they're better than
nature
uh and the reason is that the
the facets are just sharp all the edges
on these things are just sharp and crisp
whereas in the sky they have to fall and
by the time they fall and you pick them
up and you put them under a microscope
they started to evaporate a little boy
these are just
bang just just crisp
the first close-up photograph of a
snowflake in the wild was taken in 1885
by american meteorologist wilson a
bentley
it was bentley who originated the idea
that no two snowflakes are alike
and he would know
over the course of his life he took more
than 5000 photos of snowflakes a
selection of which appear in his book
snow crystals which is still in print
today
but most snowflakes don't look like the
ones bentley photographed because he
selected only those in pristine
condition with uncommon beauty and
symmetry
i mean when you're looking for
snowflakes i'll take a big piece of
cardboard and you just glance at it
crap nothing brush them aside more
no
each brush is a thousand snowflakes
they're hard to find you can't you know
they're one in a million i mean
literally we are also used to seeing
pictures like this that we are blind to
the mysteries of the snowflake like why
do they all have six-fold radial
symmetry
why are they so intricate and yet so
different from each other how do
opposite arms of a snowflake mirror each
other so perfectly i mean how does one
side of the snowflake know what the
other side is doing and why are
snowflakes flat
they're usually millimeters in diameter
but micrometers thick the edges of a
plate can be as narrow as razor blades
but the mystery goes even deeper
everyone pictures snowflakes like this
but the truth is they take all sorts of
different forms
like this
a hollow column that is a snowflake that
is a snowflake
there are also needles
cups
and bullets this is like my favorite
kind of snowflake is a capped column
started out growing as a column but then
the temperature changed and then you've
got plates growing on either end
it's just a cacophony of different
shapes all these appear spontaneously
there's no dna or any kind of blueprint
for what's going on it's just water
vapor freezing into ice and all this
happens so you've identified 35
different types of snowflakes yes
there's no really one way to define a
type of snowflake the first chart of
snowflakes was like 41 i think and then
it got bigger and 60 or 70 and the
latest one by some japanese
physicists i think at 108
different types of snowflakes and i
found 108 was too many
how does simple ice create so many
distinct forms
all snowflakes form in much the same way
water evaporates into water vapor
individual molecules bouncing around in
the atmosphere and as this vapor rises
it cools and becomes supersaturated
meaning there are more water molecules
in the air than there would be in
equilibrium at this temperature
water molecules condense onto dust
particles to form tiny droplets and
although the temperature may be below
freezing the droplets don't immediately
freeze but at some point one droplet
will freeze
inside the water molecules lock into
place forming a hexagonal crystal
this structure results from the
peculiarities of water molecules
oxygen atoms attract electrons more than
hydrogen
and since the molecule has a bent shape
it's polar with oxygen being slightly
negative and the hydrogens slightly
positive
since unlike charges attract a hydrogen
from one molecule will sit next to an
oxygen from another molecule forming a
so-called hydrogen bond and this is what
creates the hexagonal molecular lattice
but how does this microscopic lattice
grow into a hexagonal crystal that we
can see
so you start with a chunk of ice and
these little guys are meant to be water
molecules and what happens is these flat
surfaces which are the facet surfaces
and at a molecular scale they're very
smooth and flat
and so when a molecule hits a water
vapor molecule hits that smooth and flat
surface it tends to bounce off whereas
here it's rough
there are a lot of dangling molecular
bonds over here that's a rough surface
and so when these molecules hit they
tend to stick
it's a statistical thing of course but
the probabilities are high that they
stick here and low that they stick here
so if you take any shape and you just
let it grow for a little while the rough
areas fill in
and the flat areas don't grow very fast
and you end up with a faceted shape
and that's how we get from the quantum
mechanics that governs a water molecule
to a hexagonal prism of ice
this prism has two basal facets and six
prism facets which is important if the
basal facets grow fast you get a column
if the prism facets grow faster you get
a flat snowflake
once there is a seed crystal nearby
water droplets evaporate and deposit
water molecules onto the growing
snowflake
since the corners of the hexagonal prism
stick out farther into humid air they
grow faster and now they extend even
farther so they grow even faster in a
positive feedback loop
this gives rise to six radial branches
at the corners of these branches
additional branches can form for the
same reason
around a hundred thousand droplets are
required to make a single snowflake and
the process usually takes 30 to 45
minutes
in the 1930s ukahiro nakaya was
systematically studying snowflakes at
the university of hokkaido in japan
he discovered that the different types
of snowflakes don't all occur under the
same conditions
instead two factors the temperature and
level of supersaturation determine what
type of snowflake grows
his findings are summarized in the
nikaya diagram
but it's not a simple pattern
around -2 celsius you get plates
at -5 celsius columns and needles form
at -15 celsius it's plates again and
then below minus 20 you get columns and
plates
the nikaya diagram allows us to
understand a rough history of any
snowflake does each snowflake
in essence
reveal its history
through its shape
yeah absolutely to some degree
you can definitely look at a snowflake
and say yeah i know what conditions that
crystal grew under more or less
typical weather patterns fronts cold
front that produces a lot of capped
columns because the cloud moves up it
starts to get colder and
initially start to freeze around minus
six minus ten
that makes columns and as it gets colder
then it makes branches and plates and so
you get capped columns
this also explains why snowflakes are so
