Physicists still disagree over the answer to the seemingly
simple question of why ice is slippery and how ice skaters
can glide across it on their skates.
According to a recent article in the New York Times,
("Why is Ice Slippery?" By Kenneth Chang, February 21, 2006),
the usual answer that everyone knows is wrong.
The standard — and wrong — explanation is based
on an unusual property of water: that the solid form, ice, is
less dense than the liquid form. That is why ice floats on
water, while a cube of frozen alcohol — which has a
freezing temperature of minus 173 degrees Fahrenheit —
would plummet to the bottom of a glass of liquid alcohol.
The lower density of ice also means that the melting
temperature of ice can be lowered below the usual 32 degrees
by squeezing on it and thereby increasing its density.
Therefore, according to the incorrect explanation, the
pressure exerted by the blade lowers the melting temperature
of the top layer of ice, so the ice melts and the blade
glides on a thin layer of water that refreezes as soon as the
blade passes.
However ice, said Robert M. Rosenberg, an emeritus professor
of chemistry at Lawrence University in Appleton, Wis., and a
visiting scholar at Northwestern University, "is a very
mysterious solid."
Dr. Rosenberg wrote an article about the slipperiness of ice
in the December issue of Physics Today, among other
reasons because it annoyed him that the wrong explanation is
so widespread.
"People will still say that when you ask them," Dr. Rosenberg
told the New York Times. "Textbooks are full of
it."
The explanation fails because the pressure-melting effect is
too small. A 150-pound person standing on ice wearing a pair
of ice skates exerts a pressure of about only 50 pounds per
square inch on the ice. (A typical blade edge is about one-
eighth of an inch wide and about 12 inches long.) That amount
of pressure lowers the melting temperature only a small
amount, from 32 degrees to 31.97 degrees. Yet ice skaters can
easily skate when the outside temperatures are much
colder.
The pressure-melting explanation also fails to explain why
someone wearing flat-bottom shoes, with a much greater
surface area that exerts even less pressure on the ice, can
also slip on ice.
Slipperiness is just one of the puzzles about ice. Besides
everyday ice, there are about a dozen other forms, some of
which experts suspect exist in the hot interior of Earth or
on the surface of Pluto. Scientists expect to discover more
variations.
Two alternative explanations have been proposed. One says
that the rubbing of a skate blade or a shoe bottom over ice
heats the ice and melts it, creating a slippery layer.
The other, which emerged a decade ago, suggests that the
surface of ice is always a slippery liquid. The argument is
that water molecules at the ice surface vibrate more because
there are no molecules above them to help hold them in place.
They thus remain an unfrozen liquid even at temperatures far
below freezing.
Scientists debate whether friction or the liquid layer plays
the more important role. Dr. Rosenberg, asked his opinion,
answered: "I say there are two major reasons."
A liquid layer on the ice surface is not a new idea. It was
first proposed by the physicist Michael Faraday in 1850 after
a simple experiment: he pressed two cubes of ice against each
other and they fused together. Faraday argued that the liquid
layers froze solid when they were no longer at the surface.
Because the layer is so thin, however, it is hard for
scientists to see.
In recent years more sophisticated experiments have also
produced results that suggest that there is a liquid on the
surface of ice. In 1996, Gabor A. Somorjai, a scientist at
Lawrence Berkeley Laboratory, bombarded the surface of ice
with electrons and noted that their scattering pattern
indicated a surface liquid at temperatures down to minus 235
degrees. More recently a team of German scientists found
results that corroborated the Lawrence Berkeley findings.
But a colleague of Dr. Somorjai's at Lawrence Berkeley,
Miquel Salmeron argues that the liquid layer, while it is
there, is not enough to explain the slipperiness.
In 2002, Dr. Salmeron and colleagues performed an experiment
that showed, according to him, that while the top layer of
ice may be liquid, it is too thin to contribute much to
slipperiness except near the melting temperature. In his
view, friction is the primary reason ice is slippery.
Dr. Salmeron concedes, however, that he cannot definitively
prove that his view is the correct one. "It's amazing," he
said. "We're in 2006, and we're still talking about this
thing."
In everyday ice, the molecules of water line up in a
hexagonal pattern. That is why snowflakes all have six-sided
patterns. Water — H2O — is two hydrogen atoms
connected to a central oxygen atom in a V-shape. In everyday
ice, which scientists call Ice Ih, the water molecules line
up in hexagonal structures. This is why snowflakes all have
six- sided patterns. (The "h" stands for hexagonal. A
variation called Ice Ic, found in ice crystals floating high
up in the atmosphere, forms cubic crystals.)
At high pressures, the hexagonal structure breaks down, and
the bonds rearrange themselves in denser crystal structures,
labeled with Roman numerals: Ice II, Ice III, Ice IV and so
on.
At a pressure of about 30,000 pounds per square inch, Ice Ih
turns into a different type of crystalline ice, Ice II. Ice
II does not occur naturally on Earth. Even at the bottom of
the thickest portions of the Antarctic ice cap, the weight of
three miles of ice pushes down at only one-quarter of the
pressure necessary to make Ice II. But planetary scientists
expect that Ice II, and possibly some other variations, like
Ice VI, exist inside icier bodies in the outer solar system,
like the Jupiter moons Ganymede and Callisto.
Some scientists calculated that the pressure of 700,000
pounds per square inch found in rocks 100 miles beneath the
surface of the Earth could be great enough to transform any
water there into a solid phase known as Ice VII. But no one
knows whether ice can be found inside Earth.
The most recently discovered form of ice, Ice XII, was found
just a decade ago, although hints of it had been seen years
earlier. John L. Finney of University College London, one of
the discoverers of Ice XII, said that trying to understand
all the different forms of ice was important for an
understanding of how the water molecule works, and that was
important in understanding how water interacts with all the
biological molecules in living organisms.
