Kind: captions
Language: en
Let's talk about ice. Why is ice so
extremely slippery? That's actually not
a trivial question, and physicists have
been arguing about it for more than a
hundred years.
You probably already know that molecules
behave differently in different states.
Like in liquid, molecules form weak
bonds, but they still have enough energy
to slip and slide past each other to fit
the shape of their container. In solids,
molecules are packed close together,
often in a regular pattern, which helps
solids keep a distinct shape. Because
these molecules can often nestle closer
together than they can in liquids, the
solid form of a material is usually
denser than the liquid form. But ice
acts a little differently. Ice is the
solid form of liquid. Oh my gosh, that's
why ice is slippery. But ice acts a
little differently. Ice is the solid
form of liquid water. But depending on
the temperature and pressure conditions,
ice can freeze in a lot of different
ways. In fact, there are more than a
dozen variations of ice crystals, and
scientists are still finding more. The
most common ice is hexagonal ice. It's
called ice 1H. One because it was the
first kind discovered, and H because
it's the hexagonal form. In this form,
the water molecules form layers of
honeycombs that stack on top of each
other to form the whole crystal. These
crystals take up more space than the
same number of liquid water molecules.
That means ice is less dense than water.
So it floats. Another important type is
ice 1C. Here the C stands for cubic
because in this form the water molecules
create a cubic centered crystal
structure. This kind of ice forms at
colder temperatures than ice 1H and can
sometimes be found in Earth's
atmosphere. As it turns out, these
crystal structures are key to
understanding why ice is so slippery.
But scientists in the 1800s didn't know
anything about them when they started
studying the problem. So, what did they
think was going on? Some of the first
people to try to solve the mystery were
big names like Lord Kelvin, famous for
the temperature unit Kelvin, and Michael
Faraday, Mr. Electronamics. Back around
1850, Kelvin and his brothers studied
the relationship between pressure and
the melting point of water. For example,
the idea was that if you stand on an icy
sidewalk, the weight of your body could
make the ice melt a little bit, creating
a liquid layer on the surface of the
ice. That layer would reduce friction,
make the ice slippery, and suddenly
you're not standing anymore. Faraday
also thought that a thin layer of liquid
on the surface of ice seemed to be the
key to making it slippery. And in 1859,
he proposed that liquid water always
coats the surface of ice, even at
temperatures far below freezing. This
idea didn't catch on right away, and
Kelvin's pressure melting theory was the
most widely accepted explanation for a
long time. Then in 1939, a pair of
researchers proposed it was actually
friction doing the melting, not just
pressure. Their thinking was that when a
material like an ice skate is dragged
across ice, the heat generated by
friction could melt the ice enough to
form a thin layer of lubricating water.
As it turns out, while pressure melting
and frictional melting can play a role,
they don't tell the whole story. The
arguments start to fall apart when you
get to temperatures of about -20° C,
where it would take way more pressure
than an ice skate pushing down on the
ice to melt it. Plus, in an experiment
with friction in the 1960s, friction no
longer produced enough heat to melt ice
at -35° C. But the ice was still
slippery. With modern imaging tools,
we've finally been able to take a closer
look at the atomic structure of ice. And
we found that ice isn't such a perfect
crystal after all. In a study published
in 2024, scientists found that at around
-150°
C, the surface of ice actually has a
mixture of 1H and 1C crystals. And as
the temperature increases, molecules
near the surface and between the
hexagonal and cubic regions become
disordered and the ice loses a lot of
its rigid crystalline structure. The
disordered layers are still somewhat
rooted in place because they're bonded
to the bulk of the solid, but they also
have some wiggle room at the surface of
the ice. In particular, these disordered
molecules have dangling bonds that reach
outward, like one of those blowup wavy
armed guys from car dealerships. Those
wavy arms have a lot of extra mobility,
but they also have some elasticity, so
they can spring back to their original
position. In ice terms, that means the
wiggly molecules form a super thin quasi
liquid layer that's way more viscous
than liquid water, almost more like oil.
And just like oil, that layer acts as a
lubricant, which is why ice feels so
slippery. In the end, Faraday wasn't so
far off in 1859. Sure, friction and
pressure can play a role, but ice is
slippery because, well, that's just the
way it is. It's a consequence of the
imperfections in the ice crystal itself.
And knowing that is actually kind of a
big deal. A lot of the world's energy is
lost to friction. So, new ideas about
slipperiness could also mean new ideas
for lubrication and manufacturing that's
more fuel, cost, and energy efficient,
or new ways to make winter sports and
winter driving safer. After a hundred
years, there's a whole world of new
possibilities for what we can do with
ice.