intricate the temperature and humidity
at each moment of growth determines the
structures formed in that moment
the symmetry you see is not because one
side somehow knows what the other side
is doing but because both sides of a
single snowflake grow in the exact same
conditions when the crystal changes its
position the temperature will change say
in all six branches we'll see the same
temperature change and so they'll all
respond the same way
different snowflakes on the other hand
each take a unique path and therefore
they experience a unique set of
conditions which is why no two
snowflakes are alike
but in the lab you can carefully control
the conditions so theoretically it
should be possible to create almost
identical snowflakes
and indeed
ken has
what's in here
poke your flashlight in there you'll see
uh-huh
i'm imagining these are seed crystals of
a sword those are little sparkly snow
crystals yeah you know this is another
little chamber just a cold plate
and there's a little sapphire disc in
there and then i'm going to push this
thing off my stomach all the way in here
and the crystals will waft onto it and
hopefully stay there
the idea popped in it's like if i grow
two next to one they'll be kind of
identical
and i call them identical twin
snowflakes because they're like
identical twin people they're not
exactly the same but clearly more alike
than you would never expect
[Music]
is it really true that no two snowflakes
are like that's just a silly question
it's silly because no two trees are like
no two grains of sand are like no do
anything or like anything that has any
complexity is different from everything
else because
once you introduce complexity
then there's just an uncountable number
of ways to make it
if a pair of twin snowflakes are growing
too close together they end up competing
for moisture between them stunting both
of their growths
the nikaya diagram allows us to
understand a lot about snowflake
formation ken has used his experiments
to build his own version of the chart
but what it doesn't explain is why do
ice crystals form this way in the first
place i mean why do we get plates and
then columns and then plates and columns
again
this has been a mystery essentially
since nikaya introduced his diagram back
in the 1930s but ken believes he now has
an answer
anytime you have a crystal the reason
why you get these smooth flat facets is
because it's not easy to grow more
crystal on top there are so called
nucleation barriers what you need is a
critical density of additional molecules
of the substance before they can come
together to form a little island that is
stable enough to grow and add another
layer onto the crystal
when you're first forming a snowflake
you're always going to start with a
hexagonal prism with its two basal
facets and six prism facets around the
side
and the nucleation barrier for the basal
facets is different than that of the
prism facets if the nucleation barrier
is lower for the prism facets then they
grow faster and you get plate-like
structures if the nucleation barrier is
lower for the basal facets then they
grow faster and you end up with
column-like structures
now the nucleation barriers of ice are
known as a function of temperature
and this explains why around -2 the
prism facets grow faster and you get
plates because their nucleation barrier
is lower you can also see why below
minus 20 or so well then you get columns
because the basal facet nucleation
barrier is lower at those temperatures
but what doesn't make sense is why we
should get columns at around -5 celsius
and then plates again at minus 15.
so what is happening
well ken's hypothesis is that these
nucleation barriers are valid only for
large flat facets but if you had really
narrow facets the nucleation barriers
would be different so ken proposes that
narrow basal facets have a dip in their
nucleation barrier around -4 celsius and
narrow prism facets have a dip at -15 so
his hypothesis is that the graph should
look like this
this then is consistent with all the
different forms of snowflakes that grow
at different temperatures
but what accounts for these dips
well let's say we have a narrow prism
facet so we're growing a plate snowflake
water molecules that hit the basal
facets are unlikely to reach the
critical density required to overcome
the nucleation barrier so that surface
grows only slowly
but on either side of this narrow prism
facet water molecules can stick on the
rough edges and to minimize surface
energy the ideal shape of this face
would be semicircular but if only the
top few layers of water molecules are
mobile to try to lower the surface
energy many of them diffuse onto the
prism facet and in the process they
exceed the critical density required to
overcome the nucleation barrier and so
they can grow the crystal on the prism
facet so due to this narrow edge the
nucleation barrier is effectively lower
than it would be for a large prism facet
a similar effect happens for the basal
facets just at a different temperature
and ken has done experiments to
investigate whether these effects are
observed in the lab and so i did a
series of experiments using that
apparatus and man it just like boom just
like that
[Music]
when you make a model
and you sort of find it supposed to do
something and it sort of does it's just
like
this might be right
so far the results agree nicely with the
hypothesis so after 85 years maybe we
now understand the molecular physics of
ice well enough to finally explain why
snowflakes grow into such a diverse
collection of columnar and plate-like
forms
i've done a lot of my career in
astronomy
and astrophysics nobody ever asks you
what it's good for i mean never not even
once did anyone say you know what are
those black holes are going to be used
for no
saturn's rings why do you care about
saturn's rings what what's the
motivation for studying sin no but
nobody asks that every time i give a
talk people are like what are you doing
what what what on earth is this for
i tell you the real reason the real
reason that i got into this you look at
a snowflake can you kind of go um
actually
we have any idea how that works well
that's just not that doesn't work
we have to know how that works damn it
well i want to be the guy who figured
out how snowflakes work
that's always been a driver you know as
a scientist you want to figure something
out
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